CIRCUIT
IDEAS
15-STEP DIGITAL POWER SUPPLY ere is a simple circuit to obtain variable DC voltage from 1.25V to 15.19V in reasonably small steps as shown in the table. The input voltage may lie anywhere between 20V
down by closing switch S2. The output of counter IC2 is used to realise a digitally variable resistor. This section consists of four N/O reed relays that need just about 5mA current for their
ing resistor across the relay contacts gets connected to the circuit. The table shows the theoretical output for various digital input combinations. The measured output is nearly equal to the theoretically calculated output across regulator IC3 (LM317). The output voltage is governed by the following relationship as long as the input-to-output differential is greater than or equal to 2.5V: Vout = 1.25(1+R2'/R1') Where, R1' = R15 = 270 ohms (fixed)
and 35V. The first section of the circuit comprises a digital up-down counter built around IC1— a quad 2-input NAND schmitt trigger (4093), followed by IC2— a binary up-down counter (4029). Two gates of IC 4093 are used to generate up-down logic using push buttons S1 and S2, respectively, while the other two gates form an oscillator to provide clock pulses to IC2 (4029). The frequency of oscillations can be varied by changing the value of capacitor C1 or preset VR1. IC2 receives clock pulses from the oscillator and produces a sequential binary output. As long as its pin 5 is low, the counter continues to count at the rising edge of each clock pulse, but stops counting as soon as its pin 5 is brought to logic 1. Logic 1 at pin 10 makes the counter to count upwards, while logic 0 makes it count downwards. Therefore the counter counts up by closing switch S1 and counts
operation. (EFY lab note. The original circuit containing quad bilateral switch IC 4066 has been replaced by reed relays operated by transistorised switches because of unreliable operation of the former.) The switching action is performed using BC548 transistors. External resistors are connected in parallel with the reed relay contacts. If particular relay contacts are opened by the control input at the base of a transistor, the correspond-
and R2' = R11 + R12 + R13 + R14 = 220 + 470 + 820 +1500 ohms = 3,010 ohms (with all relays energised) One can use either the binary weighted LED display as indicated by LED1 through LED4 in the circuit or a 74LS154 IC in conjunction with LED5 through LED20 to indicate one of the 16 selected voltage steps of Table I. The input for IC4 is to be tapped from points
NAVEEN THARIYAN
H
RUP
ANJA
ELECTRONICS FOR YOU ❚ MAY 2001
NA
CIRCUIT
IDEAS
TABLE Binary output 0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011 1100 1101 1110 1111
Equivalent dec no. 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
LED4 R14 (W) Shorted Shorted Shorted Shorted Shorted Shorted Shorted Shorted 1500 1500 1500 1500 1500 1500 1500 1500
LED3 R13 (W) Shorted Shorted Shorted Shorted 820 820 820 820 Shorted Shorted Shorted Shorted 820 820 820 820
marked ‘A’ through ‘D’ in the figure. This arrangement can be used to replace the LED arrangement at points A, B, C, and D. This 74LS154 IC is a decoder/ demultiplexer that senses the output of IC2 and accordingly activates only one of its 16 outputs in accordance with the
LED2 R12 (W) Shorted Shorted 470 470 Shorted Shorted 470 470 Shorted Shorted 470 470 Shorted Shorted 470 470
LED1 R11 (W) Shorted 220 Shorted 220 Shorted 220 Shorted 220 Shorted 220 Shorted 220 Shorted 220 Shorted 220
R2' (W) 0 220 470 690 820 1040 1290 1510 1500 1720 1970 2190 2390 2540 2790 3010
Vout (V) 1.25 2.27 3.43 4.44 5.05 6.06 7.22 8.24 8.19 9.21 10.37 11.39 11.99 13.01 14.17 15.19
count value. LEDs at the output of this IC can be arranged in a circular way along side the corresponding voltages.
Working When the power is switched on, IC2 re-
ELECTRONICS FOR YOU ❚ MAY 2001
sets itself, and hence the output at pins 6, 11, 14, and 12 is equivalent to binary zero, i.e. ‘0000’. The corresponding DC output of the circuit is minimum (1.25V). As count-up switch S1 is pressed, the binary count of IC2 increases and the output starts increasing too. At the highest count output of 1111, the output voltage is 15.19V (assuming the in-circuit resistance of preset VR2 as zero). Preset VR2 can be used for trimming the output voltage as desired. To decrease the output voltage within the range of 1.25V to 15.2V, count-down switch S2 is to be depressed. Notes. 1. When relay contacts across a particular resistor are opened, the corresponding LED glows. 2. The output voltages are shown assuming the in-circuit resistance of preset VR2 as zero. Thus when the in-circuit resistance of preset VR2 is not zero, the output voltage will be higher than that indicated here.
2-LINE INTERCOMCUM-TELEPHONE LINE CHANGEOVER CIRCUIT
T
he circuit presented here can be used for connecting two tele phones in parallel and also as a 2-line intercom. Usually a single telephone is connected to a telephone line. If another telephone is required at some distance, a parallel line is taken for connecting the other telephone. In this simple parallel line operation, the main problem is loss of privacy besides interference from the other phone. This problem is obviated in the circuit presented here. Under normal condition, two telephones (telephone 1 and 2) can be used as intercom while telephone 3 is connected to the lines from exchange. In
changeover mode, exchange line is disconnected from telephone 3 and gets connected to telephone 2. For operation in intercom mode, one has to just lift the handset of phone 1 and then press switch S1. As a result, buzzer PZ2 sounds. Simultaneously, the side tone is heard in the speaker of handset of phone 1. The person at phone 2 could then lift the handset and start conversation. Similar procedure is to be followed for initiation of the conversation from phone 2 using switch S2. In this mode of operation, a 3-pole, 2-way slideswitch S3 is to be used as shown in the figure. In the changeover mode of operation,
switch S3 is used to changeover the telephone line for use by telephone 2. The switch is normally in the intercom mode and telephone 3 is connected to the exchange line. Before changing over the exchange line to telephone 2, the person at telephone 1 may inform the person at telephone 2 (in the intercom mode) that he is going to changeover the line for use by him (the person at telephone 2). As soon as changeover switch S3 is flipped to the other position, 12V supply is cut off and telephones 1 and 3 do not get any voltage or ring via the ring-tone-sensing unit. Once switch S3 is flipped over for use of exchange line by the person at telephone 2, and the same (switch S3) is not flipped back to normal position after a telephone call is over, the next telephone call via exchange lines will go to telephone 2 only and the ring-tonesensing circuit will still work. This enables the person at phone 3 to know that a call has gone through. If the handset of telephone 3 is lifted, it is found to be dead. To make telephone 3 again active, switch S3 should be changed over to its normal position.
ELECTRONICS PROJECTS Vol. 21
189
40-Metre Direct Conversion Receiver
U
sing the circuit of direct-conversion receiver described here, one can listen to amateur radio QSO signals in CW as well as in SSB mode in the 40-metre band. The circuit makes use of three n-channel FETs (BFW10). The first FET (T1) performs the function of ant./RF amplifier-cum-product detector, while the second and third FETs (T2 and T3) together form a VFO (variable frequency oscillator) whose output
is injected into the gate of first FET (T1) through 10pF capacitor C16. The VFO is tuned to a frequency which differs from the incoming CW signal frequency by about 1 kHz to produce a beat frequency note in the audio range at the output of transformer X1, which is an audio driver transformer of the type used in transistor radios. The audio output from transformer X1 is connected to the input of audio amplifier built around IC1 (TBA820M) via volume
control VR1. An audio output from the AF amplifier is connected to an 8-ohm, 1-watt speaker. The receiver can be powered by a 12-volt power-supply, capable of sourcing around 250mA current. Audiooutput stage can be substituted with a readymade L-plate audio output circuit used in transistor amplifiers, if desired. The necessary data regarding the coils used in the circuit is given in the circuit diagram itself.
ELECTRONICS PROJECTS Vol. 20
CIRCUIT IDEAS
MAINS-OPERATED CHRISTMAS STAR
O I THE SAN
PRINCE PHILLIPS
H
ere is a low-cost circuit of Christmas star that can be easily constructed even by a novice. The main
advantage of this circuit is that it doesn’t require any step-down transformer or ICs. Components like resistors R1 and R2,
capacitors C1, C2, and C3, diodes D1 and D2, and zener ZD1 are used to develop a fairly steady 5V DC supply voltage that provides the required current to operate the multivibrator circuit and trigger triac BT136 via LED1. The multivibrator circuit is constructed using two BC548 transistors (T1 and T2) and some passive components. The frequency of the multivibrator circuit is controlled by capacitors C4 and C5 and resistors R3 through R7. The output of the multivibrator circuit is connected to transistor T3, which, in turn, drives the triac via LED1. During positive half cycles of the multivibrator’s output, transistor T3 energises triac BT136 and the lamp glows. This circuit is estimated to cost Rs 75.
MAY 2002
ELECTRONICS FOR YOU
CIRCUIT IDEAS
INFRARED TOY CAR MOTOR CONTROLLER
SAN
I THE
O
T.K. HAREENDRAN
T
his add-on circuit enables remote switching on/off of battery-operated toy cars with the help of a TV/ video remote control handset operating at 30–40 kHz. When the circuit is energised from a 6V battery, the decade counter CD4017 (IC2), which is configured as a toggle flip-flop, is immediately reset by the power-onreset combination of capacitor C3 and resistor R6. LED1 connected to pin 3 (Q0) of IC2 via resistor R5 glows to indicate the standby condition. In standby condition, data output pin of the integrated infrared receiver/demodulator (SFH505A or TSOP1738) is at a high level (about 5 volts) and transistor T1 is ‘off’ (reverse biased). The monostable wired around IC1 is inactive in this condition. When any key on the remote control handset is depressed, the output of the IR receiver momentarily transits through low state and transistor T1 conducts. As a result, the monostable is triggered and a short pulse is applied to the clock input (pin 14) of IC2, which takes Q1 output (pin 2) of IC2 high to switch on motor driver transistor T2 via base bias resistor R7 and the motor starts rotating continously (car starts running). Resistor R8 limits the starting current. When any key on the handset is
depressed again, the monostable is retriggered to reset decade counter IC2 and the motor is switched off. Standby LED1 glows again.
example, behind the front glass, and connect its wires to the circuit board using a short 3-core ribbon cable/shielded wire. Note. Since the circuit uses modu-
This circuit can be easily fabricated on a general-purpose printed board. After construction, enclose it inside the toy car and connect the supply wires to the battery of the toy car with right polarity. Rewire the DC motor connections and fix the IR receiver module in a suitable location, for
lated infrared beam for control function, ambient light reflections will not affect the circuit operation. However, fluorescent tubelights with electronic ballasts and CFL lamps may cause malfunctioning of the circuit.
JULY 2002
ELECTRONICS FOR YOU
CCIIRRC UCIUT IITD EIADS E A S
MAINS MANAGER
SUN
IL KU
MAR
SHIBASHISH PATEL
V
ery often we forget to switch off the peripherals like monitor, scanner, and printer while switching off our PC. The problem is that there are separate power switches to turn the peripherals off. Normally, the peripherals are connected to a single of those four-way trailing sockets that are plugged into a single wall socket. If that socket is accessible, all the devices could be switched off from there and none of the equipment used will require any modification. Here is a mains manager circuit that allows you to turn all the equipment on or off by just operating the switch on any one of the devices; for example, when you switch off your PC, the monitor as well as other equipment will get powered down automatically. You may choose the main equipment to control other gadgets. The main equipment is to be directly plugged into the master socket, while all other equipment are to be connected via the slave socket. The mains supply from the wall socket is to be connected to the input of the mains manager circuit. The unit operates by sensing the current drawn by the control equipment/load from the master socket. On sensing that the control equipment is on, it powers up the other (slave) sockets. The load on the master socket can be anywhere between 20 VA and 500 VA, while the load on the slave sockets can be 60 VA to 1200 VA. During the positive half cycle of the mains AC supply, diodes D4, D5, and D6 have a voltage drop of about 1.8 volts when current is drawn from the master socket. Diode D7 carries the current during negative half cycles. Capacitor C3, in series with diode D3, is connected across the diode combination of D4 through D6, in addition to diode D7 as well as resistor R10. Thus current pulses during positive half-cycles, charge up the capacitor to 1.8 ELECTRONICS FOR YOU
JULY 2002
volts via diode D3. This voltage is sufficient to hold transistor T2 in forward biased condition for about 200 ms even after the controlling load on the master socket is switched off. When transistor T2 is ‘on’, transistor T1 gets forward biased and is switched on. This, in turn, triggers Triac 1, which then powers the slave loads. Capacitor C4 and resistor R9 form a snubber network to ensure that the triac turns off cleanly with an inductive load.
possible, plug the unit into the mains via an earth leakage circuit breaker. The mains LED1 should glow and the slave LED2 should remain off. Now connect a table lamp to the master socket and switch it ‘on’. The lamp should operate as usual. The slave LED should turn ‘on’ whenever the lamp plugged into slave socket is switched on. Both lamps should be at full brightness without any flicker. If so, the unit is working correctly and can be put into use.
LED1 indicates that the unit is operating. Capacitor C1 and zener ZD1 are effectively in series across the mains. The resulting 15V pulses across ZD1 are rectified by diode D2 and smoothened by capacitor C2 to provide the necessary DC supply for the circuit around transistors T1 and T2. Resistor R3 is used to limit the switching-on surge current, while resistor R1 serves as a bleeder for rapidly discharging capacitor C1 when the unit is unplugged. LED1 glows whenever the unit is plugged into the mains. Diode D1, in anti-parallel to LED1, carries the current during the opposite half cycles. Don’t plug anything into the master or slave sockets without testing the unit. If
Note. 1. The device connected to the master socket must have its power switch on the primary side of the internal transformer. Some electronic equipment have the power switch on the secondary side and hence these devices continue to draw a small current from the mains even when switched off. Thus such devices, if connected as the master, will not control the slave units correctly. 2. Though this unit removes the power from the equipment being controlled, it doesn’t provide isolation from the mains. So, before working inside any equipment connected to this unit, it must be unplugged from the socket.
7MHz CW/AM QRP TRANSMITTER D. PRABAHARAN
T
he circuit of a 7MHz C W / A M QRP transmitter described here can be used to transmit either CW or audio frequency modulated signal over a 7MHz carrier. The carrier frequency oscillator is crystal controlled using 7MHz crystal in its fundamental mode. The tank circuit comprises a shortwave oscillator coil which can be tuned to 7MHz frequency with the help of ½J gang capacitor VC1. Transistor T2 (with identical tank circuit connected at its collector as in case of transistor T1) serves as a power amplifier. The RF output from oscillator stage is inductively coupled to the power amplifier stage. The output from power amplifier is routed via capacitor C3 and inductor L3 to a half-wave dipole using a 75-ohm coaxial cable. ½J gang capacitor VC3 along with inductor L3 forms an antenna tuning and matching network between the output of power amplifier stage and coaxial transmis-
sion line for maximum power transfer. Suitable heatsink should be used for transistor T2. Tuning adjustments may be accomplished using a 6-volt torch bulb. Connect the bulb to the collector of transistor T1 first through a coupling capacitor and tune ½J gang VC1 for maximum brilliance. (Note: the bulb would light according to intensity of RF energy.) Same procedure may be repeated for power amplifier stage and antenna tuning network for ensuring maximum power transfer.
For CW operation, switch S1 is to be kept on for bypassing the audio driver transformer and Morse key is used for on/off-type modulation. CW would be generated during key depressions. For AF modulation, Morse key points should be closed and switch S1 should be flipped to ‘off ’ position. Any suitable mic. amplifier may be used to feed audio input to the audio driver transformer X1. (For transformer X1 you may use the transistor-radio type AF driver transformer.)
Reader Comments: ¨ I request the author for the following clarifications: 1. Please indicate the construction details of coils L1 and L3 as well as the inductor which is connected in parallel to VC2. 2. Can we use any other crystal in place of 7MHz crystal? M.A. Kamal Guwahati ¨ What is the range of this transmitter and what is the output power of this circuit?
Vaibhav Kumar Saharanpur The author D. Prabaharan, comments: In reply to the above queries, I would like to say that the transistor T1 is BF495. Power output of this circuit is about 150mW. It can be further increased by using separate power supply for the power-amplifier stage (24V, 1A). The coil details are as follows— L1 is short-wave oscillator coil; L2:14 turns on 1cm-diameter air-core tube using 26 SWG wire; L3 has 12 turns on 1.5cm-diameter
air-core tube using 26 SWG wire. The frequency allotted for amateur radio operators is 7.0 MHz to 7.1 MHz. Hence, any crystal available within this frequency can be used. Range of this QRP transmitter depends on propagation conditions. If conditions are good, the range is about 500 kms in the CW mode and 100 kms in the AM mode. It is possible to convert this transmitter to 20-meter HAM band. Any crystal available from 14 MHz to 14.350 MHz range can be used for the purpose. However, this conversion needs following
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ELECTRONICS PROJECTS Vol. 19
modifications on coils L1, L2 and L3—L1: shortwave oscillator coil; L2: 11 turns on 1cm-diameter air-core tube using 26 SWG wire; L3: 9½ turns on 1cm-diameter aircore tube using 28 SWG wire. An ammeter with a range 0-250mA or a multimeter with 0-250mA can be
connected in-between the positive of the supply and the modulation transformer. Adjust VC1, VC2 and VC3 for maximum current through ammeter (CW-200mA, AM-125mA). The power input in CW and AM mode is calculated as shown below: DC power input (CW mode) = 24V
x 250mA = 6watt (the power amplifier draws 250mA current). DC power input (AM mode) = 24V x 120mA = 2.8watt (the power amplifier draws 120mA current).
ELECTRONICS PROJECTS Vol. 19
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CIRCUIT
IDEAS
9-LINE TELEPHONE SHARER
EDI DWIV S.C.
DHURJATI SINHA
T
his circuit is able to handle nine independent telephones (using a single telephone line pair) located at nine different locations, say, up to a distance of 100m from each other, for receiving and making outgoing calls, while maintaining conversation secrecy. This circuit is useful when a single telephone line is to be shared by more members residing in different rooms/apartments. Normally, if one connects nine phones in parallel, ring signals are
heard in all the nine telephones (it is also possible that the phones will not work due to higher load), and out of nine persons eight will find that the call is not for them. Further, one can overhear others’ conversation, which is not desirable. To overcome these problems, the circuit given here proves beneficial, as the ring is heard only in the desired extension, say, extension number ‘1’. For making use of this facility, the calling subscriber is required to initially dial the normal phone number of the
ELECTRONICS FOR YOU ❚ FEBRUARY 2001
called subscriber. When the call is established, no ring-back tone is heard by the calling party. The calling subscriber has then to press the asterik (*) button on the telephone to activate the tone mode (if the phone normally works in dial mode) and dial extension number, say, ‘1’, within 10 seconds. (In case the calling subscriber fails to dial the required extension number within 10 seconds, the line will be disconnected automatically.) Also, if the dialed extension phone is not lifted within 10 seconds, the ring-back tone will cease. The ring signal on the main phone line is detected by opto-coupler MCT2E (IC1), which in turn activates the 10-second ‘on timer’, formed by IC2 (555), and energises relay RL10 (6V, 100ohm, 2 C/O). One of the ‘N/O’ contacts of the relay has been used to connect +6V rail to the processing circuitry and the other has been used to provide 220-ohm loop resistance to deenergise the ringer relay in telephone exchange, to cut off the ring. When the caller dials the extension number (say, ‘1’) in tone mode, tone receiver CM8870 (IC3) outputs code ‘0001’, which is fed to the 4bit BCD-to-10 line decimal decoder IC4 (CD4028). The output of IC4 at its output pin 14 (Q1) goes high and switches on the SCR (TH-1) and associated relay RL1. Relay RL1, in turn, connects, via its N/O contacts, the 50Hz extension ring signal, derived from the 230V AC mains, to the line of telephone ‘1’. This ring signal is available to telephone ‘1’ only, because half of the signal is blocked by diode D1 and DIAC1 (which do not conduct below 35 volts). As soon as phone ‘1’ is lifted, the ring current increases and voltage drop across R28 (220-ohm, 1/2W resistor) increases and operates opto-coupler IC5 (MCT-2E). This in turn resets timer IC2 causing: (a) interruption of the power supply for processing circuitry as well as the ring
A HIERARCHICAL PRIORITY ENCODER
A
normal priority encoder encodes only the highest-order data line. But in many situations, not only the highest but the second-highest priority information is also needed. The circuit presented here encodes both the highest-priority information as well as the second-highest priority information of an 8-line incoming data. The circuit uses the standard octal priority encoder 74148 that is an 8-line-to-3-line (4-2-1) binary encoder with active-‘low’ data inputs and outputs. The first encoder (IC1) generates the highest-priority value, say, F. The active‘low’ output (A0, A1, A2) of IC1 is inverted by gates N9 through N11 and fed to a 3-line-to-8-line decoder (74138) that requires active-‘high’ inputs. The decoded outputs are active-‘low’. The decoder identifies the highest-priority data line and
(active-‘low’). Thus Lp=0 and Lq=0. All lines above Lp and also between Lp and Lq (denoted as Lj) are at logic 1. All lines below Lq logic state are irrelevant, i.e. ‘don’t care’. Here p is the highest-priority value and q the second-highest-priority value. (Obviously, q has to be lower than p, and the minimum possible value for p is taken as ‘1’.) Priority encoder IC1 generates binary output F2, F1, F0, which represents the value of p in active-‘low’ format. The complemented F2, F1, and F0 are applied to 3-line-to-8-line (one out of eight outputs is active-‘low’) decoder 74138. Let the output lines of 74138 be denoted as M0 through M7. Now only one line is active-‘low’ among M0 through M7, and that is Mp (where the value of p is explained as above). Therefore the logic level of line Mp is ‘0’ and that of all other M
ment of Lp = 1. All other L’s are not changed because the corresponding M’s are all 1’s. Thus data lines N0 through N7 are same as L0 through L7, except that the highest-priority level in L0 through L7 is cancelled in N0 through N7. The highest-priority level in N0 through N7 is the second-highest priority leftover from L0 through L7, i.e. Nq=0 and Nj=1 for q
that data value is cancelled using XNOR gates (N1 through N8) to retain the second-highest priority value that is generated by the second encoder. To understand the logic, let the incoming data lines be denoted as L0 to L7. Lp is the highest-priority line (active-‘low’) and Lq the second-highest priority line
lines ‘1’. The highest-priority line is cancelled using eight XNOR gates as shown in the figure. Let the output lines from XNOR gates be N0 through N7. Consider inputs Lp and Mp of the corresponding XNOR gate. Since Mp = 0 and also Lp = 0, the output of this XNOR gate is Np = comple-
to 74138 is 1 1 0 and it outputs M0 through M7 = 1 1 1 1 1 1 0 1. Since M6=0, only L6 is complemented by XNOR gates. Thus the outputs of XNORs are N0 through N7 = X X X 0 1 1 1 1. Now N3=0 and the highest priority for ‘N’ is 3. This value is recovered by priority encoder 2 (IC3) as S2 S1 S0 = 1 0 0.
ELECTRONICS PROJECTS Vol. 22
ACCURATE ELECTRONIC STOP-WATCH
H
ere is a simple circuit which can be used as an accurate stop-watch to count up to 100 seconds with a resolution of 0.01 second or up to 1000 seconds with a resolution of 0.1 second. This stop-watch can be used for sports and similar other activities. A 1MHz crystal generates stable frequency which is divided by two stages of 74390 ICs (dual decade counter) and another stage employing 7490 (decade
counter) IC to obtain a final frequency of 100 Hz or 10 Hz. Due to the use of crystal, the final frequency is very accurate. The output of IC4 (7490) is counted and displayed using IC5 74C926 (4-digit counter with multiplexed 7-segment LED driver). Due to multiplexed display the power consumption is very low. Switch S2 (2-pole, 2-way) is used to select appropriate input frequency and corresponding decimal point position to display up to
either 99.99 seconds or 999.9 seconds maximum count. For proper operation, first press switch S3 (reset) and then operate switch S2, according to the resolution/range desired (0.1 sec. or 0.01 sec.)/(100 seconds or 1000 seconds). Now to start counting, press switch S1. To stop counting, press switch S1 again. The counting will stop and display will show the correct time elapsed since the start of counting.
ELECTRONICS PROJECTS Vol. 19
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T I D E A S C I RC ICR CUUII T IDEAS
ADD-ON STEREO CHANNEL SELECTOR
RUPANJANA
PRABHASH K.P.
T
he add-on circuit presented here is useful for stereo systems. This circuit has provision for connecting stereo outputs from four different sources/channels as inputs and only one of them is selected/ connected to the output at any one time. When power supply is turned ‘on’, channel A (A2 and A1) is selected. If no audio is present in channel A, the circuit waits for some time and then selects the next channel (channel B), This search operation continues until it detects audio signal in one of the channels. The inter-channel wait or delay time can be adjusted with the help of preset VR1. If still longer time is needed, one may replace capacitor C1 with a capacitor of higher value. Suppose channel A is connected to a tape recorder and channel B is connected to a radio receiver. If initially
channel A is selected, the audio from the tape recorder will be present at the output. After the tape is played completely, or if there is sufficient pause between consecutive recordings, the circuit automatically switches over to the output from the radio receiver. To manually skip over from one (selected) active channel, simply push the skip switch (S1) momentarily once or more, until the desired channel inputs gets selected. The selected channel (A, B, C, or D) is indicated by the glowing of corresponding LED (LED11, LED12, LED13, or LED14 respectively). IC CD4066 contains four analogue switches. These switches are connected to four separate channels. For stereo operation, two similar CD4066 ICs are used as shown in the circuit. These analogue switches are controlled by IC CD4017 outputs. CD4017 is a 10-bit ring
97
counter IC. Since only one of its outputs is high at any instant, only one switch will be closed at a time. IC CD4017 is configured as a 4-bit ring counter by connecting the fifth output Q4 (pin 10) to the reset pin. Capacitor C5 in conjunction with resistor R6 forms a power-on-reset circuit for IC2, so that on initial switching ‘on’ of the power supply, output Q0 (pin 3) is always ‘high’. The clock signal to CD4017 is provided by IC1 (NE555) which acts as an astable multivibrator when transistor T1 is in cut-off state. IC5 (KA2281) is used here for not only indicating the audio levels of the selected stereo channel, but also for forward biasing transistor T1. As soon as a specific threshold audio level is detected in a selected channel, pin 7 and/ or pin 10 of IC5 goes ‘low’. This low level is coupled to the base of transistor T1, through diode-resistor combination of D2-R1/D3-R22. As a result, transistor T1 conducts and causes output of IC1 to remain ‘low’ (disabled) as long as the selected channel output exceeds the preset audio threshold level. Presets VR2 and VR3 have been included for adjustment of individual audio threshold levels of left stereo channels, as desired. Once the multivibrator action of IC1 is disabled, output of IC2 does not change further. Hence, search-
C I R C U I T
ing through the channels continues until it receives an audio signal exceeding the preset threshold value. The skip
I D E A S
switch S1 is used to skip a channel even if audio is present in the selected channel. The number of channels can be eas-
98
ily extended up to ten, by using additional 4066 ICs.
CIRCUIT
ANTI-THEFT SECURITY FOR CAR AUDIOS
IDEAS
EDI DWIV S.C.
T.K. HAREENDRAN his small circuit, based on popular CMOS NAND chip CD4093, can be effectively used for protecting your expensive car audio system against theft. When 12V DC from the car battery is
T
Whenever an attempt is made to remove the car audio from its mounting by cutting its connecting wires, the optocoupler immediately turns off, as its LED cathode terminal is hanging. As a result, the oscillator circuit built around
applied to the gadget (as indicated by LED1) through switch S1, the circuit goes into standby mode. LED inside optocoupler IC1 is lit as its cathode terminal is grounded via the car audio (amplifier) body. As a result, the output at pin 3 of gate N1 goes low and disables the rest of the circuit.
gates N2 and N3 is enabled and it controls the ‘on’/‘off’ timings of the relay via transistor T2. (Relay contacts can be used to energise an emergency beeper, indicator, car horns, etc, as desired.) Different values of capacitor C2 give different ‘on’/‘off’ timings for relay RL1 to be ‘on’/‘off’. With 100µF we get approxi-
ELECTRONICS FOR YOU ❚ JULY 2001
mately 5 seconds as ‘on’ and 5 seconds as ‘off’ time. Gate N4, with its associated components, forms a self-testing circuit. Normally, both of its inputs are in ‘high’ state. However, when one switches off the ignition key, the supply to the car audio is also disconnected. Thus the output of gate N4 jumps to a ‘high’ state and it provides a differentiated short pulse to forward bias transistor T1 for a short duration. (The combination of capacitor C1 and resistor R5 acts as the differentiating circuit.) As a result, buzzer in the collector terminal of T1 beeps for a short duration to announce that the security circuit is intact. This ‘on’ period of buzzer can be varied by changing the values of capacitor C1 and/or resistor R5. After construction, fix the LED and buzzer in dashboard as per your requirement and hide switch S1 in a suitable location. Then connect lead A to the body of car stereo (not to the body of vehicle) and lead B to its positive lead terminal. Take power supply for the circuit from the car battery directly. Caution. This design is meant for car audios with negative ground only.
Audio-Visual Extra Ringer for Phone
M
any a times one needs an extra telephone ringer in an adjoining room to know if there is an incoming call. For example, if the telephone is installed in the drawing room you may need an extra ringer in the bedroom. All that needs to be done is to connect the given circuit in parallel with the existing telephone lines using twin flexible wires. This circuit does not require any external power source for its operation. The section comprising resistor R1 and diodes D5 and LED1 provides a visual indication of the ring. Remaining part of the circuit is the audio ringer based on IC1 (BA8204 or ML8204). This integr- ated circuit, specially designed for telec- om application as bell sound generator, requires very few external parts. It is readily available in
8-pin mini DIP pack. Resistor R3 is used for bell sensitivity adjustment. The bell frequency is controlled by resistor R5 and capacitor C4, and the repetition rate is controlled by resistor R4 and capacitor C3. A little experimentation with the various values
of the resistors and capacitors may be carried out to obtain desired pleasing tone. Working of the circuit is quite simple. The bell signal, approximately 75V AC, passes through capacitor C1 and resistor R2 and appears across the diode bridge comprising diodes D1 to D4. The rectified DC output is smoothed by capacitor C2. The dual-tone ring signal is output from pin 8 of IC1 and its volume is adjusted by volume control VR1. Thereafter, it is impressed on the piezo-ceramic sound generator.
ELECTRONICS PROJECTS Vol. 20
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Dual-Channel Digital Volume Control SHEENA K.
T
his circuit could be used for replacing your manual volume control in a stereo amplifier. In this circuit, push-to-on switch S1 controls the forward (volume increase) operation of both channels while a similar switch S2 controls reverse (volume decrease) operation of both channels. Here IC1 timer 555 is configured as an astable flip-flop to provide low-fre-
N ILLO . DH A.P.S
quency pulses to up/down clock input pins of pre-setable up/down counter 74LS193 (IC2) via push-to-on switches S1 and S2. To vary the pulse width of pulses from IC1, one may replace timing resistor R1 with a variable resistor. Operation of switch S1 (up) causes the binary output to increment while operation of S2 (down) causes the binary output to decrement. The maxi-
ELECTRONICS FOR YOU n AUGUST '99
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mum count being 15 (all outputs logic 1) and minimum count being 0 (all outputs logic 0), it results in maximum and minimum volume respectively. The active high outputs A, B, C and D of the counter are used for controlling two quad bi-polar analogue switches in each of the two CD4066 ICs (IC3 and IC4). Each of the output bits, when high, short a part of the resistor network comprising series resistors R6 through R9 for one channel and R10 through R13 for the other channel, and thereby control the output of the audio signals being fed to the inputs of stereo amplifier. Push-to-on switch S3 is used for resetting the output of counter to 0000, and thereby turning the volume of both channels to the minimum level.
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Infrared Cordless Headphone
OO SAG G.S.
PRADEEP G.
U
sing this low-cost project one can reproduce audio from TV without disturbing others. It does not use any wire connection between TV and headphones. In place of a pair of wires, it uses invisible infrared light to transmit audio signals from
TV to headphones. Without using any lens, a range of up to 6 metres is
possible. Range can be extended by using lenses and reflectors with IR sensors comprising transmitters and receivers. IR transmitter uses two-stage transistor amplifier to drive two series-connected IR LEDs. An audio output transformer is used (in reverse) to couple audio output from TV to the IR transmitter. Transistors T1 and T2 amplify the audio signals received from TV through the audio transformer. Lowimpedance output windings (lower
ELECTRONICS FOR YOU n AUGUST '99
gauge or thicker wires) are used for connection to TV side while high-impedance windings are connected to IR transmitter. This IR transmitter can be powered from a 9-volt mains adapter or battery. Red LED1 in transmitter circuit functions as a zener diode (0.65V) as well as supply-on indicator. IR receiver uses 3-stage transistor amplifier. The first two transistors (T4 and T5) form audio signal amplifier while the third transistor T6 is used to drive a headphone. Adjust potmeter VR2 for max. clarity. Direct photo-transistor towards IR LEDs of transmitter for max. range. A
9-volt battery can be used with receiver for portable operation.
Auto Reset Over/Under Voltage Cut-Out J. Gopalakrishnan his over/under voltage cut-out will save your costly electrical and electronic appliances from the adverse effects of very high and
T
very low mains voltages. The circuit features auto reset and utilises easily available components. It makes use of the comparators available
inside 555 timer ICs. Supply is tapped from different points of the power supply circuit for relay and control circuit operation to achieve reliability.
The circuit utilises comparator 2 for control while comparator 1 output (connected to reset pin R) is kept low by shorting pins 5 and 6 of 555 IC. The positive input pin of comparator 2 is at 1/3rd of Vcc voltage. Thus as long as negative input pin 2 is less positive than 1/3 Vcc, comparator 2 output is high and the internal flip-flop is set, i.e. its Q output (pin 3) is high. At the same time pin 7 is in high impedance state and LED connected to pin
7 is therefore off. The output (at pin 3) reverses (goes low) when pin 2 is taken more positive than 1/3 Vcc. At the same time pin 7 goes low (as Q output of internal flip-flop is high) and the ED connected to pin 7 is lit. Both timers (IC1 and IC2) are configured to function in the same fashion. Preset VR1 is adjusted for under voltage (say 160 volts) cut-out by observing that LED1 just lights up when mains voltage is slightly greater than 160V AC. At this setting the output at pin 3 of IC1 is low and transistor T1 is in cut-off state. As a result RESET pin 4 of IC2 is held high since it is connected to Vcc via 100 kilo-ohm resistor R4. Preset VR2 is adjusted for over voltage (say 270V AC) cut-out by ob-
serving that LED2 just extinguishes when the mains voltage is slightly less than 270V AC. With RESET pin 4 of IC2 high, the output pin 3 is also high. As a result transistor T2 conducts and energises relay RL1, connecting load to power supply via its N/O contacts. This is the situation as long as mains voltage is greater than 160V AC but less than 270V AC. When mains voltage goes beyond 270V AC, it causes output pin 3 of IC2 to go low and cut-off transistor T2 and de-energise relay RL1, in spite of RESET pin 4 still being high. When mains voltage goes below 160V AC, IC1’s pin 3 goes high and LED1 is extinguished. The high output at pin 3 results in conduction of transistor T1. As a result collector of transistor T1 as also RESET pin 4 of IC2 are pulled low. Thus output of IC2 goes low and transistor T2 does not conduct. As a result relay RL1 is de-energised, which causes load to be disconnected from the supply. When mains voltage again goes beyond 160V AC (but less than 270V AC) the relay again energises to connect the load to power supply.
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ELECTRONICS PROJECTS Vol. 20
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AUTO SHUT-OFF FOR CASSETTE PLAYERS AND AMPLIFIERS
MAR IL KU SUN
ARTHUR LOUIS
H
ere are two simple, low-cost circuits that can be used to shut off the mains supply to any audio or video equipment (such as tape recorder, CD player, and amplifier). These circuits are helpful to those in the habit of falling asleep with their music system on. The circuits will also protect the equipment from getting damaged due to highvoltage spikes whenever there is a resumption of power after a break. This is possible because the equipment will get switched off automatically under such conditions but will not get switched on automatically on resumption of mains supply. The circuit in Fig. 1 can be used to shut off any cassette player that has a reliable auto-stop mechanism. Whenever switch S1 is pressed momentarily, it extends the supply to the step-down transformer of the tape recorder and charges capacitor C1 through diode D1. This, in turn, makes transistor T1 conduct and energise relay RL1 to provide a parallel path to switch S1, so that supply to the step-down transformer continues even when switch S1 is released. When any button on the cassette player is pressed, the capacitor charges through diode D2. This ensures conduction of transistor T1 and thus the continuity of operation of cassette player. However, whenever the auto-stop mechanism functions at the end of a tape, the leaf switch gets opened. This cuts the charging path for the capacitor and it starts discharging slowly. After about one minute, the relay opens and interrupts main power to the transformer. The time delay can be increased by increasing the value of capacitor C1. If the appliance used is a two-in-one type (e.g. cassette player-cum-radio), just connect another diode in parallel with diodes D1 and D2 to provide an additional path for charging capacitor C1 via the tape-to-radio changeover switch, so that when radio is played the relay does not
interrupt the power supply. The other circuit, shown in Fig. 2, functions on the basis of the signal received from preamp of the appliance used. In this circuit, opamp µA741 is wired in inverting opamp configuration. It amplifies the signal received from the preamp. Timer NE555 is used to provide the necessary time delay of about one minute. Preset VR1 is used to control the sensitivity of the circuit to differentiate be-
ELECTRONICS FOR YOU ❚ APRIL 2001
tween the noise and the signal. Resistor R4 offers feedback resistance to control the gain of the opamp. By increasing or decreasing the value of resistor R4, the gain can be increased or decreased, respectively. The preset time delay of timer NE555 (which is about one minute) can be increased by increasing the value of C4. Initial energisation of relay RL2 and charging of capacitor C4 take place on depression of switch S3 in the same manner as charging of capacitor C1 (refer Fig. 1) on depression of switch S1. As a result, pins 2 and 6 of NE555 go high and the output of timer goes low to switch off mains supply from the relay to step-down transformer X2 of the appliance. Bleeder resistor R6 is used to discharge capacitor C4. Now if signals are received from the
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preamplifier, these are amplified by 741 and fed to the base of transistor T2, which keeps capacitor C4 charged through resistor R5. When there is no signal, T2 will not conduct and the capacitor slowly discharges through R6. The output of 555 goes high to switch off the relay and thus
IDEAS
the mains supply to transformer X2. Switch S2 can be depressed momentarily if the device needs to be manually switched off. Note. The 12V supply should be provided to the circuit from the equipment’s power supply. Opamp 741 should be
ELECTRONICS FOR YOU ❚ APRIL 2001
driven from the preamplifier of the gadget used, and not from its power amplifier output. Switches S1 and S2 are 2pole push-to-on switches. These can also be fabricated from 2-pole on-off switches, which are widely used in cassette players, by removing the latch pin from them.
T
Automatic Dualoutput Display
his circuit lights up ten bulbs sequentially, first in one direction and then in the opposite direction, thus presenting a nice visual effect. In this circuit, gates N1 and N2 form
an oscillator. The output of this oscillator is used as a clock for BCD up/down counter CD4510 (IC2). Depending on the logic state at its pin 10, the counter counts up or down.
During count up operation, pin 7 of IC2 outputs an active low pulse on reaching the ninth count. Similarly, during countdown operation, you again get a low-going pulse at pin 7.
This terminal count output from pin 7, after inversion by gate N3, is connected to clock pin 14 of decade counter IC3 (CD4017) which is configured here as a toggle flip-flop by returning its Q2 output at pin 4 to reset pin 15. Thus output at pin 3 of IC3 goes to logic 1 and logic 0 state alternately at each terminal count of IC2.
Initially, pin 3 (Q0) of IC3 is high and the counter is in count-up state. On reaching ninth count, pin 3 of IC3 goes low and as a result IC2 starts counting down. When the counter reaches 0 count, Q2 output of IC3 momentarily goes high to reset it, thus taking pin 3 to logic 1 state, and the cycle repeats. The BCD outputs of IC2 are con-
nected to 1-of-10 decoder CD4028 (IC4). During count-up operation of IC2, the outputs of IC4 go logic high sequentially from Q0 to Q9 and thus trigger the triacs and lighting bulbs 1 through 10, one after the other. Thereafter, during count-down operation of IC2, the bulbs light in the reverse order, presenting a wonderful visual effect.
ELECTRONICS PROJECTS Vol. 20
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AUTOMATIC EMERGENCY LIGHT
PRIYANK MUDGAL his emergency light has the following two advantages: 1. It turns on automatically
stepped down by transformer X1, rectified by a full-wave rectifier comprising diodes D1 and D2, filtered by capacitor C1 and fed to relay coil RL1. The relay energises to connect the bat-
when the mains power fails, so you need not search it in the dark. 2. Its battery starts charging as soon as the mains resumes. Operation of the circuit is quite straightforward. Mains supply is
tery to the charging circuit through its normally-opened (N/O) contacts. Freewheeling diode D3 acts as a spike buster for the relay. The charging circuit is built around npn transistor BD139 (T1). The trans-
T
114 • MARCH 2007 • ELECTRONICS FOR YOU
IVEDI S.C. DW
former output is fed to the collector of transistor T1, which provides a fixed bias voltage of 6.8V to charge the battery. When the battery is fully charged, the battery voltage becomes equal to the breakdown voltage of the zener diode (ZD1). Zener diode ZD1 conducts to provide an alternative path for the current to ground and battery charging stops. When mains fails, relay RL1 de-energises. The battery now gets connected to the white LED array (comprising LED1 through LED6) through current-limiting resistor R2. The LEDs glow to light up the room. To increase the brightness in your room, you can increase the number of white LEDs after reducing the value of resistor R2 and also use a reflector assembly.
WWW.EFYMAG.COM
AUTOMATIC EMERGENCY TORCH
J
ust don’t think that this is yet another addition to other emergency light circuits published in EFY earlier. This circuit is a hit different. Its main features are: 1. Very reliable operation. 2. As transformer is not used, it is compact and cost-effective. 3. The torch bulb glows automatically at power off and goes out on restoration of power. 4. Since Ni-Cd battery is used, no maintenance is required. Also, battery life is very long, nearly 4-5 years (though this depends on frequency of usage and also on ampere-hour rating of the battery used). Sounds interesting, doesn’t it? Read on then. The circuit is very simple, comprising just a handful of components. This implies that the circuit operation also is very simple. The circuit consists of two parts: 1. Power supply for charging the NiCd battery. 2. Switchover circuit which detects mains failure and switches the bulb ‘on’. In the power supply section, capacitors C1 and C2 function as non-dissipating, reactive impedances which limit the current to a safe value. With the values of capacitors as shown, the maximum current that can be drawn is limited to about 70 mA at 230V AC. Resistor R2 limits the initial surge current and resistor R1 assists in discharging the capacitors after switch off. Diodes D1 through D4 form a conventional bridge rectifier while capacitor C3 is the filter capacitor. Fuse F1 is for protection and is very helpful in the event of any component giving up the ghost. This supply charges the battery as long as mains is present. In the ‘switchover’ section, transistor T1 is used as switch. Normally, when AC mains supply is present, the rectifier output
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charges the battery through resistor R4 and LED D5 combination at about 50mA rate. The glowing LED (D5) also gives an indication of mains presence. Further, due to the LED (D5), base of transistor T1 is about 1.6V (drop across D5) more positive than its emitter. This voltage is more than sufficient to keep the transistor at cut-off. As soon as the mains voltage fails, the base of transistor T1 is pulled low through resistor R3 which drives transistor T1 to saturation thereby turning the bulb ‘on’. Since the transistor is in its saturated state, the voltage drop across it is very low. Hence the bulb glows with full brilliance. The bulb can be switched off by the ON/OFF switch, when not required. With this bulb (2.2V, 250mA) the torch can work continuously for about two hours. The batteries should be charged for about 14 hours after they are discharged. You can verify following voltages in the circuit: 1. Base voltage of the transistor must be 1.8V to 2.0V, i.e. about 0.6V less than the battery voltage. 2. Emitter voltage must be equal to the battery voltage.
3. Collector voltage must be 2.0V to 2.2V, i.e. nearly equal to the battery voltage. All above voltages should be checked with AC mains off. If any of the abovementioned voltages is absent it indicates that the transistor is bad and it should be replaced by a good one. Here is a word of caution now. Since the circuit is not isolated from AC mains. it may be hazardous to touch any component when the mains supply is on, especially if the supply wires (live and neutral) get interchanged. It is strongly recommended to use an all-plastic enclosure (including the reflector for the bulb) for the circuit. Also the ON/OFF switch used should have a plastic lever. Take proper care and precautions while building, testing and using the circuit, and never ever allow the supply wires to interchange. It is advisable to provide a plug for the mains input on the box itself so that it can be plugged directly into a mains outlet. This reduces the chances of mains supply wires getting interchanged. With proper precautions and a little care, it is hoped that this small circuit will help make life a bit more comfortable.
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AUTOMATIC HEAT DETECTOR SUKANT KUMAR BEHARA
T
his circuit uses a complementary pair comprising npn metallic transistor T1 (BC109) and pnp germanium transistor T2 (AC188) to detect heat (due to outbreak of fire, etc) in the vicinity and energise a siren. The collector of transistor T1 is connected to the base of transistor T2, while the collector of transistor T2 is connected to relay RL1. The second part of the circuit comprises popular IC UM3561 (a siren and machine-gun sound generator IC), which can produce the sound of a fire-brigade siren. Pin numbers 5 and 6 of the IC are connected to the +3V supply when the relay is in energised state, whereas pin 2 is grounded. A resistor (R2) connected across pins 7 and 8 is used to fix the frequency of the inbuilt oscillator. The output is available from pin 3. Two transistors BC147 (T3) and BEL187 (T4) are connected in Darlington configuration to amplify the
lay is in energised state. LED1, connected in series with 68-ohm resistor EDI R1 across resistor R4, glows when the V I W D S.C. siren is on. To test the working of the circuit, bring a burning matchstick Pin Designation Sound Effect close to transistor T1 (BC109), SEL1 SEL2 which causes the resistance of its No Connection No Connection Police Siren emitter-collector junction to go low +3V No Connection Fire Engine Siren Ground No Connection Ambulance Siren due to a rise in temperature and it Do not care +3V Machine Gun starts conducting. Simultaneously, transistor T2 also conducts because its base is connected to the collector of transistor T1. As a result, relay RL1 energises and switches on the siren circuit to produce loud sound of a firebrigade siren. Lab note. We have added a sound from UM3561. Resistor R4 in se- table to enable readers to obtain all posries with a 3V zener is used to provide sible sound effects by returning pins 1 the 3V supply to UM3561 when the re- and 2 as suggested in the table.
ELECTRONICS FOR YOU ❚ MARCH 2001
Automatic Room Power Control
A
n ordinary automatic room power control circuit has only one light sensor. So when a person enters the room it gets one pulse and the lights come ‘on.’ When the person goes out it gets another pulse and the lights go ‘off.’ But what happens when two persons enter the room, one after the other? It gets two pulses and the lights remain in ‘off’ state. The circuit described here overcomes the above-mentioned problem. It has a small memory which enables it to automatically switch ‘on’ and switch ‘off’ the lights in a desired fashion. The circuit uses two LDRs which are placed one after another (separated by a distance of say half a metre) so that they may separately sense a person going into the room or coming out of the room. Outputs of the two LDR sensors, after processing, are used in conjunction with a bicolour LED in such a fashion that when a person gets into the room it emits green light and when a person goes out of the room it emits red light, and vice versa. These outputs are simultaneously applied to two counters. One of the counters will count as +1, +2, +3 etc when persons are coming into the room and the other will count as -1, -2, -3 etc when persons are going out of the room. These counters make use of Johnson decade counter CD4017 ICs. The next stage comprises two logic ICs which can combine the outputs of the two counters and determine if there is any person still left in the room or not. Since in the circuit LDRs have been used, care should be taken to protect them from ambient light. If desired, one may use readily available IR sensor modules to replace the LDRs. The sensors are installed in such a way that when a person enters or leaves the room, he intercepts the light falling on them sequentially—one after the other. When a person enters the room, first he would obstruct the light falling on LDR1, followed by that falling on LDR2. When a person leaves the room it will be the other way round. In the normal case light keeps falling on both the LDRs, and as such their resistance is low (about 5 kilo-ohms). As a ELECTRONICS PROJECTS Vol. 20
result, pin 2 of both timers (IC1 and IC2), which have been configured as monostable flip-flops, are held near the supply voltage (+9V). When the light falling on the LDRs is obstructed, their resistance becomes very high and pin 2 voltages drop to near ground potential, thereby triggering the flip-flops. Capacitors across pin 2 and ground have been added to avoid false triggering due to electrical noise. When a person enters the room, LDR1 is triggered first and it results in triggering of monostable IC1. The short output pulse immediately charges up capacitor C5, forward biasing transistor pair T1-T2. But at this instant the collectors of transistors T1 and T2 are in high impedance state as IC2 pin 3 is at low potential and diode D4 is not conducting. But when the same person passes LDR2, IC2 monostable flip-flop is triggered. Its pin 3 goes high and this potential is coupled to transistor pair T1-T2 via diode D4. As a result transistor
pair T1-T2 conducts because capacitor C5 retains the charge for some time as its discharge time is controlled by resistor R5 (and R7 to an extent). Thus green LED portion of bi-colour LED is lit momentarily. The same output is also coupled to IC3 for which it acts as a clock. With entry of each person IC3 output (high state) keeps advancing. At this stage transistor pair T3-T4 cannot conduct because output pin 3 of IC1 is no longer positive as its output pulse duration is quite short and hence transistor collectors are in high impedance state. When persons leave the room, LDR2 is triggered first, followed by LDR1. Since the bottom half portion of circuit is identical to top half, this time, with the departure of each person, red portion of bicolour LED is lit momentarily and output of IC4 advances in the same fashion as in case of IC3. The outputs of IC3 and those of IC4 (after inversion by inverter gates N1
through N4) are ANDed by AND gates (A1 through A4) and then wire ORed (using diodes D5 through D8). The net effect is that when persons are entering, the output of at least one of the AND gates is high, causing transistor T5 to conduct and energise relay RL1. The bulb connected to the supply via N/O contact of relay RL1 also lights up. When persons are leaving the room, and till all the persons who entered the room have left, the wired OR output continues to remain high, i.e. the bulb continues to remains ‘on,’ until all persons who entered the room have left. The maximum number of persons that this circuit can handle is limited to four since on receipt of fifth clock pulse the counters are reset. The capacity of the circuit can be easily extended to handle up to nine persons by removing the connection of pin 1 from reset pin (15) and utilising Q1 to Q9 outputs of CD4017 counters. Additional inverters, AND gates and diodes will, however, be required.
ELECTRONICS PROJECTS Vol. 20
AUTOMATIC SPEED-CONTROLLER FOR FANS AND COOLERS
D
uring summer nights, the temperature is initially quite high. As time passes, the temperature starts dropping. Also, after a person falls asleep, the metabolic rate of one’s body decreases. Thus, initially the fan/cooler needs to be run at full speed. As time passes, one has to get up again and again to adjust the speed of the fan or the cooler. The device presented here makes the fan run at full speed for a predetermined time. The speed is decreased to medium after some time, and to slow later on. After a period of about eight hours, the fan/ cooler is switched off. Fig. 1 shows the circuit diagram of the system. IC1 (555) is used as an astable multivibrator to generate clock pulses. The pulses are fed to decade dividers/counters formed by IC2 and IC3. These ICs act as
divide-by-10 and divide-by-9 counters, respectively. The values of capacitor C1 and resistors R1 and R2 are so adjusted that the final output of IC3 goes high after about eight hours. The first two outputs of IC3 (Q0 and Q1) are connected (ORed) via diodes D1 and D2 to the base of transistor T1. Initially output Q0 is high and therefore relay RL1 is energised. It remains energised when Q1 becomes high. The method of connecting the gadget to the fan/cooler is given in Figs 3 and 4. It can be seen that initially the fan
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shall get AC supply directly, and so it shall run at top speed. When output Q2 becomes high and Q1 becomes low, relay RL1 is turned ‘off’ and relay RL2 is switched ‘on’. The fan gets AC through a resistance and its speed drops to medium value. This continues until output Q4 is high. When Q4 goes low and Q5 goes high, relay RL2 is switched ‘off’ and relay RL3 is activated. The fan now runs at low speed.
Throughout the process, pin 11 of the IC3 is low, so T4 is cut off, thus keeping T5 in saturation and RL4 ‘on’. At the end of the cycle, when pin 11 (Q9) becomes high, T4 gets saturated and T5 is cut off. RL4 is switched ‘off’, thus switching ‘off’ the fan/ cooler. Using the circuit described above, the fan shall run at high speed for a comparatively lesser time when either of Q0 or Q1 output is high. At medium speed, it will run for a moderate time period when any of three outputs Q2 through Q4 is
high, while at low speed, it will run for a much longer time period when any of the four outputs Q5 through Q8 is high. If one wishes, one can make the fan run at the three speeds for an equal amount of time by connecting three decimal decoded outputs of IC3 to each of the transistors T1 to T3. One can also get more than three speeds by using an additional relay, transistor, and associated
components, and connecting one or more outputs of IC3 to it. In the motors used in certain coolers there are separate windings for separate speeds. Such coolers do not use a rheostat type speed regulator. The method of connection of this device to such coolers is given in Fig. 4. The resistors in Figs 2 and 3 are the tapped resistors, similar to those used in manually controlled fan-speed regulators. Alternatively wire-wound resistors of suitable wattage and resistance can be used.
AUTOMATIC TEMPERATURE CONTROLLED FAN
H
ere is a circuit through which the speed of a fan can be linearly controlled automatically, depending on the room temperature. The circuit is highly efficient as it uses thyristors for power control. Alternatively, the same circuit can be used for automatic temperature controlled AC power control. In this circuit, the temperature sensor used is an NTC thermistor, i.e. one having a negative temperature coefficient. The value of thermistor resistance at 25°C is about 1 kilo-ohm. Op-amp A1 essentially works as I to V (current-to-voltage) converter and converts temperature variations into voltage variations. To amplify the change in voltage due to change in temperature, instrumentation amplifier formed by op-amps A2, A3 and A4 is used. Resistor R2 and zener diode
D1 combination is used for generating reference voltage as we want to amplify only change in voltage due to the change in temperature. Op-amp µA741 (IC2) works as a comparator. One input to the comparator is the output from the instrumentation amplifier while the other input is the stepped down, rectified and suitably attenuated sample of AC voltage. This is a negative going pulsating DC voltage. It will be observed that with increase in temperature, pin 2 of IC2 goes more and more negative and hence the width of the positive going output pulses (at pin 6) increases linearly with the temperature. Thus IC2 functions as a pulse width modulator in this circuit. The output from the comparator is coupled to an optocoupler, which in turn controls the AC
power delivered to fan (load). The circuit has a high sensitivity and the output RMS voltage (across load) can be varied from 120V to 230V (for a temp. range of 22°C to 36°C), and hence wide variations in speed are available. Also note that speed varies linearly and not in steps. Besides, since an optocoupler is used, the control circuit is fully isolated from power circuit, thus providing added safety. Note that for any given temperature the speed of fan (i.e. voltage across load) can be adjusted to a desired value by adjusting potmeters VR1 and VR2 appropriately. Potmeter VR1 should he initially kept in its mid position to realise a gain of approximately 40 from the instrumentation amplifier. It may be subsequently trimmed slightly to obtain linear variation of the fan speed.
ELECTRONICS PROJECTS Vol. 19
189
blown fuse indicator
G
enerally, when an equipment indicates no power, the cause may be just a blown fuse. Here is a circuit that shows the condition of fuse through LEDs. This compact circuit is very useful and reliable. It uses very few components, which makes it inexpensive too. Under normal conditions (when fuse is alright), voltage drop in first arm is 2V + (2 x 0.7V) = 3.4V, whereas in
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ELECTRONICS PROJECTS Vol. 22
second arm it is only 2V. So current flows through the second arm, i.e. through the green LED, causing it to glow; whereas the red LED remains off. When the fuse blows off, the supply to green LED gets blocked, and because only one LED is in the circuit, the red LED glows. In case of power failure, both LEDs remain ‘off’. This circuit can be easily modified to produce a siren in fuse-blown condition
(see Fig. 2). An optocoupler is used to trigger the siren. When the fuse blows, red LED glows. Simultaneously it switches ‘on’ the siren. In place of a bicolour LED, two LEDs of red and green colour can be used. Similarly, only one diode in place of D1 and D2 may be used. Two diodes are used to increase the voltage drop, since the two LEDs may produce different voltage drops.
CD-ROM Drive as Digital -audio CD-Player
A
CD-ROM drive can be used as a stand-alone unit for playing digital audio CDs without interfacing with a computer. The stereo output of CD player available at the audio jack can be amplified using audio input facility which is normally available on a tape-deck/tape-recorder or a stereo amplifier. Audio socket on front/rear of the CD-ROM drive is capable of driving headphones or speakers of less than 500 mW. Proper stereo jacks for interconnection between CD-ROM drive and tape deck are available from computer/tape recorder spares vendors. The principle of operation is illustrated here with the help of block diagram. The 4-pin power supply socket available at the rear of a CD-ROM player is meant for +5V, ground (two middle pins) and +12V inputs. The power supply can be easily derived using a conventional power supply circuit as shown in the figure. If you have an external CD-ROM drive, it can be simply plugged into the mains
since it has self-contained power supply circuit inside. While there may be minor differences amongst the available CD-ROM drives’ external controls, a typical drive’s controls are shown in the figure here. Please ensure that a proper power supply connector available from computer spare parts vendor is used for connection to CD-ROM drive. To identify +5V and +12V pins on the drive connector, please note that in the computer +12V
is routed using a yellow wire and for +5V a red wir is used, while for ground black wires are used with the supply connector. Once the power supply has been connected correctly, you will notice that LED indicator on the drive starts flashing. Now the digital audio CD can be loaded after pushing the eject button. A second push of the same button causes retraction of CD carriage into the drive. One can change the track (song) on the CD using play switch on the CD-ROM drive.
ELECTRONICS PROJECTS Vol. 20
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Drinking Water Alarm
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s.c. dwiv
Dr C.H. Vithalani
T
he State Jal Boards supply water for limited duration in a day. Time of water supply is decided by the management and the public does not know the same. In such a situation, this water alarm circuit will save the people from long wait as it will inform them as soon as the water supply starts. At the heart of this circuit is a small water sensor. For fabricating this water sensor, you need two foils—an aluminium foil and a plastic foil. You can assemble the sensor by rolling aluminium and plastic foils in the shape of a concentric cylinder. Connect one end of the insulated flexible wire on the aluminium foil and the other end to resistor R2. Now mount this sensor inside the water tap such that water can flow through it uninterrupted. To complete the circuit, connect another wire from the junction of pins 2 and 6 of IC1 to the water pipeline or the water tap itself. The working of the circuit is simple. Timer 555 is wired as an astable multivibrator. The multivibrator will
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work only when water flows through the water tap and completes the circuit connection. It oscillates at about 1 kHz. The output of the timer at pin 3 is connected to loudspeaker LS1 via capacitor C3. As soon as water starts flowing through the tap, the speaker starts sounding, which indicates resumption of water supply. It remains ‘on’ until you switch off the circuit with switch S1 or remove the sensor
from the tap. The circuit works off a 9V battery supply. Assemble the circuit on any general-purpose PCB and house in a suitable cabinet. The water sensor is inserted into the water tap. Connect the lead coming out from the junction of 555 pins 2 and 6 to the body of the water tap. Use on/off switch S1 to power the circuit with the 9V PP3 battery.
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CIRCUIT IDEAS
FLASHING BEACON ASHOK K. DOCTOR
A
flashing beacon has many uses. It can be employed as a distress signal on highways or as a direction pointer for parking lots, hospitals, hotels, etc. Here we present a flashing beacon that uses well-known regulator IC LM317T. As LM317T regulator can deliver more than 1 amp. A small 12V, 10W bulb with a high-quality reflector can serve as a good visible blinker. A 12-15V, 1A DC supply is connected to the input pin of the IC. A 12V, 10W bulb and a combination of resistors and capacitors are connected between the output pin and ADJ pin of the IC as shown in
I VED DWI S.C.
the figure. The IC is provided with an aluminium heat-sink to dissipate the heat generated while delivering full current. Since the IC has an inbuilt switch-on current limiter, it extends the bulb life. For the shown values of resistors and capacitors, the bulb flashes at approximately 4 cycles per second. The number of flashes depends on the charge-discharge time of the capacitors. Different values of resistors and capacitors can be used to increase or de-
crease the number of flashes. This circuit costs around Rs 50.
NOVEMBER 2002
ELECTRONICS FOR YOU
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GLOW PLUG CONTROLLER
T.A. Babu
I
n diesel engines, the air in the cylinders is not hot enough to ignite the fuel under cold conditions. Therefore each cylinder of these
charges capacitor C1 rapidly via resistor R1. When the voltage on capacitor C1 exceeds the threshold voltage of the gate (G) of MOSFET T1, it starts charging reservoir capacitor C2 and simultaneously energises relay RL1.
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the glow plug gets the power supply through its contact. The red LED (LED1) indicates that the heating process of glow plugs is
Fig. 2: Pin configurations of bs170 and bc548
Fig. 1: Glow plug controller
engines is fitted with an electric heater known as ‘glow plug.’ A control circuit is necessary to optimise the functioning of glow plugs. It raises the air temperature inside the engine cylinder for quick and reliable starting, extended battery life and reduced diesel consumption. The glow plug controller (Fig. 1) uses a simple timer circuit built around MOSFET T1 for reliability and simplicity. Momentary pushing of switch S2
MOSFET T1 remains conducting as long as the voltage on C1 is greater than the threshold voltage of the MOSFET gate. The ‘on’ time period depends on the value of capacitor C1 and resistor R2, which govern the discharge current of capacitor C2. The component values given here will produce ‘on’ time of around 25 seconds. In effect, when you press switch S2 momentarily, the relay energises for about 25 seconds and
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‘on.’ When the ‘on’ time is over, the green LED (LED2) turns on for a while, followed by a short beep from the buzzer, which indicates that the engine is ready for starting. Glow plugs draw a heavy current, hence high-current-rating contacts of an automotive relay are required. Assemble the circuit on any general-purpose PCB and house in a suitable case. Connect the glow plug wire to the relay contact. 12V battery already available with the vehicle is used to power the circuit. Connect the piezobuzzer and LED1 and LED2 through an external connection and place it at a convenient location for the driver to operate.
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CIRCUIT IDEAS
KNOCK ALARM PRADEEP G.
T
his circuit (Fig. 1), used in conjunction with a thin piezoelectric plate, senses the vibration generated on knocking a surface (such as a door or a table) to activate the alarm. It uses readilyavailable, low-cost components and can also be used to safeguard motor vehicles. The piezoelectric plate is used as the sensor. It is the same as used in ordinary
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circuit. When someone knocks on the door, the piezoelectric sensor generates an electrical signal, which is amplified by transistors T1 through T3. The amplified signal is rectified and filtered to produce a low-level DC voltage, which is further amplified by the remaining transistors. The final output from the collector of pnp transistor T6 is applied to reset pin 4 of 555 (IC1) that is wired as an Fig. 2: Proposed installation of knock alarm
Fig. 1: The circuit of knock alarm
piezobuzzers and is easily available in the market. The piezoelectric plate can convert any mechanical vibration into electrical variation. As it doesn’t sense sound from a distance like a microphone, it avoids false triggering. The plate can be fixed on a door, cash box, cupboard, etc using adhesive. A 11.5m long, shielded wire is connected between the sensor plate and the input of the
ELECTRONICS FOR YOU
NOVEMBER 2002
astable multivibrator. Whenever the collector of transistor T6 goes high, the astable multivibrator activates to sound an alarm through the speaker. The value of resistor R12 is chosen between 220 and 680 ohms such that IC1 remains inactive in the absence of any perceptible knock. When the circuit receives an input signal due to knocking, the alarm gets activated for about 10 seconds. This is the
time that capacitor C5 connected between the emitter of transistor T4 and ground takes to discharge after a knock. The time delay can be changed by changing the value of capacitor C5. After about 10 seconds, the alarm is automatically reset. The circuit operates off a 9V or a 12V battery eliminator. The proposed installation of the knock alarm is shown in Fig. 2. This circuit costs around Rs 75.
CIRCUIT
IDEAS
MEDIUM-POWER FM TRANSMITTER
PRADEEP G.
T
he range of this FM transmitter is around 100 metres at 9V DC supply. The circuit comprises three stages. The first stage is a microphone preamplifier built around BC548 transistor. The next stage is a VHF oscillator wired around another BC548. (BC series transistors are generally used in low-frequency stages. But these also work fine
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in RF stages as oscillator.) The third stage is a class-A tuned amplifier that boosts signals from the oscillator. Use of the additional RF amplifier increases the range of the transmitter. Coil L1 comprises four turns of 20SWG enamelled copper wire wound to 1.5cm length of a 4mm dia. air core. Coil L2 comprises six turns of 20SWG enamelled copper wire wound on a 4mm dia. air core. Use a 75cm long wire as the an-
Fig. 2: Pin configurations of transistors BC548 and C2570
tenna. For the maximum range, use a sensitive receiver. VC1 is a frequency-adjusting trimpot. VC2 should be adjusted for the maximum range. The transmitter unit is pow-
Fig. 3: Walkie-talkie arrangement
ered by a 9V PP3 battery. It can be combined with a readily available FM receiver kit to make a walkie-talkie set as shown in Fig. 3. z
Fig. 1: FM transmitter
80 • AUGUST 2005 • ELECTRONICS FOR YOU
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IDEAS
MOBILE SHIELD
D. MOHAN KUMAR
MALAYAP
PASAMY
rotect your mobile phone from unauthorised use or theft using this simple circuit. It can generate a loud chirping sound when somebody attempts to take away the mobile handset. The added feature is that the circuit also works as a mobile charger. The circuit is powered by a step-down transformer X1 with rectifier diodes D1 and D2 and filter ca-
greater sensitivity and enables the circuit with hand capacitance effect. Output pulses from the oscillator are directly given to trigger pin 2 of the monostable. The monostable uses a low-value capacitor C6, resistors R3 and preset VR1 for timing. The output frequency of the monostable is adjusted using preset VR1 such that it is slightly less than that of the astable. This makes the circuit standby, when there is no hand capacitance present. So in the
lates. This produces chirping sound from the buzzer and also makes the LED1 blink. The circuit can also be used as a mobile charger. It provides output of 6V at 180 mA through regulator IC 7806 (IC4) and resistor R5 for charging the mobile phone. Diode D3 protects the output from polarity reversal. The circuit can be wired on a common PCB. Enclose it in a suitable case with provision for charger out-
pacitor C1. Regulator IC 7812 (IC1) along with noise filter capacitors C2 and C3 provides regulated power supply. The circuit utilises two NE555 timer ICs: One as a simple astable multivibrator (IC2) and the second as a monostable (IC3). The astable multivibrator has timing resistors R1 and R2 but no timing capacitor as it works with stray capacitance. Its pins 6 and 2 are directly connected to a protecting shield made up of 10cm×10cm copper-clad board. The inherent stray capacitance of the circuit is sufficient to given an output frequency of about 25 kHz with R1 and R2. This arrangement provides
standby mode, the astable’s output will be low. This makes the trigger input of monostable low and output high. The warning LED1 and buzzer are connected such that they become active only when the output of the monostable sinks current. In the standby state, the LED1 remains ‘off’ and the buzzer is silent. As somebody tries to take the mobile phone from the protecting shield, his hand comes near the shield or makes contact with the shield, which introduces hand capacitance in the circuit. As a result, the astable’s frequency changes, which makes the trigger pin of the monostable low and its output oscil-
put leads. Make the protective shield using 10cm×10cm copper-clad board or aluminium sheet. Connect it to the circuit using a 15cm plastic wire. Leads of all capacitors should be short. Adjust VR1 slowly using a plastic screwdriver until the buzzer stops sounding. Bring the hand close to the shield and adjust VR1 until the buzzer sounds. With trial-and-error procedure, set it for the maximum sensitivity such that as soon the hand comes near the shield, the buzzer starts chirpring and the LED blinks. Instead of using the copper cladding for shield, a metallic mobile phone holder can be used as the shield.
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92 • AUGUST 2007 • ELECTRONICS FOR YOU
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Three-Phase Appliance Protector R.G. Thiagaraj Kumar and P. Kasi Rajan
M
any of our costly appliances require three-phase AC supply for operation. Failure of any of the phases makes the appliance prone to erratic functioning and may even lead to failure. Hence it is of paramount importance to monitor the availability of the three-phase supply and switch off the appliance in the event of failure of one or two phases. The power to the appliance should resume with the availability of all phases of the supply with certain time delay in order to avoid surges and momentary fluctuations. The complete circuit of a threephase appliance protector is described here. It requires three-phase supply, three 12V relays and a timer IC NE555 along with 230V coil contactor having
four poles. Relays RL1 and RL2 act as a sensing devices for phases Y and B, respectively. These relays are connected such that each acts as an enabling device for the subsequent relay. Therefore the combination of the relays forms a logical AND gate connected serially. The availability of phase R energises relay RL1 and its normallyopened (N/O) contacts close to connect phase Y to the input of transformer X2. The availability of phase Y energises relay RL2 and its N/O contacts close to connect phase B to the input of transformer X3, thus applying a triggering input to timer IC NE555 (IC1). Therefore the delay timer built around NE555 triggers only when all the phases (R, Y and B) are available. It provides a delay of approxi-
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mately four seconds, which energises relay RL3 and its N/O contact closes to connect the line to the energising coil of four-pole contactor relay RL4. Contactor RL4 closes to ensure the availability of the three-phase supply to the appliance. The rating of contactor RL4 can be selected according to the full-load current rating of the appliances. Here the contact current rating of the four-pole contactor is up to 32A. The availability of phases R, Y and B is monitored by appropriate LEDs connected across the secondary windings of transformers X1, X2 and X3, respectively. Hence this circuit does not require a separate
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indicator lamp for monitoring the availability of the three phases. When phase R is available, LED1 glows. When phase Y is available, LED2 glows. When phase B is available, LED3 glows. The main advantage of this protector circuit is that it protects three-phase appliances from failure of any of the
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phases by disconnecting the power supply through the contactor and automatically restores the three-phase supply to the appliance (with reasonable time delay) when all the phases are available. Assemble the circuit on a general-purpose PCB and enclose in a cabinet with the relays and contactor
mounted on the backside of cabinet. Connect the appliance through external wires. Caution. To avoid the risk of electric shock, ensure that AC mains is disconnected during assembly of the circuit and double check everything before connecting your circuit to the mains.
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Constant-Current Battery Charger
Monoj Das
T
here are many ways of battery charging but constant-current charging, in particular, is a popular method for lead-acid and NiCd batteries. In this circuit, the battery is charged with a constant current that is generally one-tenth of the battery capacity in ampere-hours. So for a 4.5Ah battery, constant charging current would be 450 mA. This battery charger has the following features: 1. It can charge 6V, 9V and 12V batteries. Batteries rated at other voltages can be charged by changing the values of zener diodes ZD1 and ZD2. 2. Constant current can be set as per the battery capacity by using a potmeter and multimeter in series with the battery. 3. Once the battery is fully charged, it will attain certain voltage level (e.g.
13.5-14.2V in the case of a 12V battery), give indication and the charger will switch off automatically. You need not remove the battery from the circuit. 4. If the battery is discharged below a limit, it will give deep-discharge indication. 5. Quiescent current is less than 5 mA and mostly due to zeners. 6. DC source voltage (VCC) ranges from 9V to 24V. 7. The charger is short-circuit protected. D1 is a low-forward-drop schottky diode SB560 having peak reverse voltage (PRV) of 60V at 5A or a 1N5822 diode having 40V PRV at 3A. Normally, the minimum DC source voltage should be ‘D1 drop+Full charged battery voltage+VDSS+ R2 drop,’ which is approximately ‘Full charged battery voltage+5V.’ For example, if we take full-charge voltage as 14V for a 12V battery, the source voltage should be
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14+5=19V. For the sake of simplicity, this constant-current battery charger circuit is divided into three sections: constantcurrent source, overcharge protection and deep-discharge protection sections. The constant-current source is built around MOSFET T5, transistor T1, diodes D1 and D2, resistors R1, R2, R10 and R11, and potmeter VR1. Diode D2 is a low-temperature-coefficient, highly stable reference diode LM236-5. LM336-5 can also be used with reduced operating temperature range of 0 to +70°C. Gate-source voltage (VGS) of T5 is set by adjusting VR1 slightly above 4V. By setting VGS, charging current can be fixed depending on the battery capacity. First, decide the charging current (one-tenth of the battery’s Ah capacity) and then calculate the nearest standard value of R2 as follows: R2 = 0.7/Safe fault current
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R2 and T1 limit the charging current if something fails or battery terminals get short-circuited accidentally. To set a charging current, while a multimeter is connected in series with the battery and source supply is present, adjust potmeter VR1 slowly until the charging current reaches its required value. Overcharge and deep-discharge protection have been shown in dotted areas of the circuit diagram. All components in these areas are subjected to a maximum of the battery voltage and not the DC source voltage. This makes the circuit work under a wide range of source voltages and without any influence from the charging current value. Set overcharge and deep-discharge voltage of the battery using potmeters VR1 and VR2 before charging the battery. In overcharge protection, zener
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diode ZD1 starts conducting after its breakdown voltage is reached, i.e., it conducts when the battery voltage goes beyond a prefixed high level. Adjust VR2 when the battery is fully charged (say, 13.5V in case of a 12V battery) so that VGS of T5 is set to zero and hence charging current stops flowing to the battery. LED1 glows to indicate that the battery is fully charged. When LED1 glows, the internal LED of the optocoupler also glows and the internal transistor conducts. As a result, gate-source voltage (VGS) of MOSFET T5 becomes zero and charging stops. Normally, zener diode ZD2 conducts to drive transistor T3 into conduction and thus make transistor T4 cut-off. If the battery terminal voltage drops to, say, 11V in case of a 12V battery, adjust potmeter VR3 such that transistor T3 is cut-off and T4 conducts.
LED2 will glow to indicate that the battery voltage is low. Values of zener diodes ZD1 and ZD2 will be the same for 6V, 9V and 12V batteries. For other voltages, you need to suitably change the values of ZD1 and ZD2. Charging current provided by this circuit is 1 mA to 1 A, and no heat-sink is required for T5. If the maximum charging current required is 5A, put another LM236-5 in series with diode D2, change the value of R11 to 1 kilo-ohm, replace D1 with two SB560 devices in parallel and provide a good heat-sink for MOSFET T1. TO-220 package of IRF540 can handle up to 50W. Assemble the circuit on a general-purpose PCB and enclose in a box after setting the charging current, overcharge voltage and deep-discharge voltage. Mount potmeters VR1, VR2 and VR3 on the front panel of the box.
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Battery-level indicator
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ormally, in mobile phones, the battery level is shown in dot or bar form. This lets you easily recognise the battery level. Here we present a circuit that lets you know the battery level of a device from the number of LEDs that are glowing. It uses ten LEDs in all. So if three LEDs glow, it indicates battery capacity of
makes it easier to recognise the voltage level on the basis of the calibration made. Red LEDs (LED1 through LED3) indicate battery capacity of less than 40 per cent. Orange LEDs (LED4 through LED6) indicate battery capacity of 40 to less than 70 per cent and green LEDs (LED7 through LED10) indicate battery capacity of 70 to under 100 per cent. The brightness of the
initially set it at 3V. Slowly adjust VR1 until LED1 glows. Now, increase the input voltage to 15V in steps of 1.2V until the corresponding LED (LED2 through LED10) lights up. Now the circuit is ready to show any voltage value with respect to the maximum voltage. As the number of
30 per cent. Unlike in mobile phones where the battery-level indicator function is integrated with other functions, here only one comparator IC (LM3914) does it all. The LM3914 uses ten comparators, which are internally assembled in the voltage divider network based on the current-division rule. So it divides the battery level into ten parts. The circuit derives the power supply for its operation from the battery of the device itself. It uses ten LEDs wired in a 10-dot mode. The use of different coloured LEDs
LEDs can be adjusted by varying the value of preset VR2 between pins 6 and 7. Diode D1 prevents the circuit from reverse-polarity battery connection. The tenth LED glows only when the battery capacity is full, i.e., the battery is fully charged. When the battery is fully charged, relay-driver transistor T1 conducts to energise relay RL1. This stops the charging through normally-open (N/O) contacts of relay RL1. For calibration, connect 15V variable, regulated power supply and
LEDs is ten, we can easily consider one LED for 10 per cent of the maximum voltage. Connect the voltage from any battery to be tested at the input probes of the circuit. By examining the number of LEDs glowing you can easily know the status of the battery. Suppose five LEDs are glowing. In this case, the battery capacity is 50 to 59 per cent of its maximum value. Assemble the circuit on a generalpurpose PCB. Calibrate it and then enclose in a box.
Aniruddh K.S.
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Pyroelectric fire alarm
D. Mohan Kumar
H
ere is an ultra-sensitive fire sensor that exploits the direct piezoelectric property of an ordinary piezo element to detect fire. The lead zirconate titanate crystals in the piezo element have the property to deform and generate an electric
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gate protected p-channel MOSFETs in the inputs. It has high speed of performance and low input current requirements. There are two inputs—the non-inverting input (pin 3) connected to the piezo element through diode D7 (OA71) that carries the voltage signal from the piezo element and the inverting input (pin 2) that gets a preset volt-
momentarily changes the voltage level at pin 3 of IC1 and its output swings high. Transistor T1 conducts taking the reset pin 12 of IC2 to ground. IC2 is now enabled and starts oscillating. With the shown values of the oscillating components C3 (0.22µ) and R6
age through VR1. By adjusting VR1, it is easy to set the reference voltage level at pin 2. In normal condition, IC1 gives a low output and the remaining circuitry is in a standby state. Capacitor C2 keeps the non-inverting input of IC1 stable, so that even a slight change in voltage level in the inputs can change the output to high. Normally, IC1 gives a low output, keeping transistor T1 non-conducting. Reseting pin 12 of IC2 (CD4060) connected to the collector of transistor T1 gets a high voltage through R5 and IC2 remains disabled. When the piezo element gets heat from fire, asymmetry in its crystals causes a potential change, enabling capacitor C2 to discharge. It
(1M), the first output (Q3) turns high after a few seconds and a red LED2 starts flashing. If heat near the piezo persists, Q7 (pin 14) output of IC2 becomes high after one minute, and the alarm starts beeping. If heat continues, Q9 (pin 15) turns high after four minutes and turns on the relay driver transistor T2. At the same time, diode D8 conducts and IC2 stops oscillating and toggles. The solenoid pump connected to the N/O (normally opened) contact of the relay starts spraying the fire-ceasing foam or water to the possible sites of fire. Power supply circuit. Power supply section (Fig. 2) comprises a 0-12V, 1A step-down transformer with a standard full-wave rectifier formed by D1 through D4 and filter capacitor C1. A battery backup is provided if the mains supply is cut-off due to short-circuit and fire. A 12V, 4.5Ah rechargeable battery is used for backup to give sufficient current to the solenoid pump. When mains
Fig. 1: Pyroelectric fire sensor
Fig. 2: Power supply with battery backup
potential when heated, thus converting the piezo element into a heat sensor. The circuit described here is very sensitive. It gives a warning alarm if the room temperature increases more than 10°C. The entire circuit has two sections—the sensor and the power supply section. Sensor side circuit. Fig. 1 shows the fire sensor circuit. The front end of the circuit has a sensitive signal amplifier built around IC1 (CA3130). It gives a high output when temperature near the piezo element increases. IC CA3130 is a CMOS operational amplifier with
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power is available, diode D5 forward biases. It provides power to the circuit and also charges the battery through resistor R2, and it limits the charging current to 120 mA. When power fails, diode D5 reverse biases and diode D6 forward biases, giving instant backup
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to the circuit. LED1 indicates the availability of mains power. Assemble the circuit on a generalpurpose PCB and enclose it in a suitable case. Connect the piezo element to the circuit using a thin insulated wire. Glue the flat side of the piezo el-
ement on a 30×30cm aluminium sheet to increase its sensitivity. Fix the sheet with the piezo sensor to the site where protection is needed. The remaining circuit can be fixed at a suitable place. If only the alarm generator is needed, omit the relay driver section.
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Guitar Effect Pedal Power
Raj K. Gorkhali
A
friend of mine plays guitar with several guitar effect pedals. He had a problem with battery eliminators and cables of the pedals cluttering the stage and so he asked for help. The solution is simple as described here. A small box is fitted to the rear of the amplifier providing a 9V output for the effect pedal. The amplifier section gets 9V through a pedal switch (refer Fig. 1). This power output and guitar signal input lines are combined into a single unit with Fig. 1: A typical guitar pedal switch multi-way cable
connecting points as shown in Fig. 2. The circuit (Fig. 2) can be divided into two sections: power supply and signal handling. The power supply section is built around transformer X1, regulators 7805 and 7905, bridge rectifier comprising diodes D1 through D4, and a few discrete components. The signal-handling circuit is built around two OP27 op-amps (IC3 and IC4). The power supply of about 9V for the effect pedals is derived from step-down transformer X1. MOV1 is a metal-oxide varistor that absorbs any large spike in mains power. IC 7905 (IC1) is a -5V low-power regulator. By using a 3.9V zener diode (ZD1) at its ground terminal, you get -8.9V output. The same technique is also applied to IC 7805 (IC2)—a +5V regulator to get 8.9V. Use good-quality components and heat-sinks for the
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regulators. This supply is more than enough for the five effect pedals. The greater the voltage drop across the regulator, the lower the output current potential. Resistors R1 and R2 provide a constant load to ensure that the regulators keep regulating. Capacitors C3 through C8 ensure that the supplies are as clean as possible. It is very important to use proper heatsinks for IC1 and IC2. Otherwise, these could heat up. Working of the circuit is simple. The input signal stage uses a basic differentiation amplifier to accept the incoming signal and a voltage follower to buffer the output to the power amplifier. The differential amplifier is built around IC3. It works by effectively looking at the signals presented to its inputs. If the input signals are of different amplitudes, IC3 amplifies the difference by a factor determined by R4/R3 (where R4=R6 and R3=R5). If the input signals have same amplitudes, these are attenuated by the common-mode rejection ratio (CMRR) of the circuit. The value of CMRR is determined by the choice of the op-amp the auxiliary components used and circuit topology. You can use standard resistors. With the values shown, you get an overall gain of unity. The combination of resistor R7 and C13 serves as a passive lowpass filter, progressively attenuating unwanted high-frequency signals. The second op-amp (IC4) w w w. e f y m ag . co m
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forms a simple voltage follower (its output follows its input), providing a low output impedance to drive into the standard power amplifier. Assemble the circuit on a generalpurpose PCB and fit it to the rear of an
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amplifier. The unit must be compact, yet robust. So use a very sturdy aluminium extrusion for the cabinet in order to neatly house the assembled PCB. To ensure simple operation, there are only three connections to the unit.
First, mains power is tapped from the transformer. The second lead carries the 9V output to the amplifier. The third is the guitar signal input at the five-way socket for connection to the effect pedal.
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VERSATILE POWER SUPPLY
SUNIL KUMAR
U
sing this circuit, you can obtain the following voltages (approx.) at a current limited to one ampere: 3.3V, 5V, 6V, 9V, 12V and 15V. The AC mains is stepped down by transformer X1 to deliver the secondary output of 18V AC at a maximum current of 1A dependant upon the load. The transformer output is rectified by the bridge rectifier comprising diodes D1 through D4, filtered by capacitor C1 and fed to regulator IC LM317, which is a 3-terminal posi-
tive regulator capable of providing 1.2V to 37 volts at 1.5A current to the load. Resistor R13 and selected combinations of resistors R1 through R12 are used to produce approximately 3.3V, 5V, 6V, 9V, 12V and 15V at the output. The desired resistors are selected by switching into conduction one of the six pnp transistors T1 through T6 by grounding the corresponding transistor base using rotary switch S1. For example, to get regulated 3.3V, simply rotate the knob of rotary switch to 3.3V position. Consequently, tran-
84 • FEBRUARY 2006 • ELECTRONICS FOR YOU
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sistor T1 is forward biased to switch resistors R1 and R2 (in series) across Adj pin of LM317 and ground to produce 3.3V. Other voltages can be produced in the same way by using rotary switch S1. Capacitor C2 bypasses any ripple in the output. Diode D5 is used as the protection diode. Use a heat-sink for dissipation of heat from IC LM317. The fuse-rated lamp provides protection against short circuit. This 1A rated power supply can be used for testing of various circuit ideas as well as construction projects published in EFY. z
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CLOCK TIMER
D. MOHAN KUMAR
W
ith this simple clockcontrolled timer, you will never again miss your favourite TV or radio programme. The TV or radio will switch on automatically at the time preset by you and will remain ‘on’ until the power supply fails or is disconnected. The circuit uses the AC signals generated at the buzzer terminals of an alarm clock. The AC signals are amplified by transistors T1 and T2 and the amplified output from the emitter of T2 is fed to the inverting input of negative-voltage comparator IC LM311 (IC1). The non-inverting input of IC1 gets a presettable voltage through pre-
IVEDI S.C. DW
set VR1. The inverting and non-inverting inputs of LM311 are different from other op-amps and it outputs sink current through pin 7 or source current through pin 1. When pin 3 of IC1 is at a higher voltage than pin 2, its output sinks as indicated by LED1. This gives a short negative pulse to the monostable wired around timer NE555. Resistor R5 keeps trigger pin 2 of IC2 high. The short-interval monostable outputs a high signal for a brief period to the gate of SCR1 (BT169) and relay RL1 energises. The latching action of SCR1 keeps the relay pulled even when the output of the monostable turns low. The relay can be de-energised by disconnecting the supply to the circuit
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via switch S1. The circuit works off a 9V battery. Assemble it on a general-purpose PCB and enclose in a suitable cabinet. Provide an AC outlet in the cabinet to switch on the appliance using the circuit. As mentioned earlier, the input signal is obtained from the buzzer terminals of the clock. Remove the small buzzer of the clock and connect point ‘A’ to the positive terminal and point ‘B’ to the negative terminal of the buzzer. Connect the mains AC terminal outlet to the normally-opened (N/O) contact of relay RL1. So when the relay energises, 230V AC operates the connected appliance. Set the desired time in the clock by adjusting the alarm set-up and switch on the circuit. When the set time reaches, the appliance will switch on automatically. The circuit can also be connected to digital clocks.
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CMYK
CIRCUIT
IDEAS
AUDIO AMPLIFIER FOR PERSONAL STEREO
M. VENK ATESWAR AN TESWARAN
IVEDI S.C. DW
n the output stages of most broadcast receivers and some amplifiers, there is a limit up to which maximum power can be developed without distortion. In the widely accepted output circuit, two output transistors are connected in series between the positive and ground and biasing is ad-
pushpull amplifier, each transistor (T2 or T3) gets double the voltage when activated. Connect the low audio signal from the stereo system at input terminals A and B of the audio amplifier and provide mains AC to activate the circuit. During the first half cycle of an AF cycle, transistor T2 conducts and the current flows from positive rail to
tor T2 and R5 and R7 for transistor T3) so that the acceptable output without overheating is obtained. You can also replace these transistors with another pair of suitable highpower transistors. For driving transistors T2 and T3, a 9V audio driver transformer having six leads is used. It is readily available in the market and reasonably matches
justed so that each transistor gets half the supply voltage. The circuit presented here is a simple audio amplifier for a personal stereo system. In this, supply voltage to each transistor can be enhanced to produce a larger output. The audio driver transformer drives the transistors adequately. A 9V-0-9V, 300mA transformer has been used in the set-up. Out of the four diodes (D1 through D4), two are used for developing the positive voltage rail (+9V) and the other two are used for developing the negative voltage rail (–9V). In the
ground rail (centre tap of transformer X1) via the loudspeaker coil (connected between the emitter of transistor T2 and ground) in one direction. While in the second half cycle, transistor T3 conducts and the current flows from ground rail to negative rail via the loudspeaker coil (connected between ground and the collector of transistor T3) in a direction opposite to the previous flow. Transistors T2 and T3 of the pushpull audio amplifier should be matched correctly. If these transistors get heated, change the bleeding resistor pairs (R3 and R4 for transis-
the output and input impedances of the preceeding and succeeding stages. To test the quality of the audio output, connect the stereo’s outputs to the respective terminals A and B. Now increase the volume level of the stereo slowly. If you get a high-level, high-quality sound across loudspeaker L1, the amplifier is working well. If the sound quality is not good, decrease the volume level until the audio amplifier gives good results. Note that this audio amplifier works well for low-level audio signals. z
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CIRCUIT
IDEAS
IR MUSIC TRANSMITTER AND RECEIVER
PRADEEP G.
U
sing this circuit, audio musical notes can be generated and heard up to a distance of
Fig. 1: Transmitter circuit
10 metres. The circuit can be divided into two parts: IR music transmitter and receiver. The IR music transmitter works off a 9V battery, while the IR music receiver works off regulated 9V to 12V. Fig. 1 shows the circuit of the IR music transmitter. It uses popular melody generator IC UM66 (IC1) that can continuously generate musical tones. The output of IC1 is fed to the IR
Fig. 2: IR audio receiver circuit
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IVEDI S.C. DW
driver stage (built across the transistors T1 and T2) to get the maximum range. Here the red LED (LED1) flickers according to the musical tones generated by UM66 IC, indicating modulation. IR LED2 and LED3 are infrared transmitting LEDs. For maximum sound transmission these should be oriented towards IR phototransistor L14F1 (T3). The IR music receiver uses popular op-amp IC µA741 and audio-frequency amplifier IC LM386 along with phototransistor L14F1 and some discrete components (Fig. 2). The melody generated by IC UM66 is transmitted through IR LEDs, received by phototransistor T3 and fed to pin 2 of IC µA741 (IC2). Its gain can be varied using potmeter VR1. The output of IC µA741 is fed to IC LM386 (IC3) via capacitor C5 and potmeter VR2. The melody produced is heard through the receiver’s loudspeaker. Potmeter VR2 is used to control the volume of loudspeaker LS1 (8-ohm, 1W). Switching off the power supply stops melody generation.
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IDEAS
AUTOMATIC LOW-POWER EMERGENCY LIGHT
S.C. DWIVEDI
H
ere is a white-LED-based emergency light that offers the following advantages: 1. It is highly bright due to the use of white LEDs. 2. The light turns on automatically when mains supply fails, and turns off when mains power resumes. 3. It has its own battery charger. When the battery is fully charged, charging stops automatically. The circuit comprises two sections: charger power supply and LED driver. The charger power supply section is
current through diode D5 and limiting resistor R16. By adjusting preset VR1, the output voltage can be adjusted to deliver the required charging current. When the battery gets charged to 6.8V, zener diode ZD1 conducts and charging current from regulator IC1 finds a path through transistor T1 to ground and it stops charging of the battery. The LED driver section uses a total of twelve 10mm white LEDs. All the LEDs are connected in parallel with a 100-ohm resistor in series with each. The common-anode junction of
Fig. 1: Automatic high intensity LED-based emergency light
built around 3-terminal adjustable regulator IC LM317 (IC1), while the LED driver section is built around transistor BD140 (T2). In the charger power supply section, input AC mains is stepped down by transformer X1 to deliver 9V, 500 mA to the bridge rectifier, which comprises diodes D1 through D4. Filter capacitor C1 eliminates ripples. Unregulated DC voltage is fed to input pin 3 of IC1 and provides charging
all the twelve LEDs is connected to the collector of pnp transistor T2 and the emitter of transistor T2 is directly connected to the positive terminal of 6V battery. The unregulated DC voltage, produced at the cathode junction of diodes D1 and D3, is fed to the base of transistor T2 through a 1kilo-ohm resistor. When mains power is available, the base of transistor T2 remains high and T2 does not conduct. Thus LEDs are
126 • JANUARY 2008 • ELECTRONICS FOR YOU
UMAR SUNIL K
Fig. 2: Pin configurations of LM317, BD140 and BC548
off. On the other hand, when mains fails, the base of transistor T2 becomes low and it conducts. This makes all the LEDs (LED1 through LED12) glow. The mains power supply, when available, charges the battery and keeps the LEDs off as transistor T2 remains cut-off. During mains failure, the charging section stops working and the battery supply makes the LEDs glow. Assemble the circuit on a general-purpose PCB and enclose in a cabinet with enough space for battery and switches. Mount the LEDs on the cabinet such that they light up the room. A hole in the cabinet should be drilled to connect 230V AC input for the primary of the transformer. EFY lab note. We have tested the circuit with twelve 10mm white LEDs. You can use more LEDs provided the total current consumption does not exceed 1.5A. Driver transistor T2 can deliver up to 1.5A with proper heat-sink arrangement. WWW.EFYMAG.COM
circuit
ideas
CASH BOX GUARD
T.K. Hareendran
M
ost thefts happen after midnight when people enter the second phase of sleep called ‘paradoxical sleep.’ Here is a smart security circuit for your cash box that thwarts the theft attempt by activating an emergency beeper. The circuit can also be used to trigger any external burglar alarm unit. The cash box guard circuit (shown in Fig. 1) is built around IC CD4060 (IC1), which has an inbuilt oscillator and divider. The basic oscillator is configured by a simple resistor-capacitor (R-C) network. IC CD4060 divides this oscillator frequency into binary divisions, which are available as outputs. In light, reset pin 12 of IC1 remains low, which enables the oscillator built around IC1. However, in the dark, it
goes high, which resets the counter making all the outputs low. This also stops oscillations of the internal oscillator. Working of the circuit is simple. If the cash box is closed, the interior will be dark. Hence in the dark, the lightdependant resistor (LDR1) resets IC1 and it stops oscillating and counting. At the same time, pins 13 and 14 of IC1 go low. So neither the piezobuzzer (PZ1) sounds, nor the relay (RL1) energises, indicating that the cash box is closed. If someone tries to open the door of the cash box, light—most probably from the burglar’s pen torch—falls on LDR1 fitted into the cash box. As a result, LDR1 conducts and pin 12 of IC1 goes low. IC1 starts oscillating and counting. With the present timing R-C components (at pins 9, 10 and
Fig. 1: Cash box guard circuit
Fig. 2: Assembled unit w w w. e f y m ag . co m
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Fig. 3: Unit fitted inside the cash box and also connected to an external alarm
11), the output timing at pin 14 of IC1 is two-three seconds. Hence pin 14 of IC1 goes high for two seconds after the door is opened and goes low for another two seconds. So the piezobuzzer (PZ1) sounds for two seconds and then falls silent for the following two seconds. This cycle repeats until the cash box is closed. An optional relay is added for a remotely located audio/visual alert system. For that, a relay driver circuit built around npn transistor BC548 (T2) is used. The relay is energised by the output from pin 13 of IC1 for about four seconds after the door is opened and then de-energised for the following four seconds. You can use this relay to activate another remotely located audio/visual alert system. After assembling the circuit on a small PCB, house it in a small tamperproof box (refer Fig. 2) leaving a little window for LDR1 and a small opening for the piezobuzzer (PZ1). Now fit the unit inside the cash box (refer Fig. 3) with LDR1 pointing towards the door of the cash box. EFY note. 1. The relay latching facility can be added to the circuit by replacing transistor T2 with a suitable silicon-controlled rectifier such as BT169. 2. By changing the value of resistor R1, you can adjust the light detection sensitivity of the circuit. 3. If you want to use a 3-pin piezobuzzer device, remove buzzerdriver npn transistor T1 and connect trigger pin of the buzzer directly to pin 14 of IC1. Also connect the positive and negative terminals of the buzzer to respective positive and negative points of the circuit. 4. Photo-transistor 2N5777 can be used in place of the 10mm LDR1. 5. The complete kit for this circuit is available with Kits’n’Spares.
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REMOTE-CONTROLLED FAN REGULATOR
Dr C.H. VITHALANI
U
sing this circuit, you can change the speed of the fan from your couch or bed. Infrared receiver module TSOP1738 is used to receive the infrared signal transmitted by remote control. The circuit is powered by regulated 9V. The AC mains is stepped down by transformer X1 to deliver a secondary output of 12V-0-12V. The transformer output is rectified by full-wave rectifier comprising diodes D1 and D2, filtered by capacitor C9 and regulated by 7809 regulator to provide 9V regulated output. Any button on the remote can be used for controlling the speed of the fan. Pulses from the IR receiver module are applied as a trigger signal to timer NE555 (IC1) via LED1 and resis-
tor R4. IC1 is wired as a monostable multivibrator to delay the clock given to decade counter-cum-driver IC CD4017 (IC2). Out of the ten outputs of decade counter IC2 (Q0 through Q9), only five (Q0 through Q4) are used to control the fan. Q5 output is not used, while Q6 output is used to reset the counter. Another NE555 timer (IC3) is also wired as a monostable multivibrator. Combination of one of the resistors R5 through R9 and capacitor C5 controls the pulse width. The output from IC CD4017 (IC2) is applied to resistors R5 through R9. If Q0 is high capacitor C5 is charged through resistor R5, if Q1 is high capacitor C5 is charged through resistor R6, and so on. Optocoupler MCT2E (IC5) is wired as a zero-crossing detector that supplies trigger pulses to monostable
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multivibrator IC3 during zero crossing. Opto-isolator MOC3021 (IC4) drives triac BT136. Resistor R13 (47ohm) and capacitor C7 (0.01µF) combination is used as snubber network for triac1 (BT136). As the width of the pulse decreases, firing angle of the triac increases and speed of the fan also increases. Thus the speed of the fan increases when we press any button on the remote control. Assemble the circuit on a generalpurpose PCB and house it in a small case such that the infrared sensor can easily receive the signal from the remote transmitter.
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CMYK
circuit
ideas
Aquarium Probe
D. Mohan Kumar
A
number of environmental factors including light and temperature affect fish culture. The temperature of water has profound effect because fish cannot breed above or below the critical temperature limits. Temperature between 24°C and 33°C is found to be the best to induce spawning in fishes. This particular temperature range is also necessary for the healthy growth of nursery fish fries (young fishes). Rise of water temperature due to sunlight may adversely affect the fish rearing process.
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temperature, the diode generates 2mV output voltage. That is, at 5°C, it is 10 mV, which rises to 70 mV when the temperature is 35°C. This property is exploited in the circuit to sense the temperature variation in aquarium water. Fig. 1 shows the circuit diagram of the aquarium probe. Since the output from the diode sensor is too low, a high-gain inverting DC amplifier is used to amplify the voltage. CA3140 (IC1) is the CMOS version op-amp that can operate down to zero-volt output. The highest output available from IC1 is 2.25V less than the input voltage at pin 7. With resistor
Fig. 1: Circuit for aquarium probe
The circuit of aquatic probe described here can monitor the temperature of water and indicate the rise in temperature through audiovisual indicators. A readily available signal diode 1N34 is used in the circuit as the temperature sensing probe. The resistance of the diode depends on the temperature in its vicinity. Typically, the diode can generate around 600 mV when a potential difference is applied to its terminals. For each degree centigrade rise in 9 2 • j u ly 2 0 0 8 • e l e c t ro n i c s f o r yo u
R4 and VR2, the variation in diode Fig. 2: Diode sensor assembly voltage can be amplified to the required level. Resistor R1 restricts current flow through diode D1 and preset VR1 (1-kilo-ohm) sets the input voltage at pin 3. IC3 (7805) provides regulated 5 volts to the inputs of IC1, so that the input voltage is stable for accurate measurement of temperature.
The output from IC1 is fed to display driver LM3915 (IC2) through preset VR3 (50-kilo-ohm). With careful adjustments, the wiper of VR3 can provide 0-400 millivolts to the input of IC2. The highly sensitive input of IC2 accepts as low as 50 mV if the reference voltage at its pin 7 is adjusted using a variable resistor. To increase the sensitivity of IC2, preset VR4 is connected at one end to ‘reference voltage end’ pin 7 and its wiper is connected to ‘high end’ pin 6 of the internal resistor chain. When approximately 70 mV is provided to the input of IC2 by adjusting preset VR3, LED1 (green) lights up to indicate that the temperature is approximately 35°C, which is the crossing point. When the input receives 100 mV, LED2 (red) lights up to indicate approximately 50°C. Finally, the buzzer starts beeping if the input receives 130 mV corresponding to a temperature of 65°C. In short, LEDs and the buzzer remain standby when the temperature of the water is below 35°C (normal). With each step increase of 30 mV in the input (corresponding to 15°C rise in temperature), LEDs and the buzzer become active. Pin 16 of IC2 is used to drive the piezobuzzer through transistor T1. When pin 16 of IC2 becomes low, T1 conducts to beep the piezobuzzer. Resistor R7 keeps the base of transistor T1 high to avoid false alarm. IC4 provides regulated 9V DC to the circuit. Assemble the circuit on a common PCB and enclose in a suitable case. Glass signal diode D1 is immersed in water to sense the temperature of water. Its leads should be coated with enamel paint to avoid shorting in water. Alternatively, enclose the diode in a small glass tube or test w w w. e f y m ag . co m
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tube having sufficient internal space to fit the diode as shown in Fig. 2. Make the sensor assembly waterproof using wax. Take care while calibrating and setting the circuit. With 5V DC supply to diode D1 and an ambient temperature of about 35°C, D1 generates around 70 mV. Adjust VR3 until the voltage in its wiper increases to 70
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mV, so that the input of IC2 (pin 5) receives 70 mV corresponding to the diode output voltage at 35°C. At this stage, green LED1 should turn on. If it doesn’t, adjust VR4 until LED1 just lights up. Immerse the diode in temperature-adjusted hot water (35°C) and adjust VR3 and VR4 until green LED1 lights up. Increase the water temperature to 50°C by adding
hot water. Now red LED2 will glow. At this position, the voltage at pin 6 of IC1 will be around 100 mV. When the temperature of water increases further to 65°C, the buzzer starts beeping. After calibration, immerse the diode assembly in the aquarium tank just below the water surface and fix it permanently to avoid floating.
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Generator Room Light
Manuj Paul
A
t night when power fails, one finds it difficult to reach the generator to start it. Here is the circuit for a generator room light that automatically turns on at night, facilitating easy access to the generator. During daytime, the light remains off. Fig. 1 shows the circuit for gen-
erator room light, while Fig. 2 shows the battery charger circuit, which is optional and can be omitted if the generator is self-start type and has built-in battery. At the heart of the generator room light circuit (Fig.1) is a light-dependent resistor (LDR1) that senses the ambient light as well as light from glowing LED1.
Fig. 1: Circuit for generator room light
Fig. 2: Battery charger circuit (optional) 8 4 • J u ly 2 0 0 9 • e l e c t ro n i c s f o r yo u
During daytime, sunlight or light from LED1 reduces the resistance of LDR1. As a result, the voltage drop across LDR1 decreases and npn transistor T1 does not conduct. The collector of T1 and therefore pins 2 and 6 of
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IC1 remain high, making output pin 3 of IC1 low and transistor T2 cut-off. So lamp L1 connected between the collector of T1 and the positive terminal of 12V supply does not glow. As the ambient light fades during sunset, the resistance of LDR1 increases. As a result, the voltage drop across LDR1 increases and npn transistor T1 conducts. Pins 2 and 6 of IC1 go low to make its output pin 3 high, and lamp L1 glows. You can replace incandescent lamp L1 with bright white LEDs using proper current-limiting resistors. Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. Install the unit near the generator. Arrange LED1 and LDR1 such that during the availability of mains, light emitted from LED1 falls directly on LDR1. Also, make sure that during daytime the ambient light falls on the LDR. For powering the battery charger circuit (Fig. 2), 15V AC secondary voltage is derived from step-down transformer X1. For fast charging of the battery, you may increase the current rating of transformer X1. The charger charges the battery through a thyristor (SCR1) when the battery voltage is low. The thyristor gets a regulated gate voltage from the zener diode, and goes to tickle charging mode when the battery voltage nears the zener voltage. Assemble the charger circuit on a general-purpose PCB and enclose in a suitable cabinet. Use two crocodile clips (red for positive and black for negative) for connecting the battery terminal to the charger circuit. w w w. e f y m ag . co m
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Mobile Electronic Workbench
Abhijeet Deshpande
T
ypically, implementing and testing even a small circuit requires an elaborate setup that includes breadboards, a dual DC power supply, hookup wires, ICs and resistors of different values. This setup can be quite messy and difficult to clean up at the end of the experiment. Also, the power supply can make the
Fig. 1: Circuit for mobile electronic workbench
Fig. 2: Photograph of electronic workbench 9 6 • J u ly 2 0 1 0 • e l e c t ro n i c s f o r yo u
setup non-portable. Here we present a mobile electronic workbench that makes it easier for you to assemble and test circuits. This mobile workbench is useful for students in schools, colleges, research institutions and industries alike. It can be used conveniently wherever you want. It is also cost-effective and very useful for giving demos. As the power is supplied by the batteries, the
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voltage is noise-free. Fig. 1 shows the circuit of the mobile electronic workbench. Two low-drop-out (LDO) regulators (one positive and the other negative) are used here to provide regulated +5V and -5V for digital ICs. When switch S1 is pushed to ‘on’ position, LEDs indicate the availability of voltages on the breadboard. When it is in ‘off’ position, the battery terminals connect to the sockets for charging the batteries. Apart from +6V and +5V supplies, you can also have a 12V source between +6V and -6V terminals. As shown in Fig. 2, the mobile workbench consists of a big melamine tray. At the centre of this tray, mount the breadboard. On the sides of the breadboard, stick two 6V, 4.5Ah maintenance-free lead-acid batteries (Batt.1 and Batt.2). On a wooden batten, mount two-pole, two-way toggle switch S1 and two fuses and two sockets symmetrically. Mount LED1 and LED2 on the sides of S1. If you do not want this mobile workbench on a breadboard, you can assemble it on a general-purpose PCB and enclose in a suitable cabinet. Fix LEDs and switch S1 on the front panel of the cabinet and the fuses at the back side of the box. In place of LM2990-5, you can use a 5.1V, 2W zener diode with 100ohm, 2W series limiting resistor as shown in Fig. 3. During testing, we used a zener diode for negative 5V regulation instead of Fig. 3: Alternative –5V LM2990-5. circuit w w w. e f y m ag . co m
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PARROT-SOUNDING AC DOORBELL
IVEDI S.C. DW
SANI THEO
H
ere is a mains-operated doorbell that produces parrot-like sweet voice without requiring any musical IC. The circuit is cheap and easy to construct. The AC mains is fed to the circuit without using any step-down transformer. The complete circuit is shown in Fig. 1. The main components of the circuit are a resistor-capacitor network, transistor BC337 and audio output transformer X1. The oscillation frequency depends on the combination of resistors R4 and R5 and capacitors C3, C4 and C5. When switch S1 is closed, the audio signal generated due to oscillations is amplified by transistor BC337 and parrot-like sound is reproduced from loudspeaker LS1 connected across the secondary of transformer X1. Here we have used an 8-ohm, 0.5W loudspeaker. The audio output transformer (X1) is normally used in transistor radio. The function of the audio output transformer is to transform the high impedance of the output amplifier to match the much lower impedance of the speaker. This is necessary to get an efficient transfer of the audio signal to the speaker. If a wrong audio transformer is used, the result can
Fig. 1: Circuit of parrot-sounding doorbell
Fig. 2: Dimensions of audio transformer
be low output and loss of tone quality. The audio frequency tone across the speaker terminal is about 3 kHz. The dimensions of the audio trans-
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former used in the experimental setup are shown in Fig. 2. The circuit is powered directly from 220V AC mains. The operating DC voltage obtained at the cathode of diode D1 is about 6V. However, if you press switch S1 continuously for a few seconds, the maximum voltage developed at this point may go up to 20 volts, which must be avoided to prolong the life of the circuit. R1 limits surge current in the circuit. The parallel combination of resistor R1 and capacitor C1 limits the circuit Fig. 3: Pin current to a safe level configuration of BC337 for circuit operation. R2 across C1 provides DC path for the current as well as a discharge path when the circuit is switched off. This is to prevent a possible shock to the operator by charged capacitor C1.
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Poor Man’s Hearing Aid
EFY LAB
T
his miniature stereo preamplifier-cum-headphone amplifier circuit works off a 3V battery (lithium non-rechargeable coin cell). Although its performance is not comparable to that of commercially available sophisticated hearing aids, still it can serve the purpose well for persons with a low degree of hearing impairment. Its maximum power output at 1 kHz is around 8 mW, which is adequate for driving the headphones. The circuit, as shown in Fig. 1, is wired around Sanyo’s MSI (medium-scale-integrated) surface-mount 10-pin DIL IC LA4537M, which measures just 8×6.4×1.5 mm 3 . A functional block diagram of LA4537M IC is shown in Fig. 2. Since the MSI’s pin-to-pin (centre-to-centre) distance is only 1 mm, the circuit has to be assembled on a properly designed PCB using soldering iron with a pointed bit. Two ICs (LA4537M) have been cascaded to increase the overall sensitivity and thereby the reception range. You can adjust the volume of the stereo channels indi-
vidually, as per your requirement, using presets VR1 and VR2, respectively. With 3V supply voltage, you can afford to use 1/8-watt resistors, while the electrolytic capacitors’ voltage rating can be as low as 5V. This will allow the assembled circuit to occupy very little space. Apart from the usual battery ‘on’/‘off’ switch S1, muting ‘on’/‘off’ switch S2 has also been provided. Both S1 and S2 could be PCB-mount slide switches. After assembling the main circuit,
Fig. 2: Functional diagram of ic la4537m
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house it in a thin metallic case, which can then be mounted in the middle of a metallic/plastic headband (generally used by telephone operators), while the two microphones with their associated earpieces are to be extended using screened wires so that these (microphone-earpiece sets) can be kept closest to the respective earlobes. Caution. Ensure that shielded microphone wires do not touch (short) the shielded earpiece wires, as these are connected to different pins (reference input pin 5 and ground pins 3 and 8, respectively) of ICs LA4537M. You may use an insulating sleeve over each of the shielded wires.
Fig. 1: Circuit for the hearing aid
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PC POWER MANAGER
T.K. Hareendran
V
ery often we forget to switch off the connected peripherals like monitor, scanner and printer while switching off our PC. This leads to needless energy consumption and possible shortening of the life of the peripheral. PCs with an ATX switch-mode power supply (SMPS) unit are not provided with a mains switch outlet. It is therefore not possible to achieve automatic switching (on/off) of peripheral units with the computer power switch. Here is a simple circuit that turns the connected peripherals on/off along with your PC. It consists of a regulated power supply, a simple USB interface and two electromagnetic relays used as power switches. The power supply for the circuit is derived from the AC mains via transformer X1. The 15V AC available at the secondary winding of transformer X1 is first rectified by a bridge recti-
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fier comprising diodes D1 through D4, smoothed by capacitors C1 and C2, and regulated by IC LM7812 (IC1). The To activate the PC manager circuit, regulated 12V DC is used to energise proceed as follows: Press ‘start’ switch relay RL1. LED1 works as a powerS1 and hold it in this position for a ‘active’ indicator. few minutes. When power-‘active’ To set up the circuit, first connect indicator LED1 lights up, relay RL1 the input socket (SOC1) of the circuit energises and the 230V mains power to a proper AC mains wall outlet ussupply from SOC1 is fed to output ing a three-core power cable. Now socket SOC2 through the contacts of connect one end of a standard USB relay RL1. cable to the B-type USB input socket and the other end of the cable to any vacant USB port (A-type) of the PC. Finally, plug one standard four-way switchboard (extension cord) into the supply output socket (SOC2) of the circuit and take power from this switchboard to activate all loads like monitor, scanner, printer and Fig. 2: Wiring diagram for PC power manager even your PC.
Fig. 1: Circuit of PC power manager
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Now start your computer as usual, by pressing the power button on the front panel. When the PC runs, there will be 5V DC at the USB interface socket. As a result, relay RL2 energises via diode D6. The contacts of relay RL2 close switch S1 permanently, and LED2 glows continuously. Release ‘start’ switch S1. Now your
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PC manager is ready to use. When you switch off your PC, relay RL2 de-energises. As a result, electric power from the switchboard (to which all peripherals are connected) is cut off. Switch S2 works here as an emergency bypass switch. Assemble the circuit on a generalpurpose PCB and enclose in a suitable
cabinet. Connect SOC1, SOC2 and USB socket along with switches S1 and S2 and LEDs (LED1 and LED2) on the front panel of the cabinet. Refer Fig. 2 for connections. EFY note. Take care during fabrication and testing, as the circuit is at mains potential and may give you lethal shock.
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Weekly Reminder
Raj K. Gorkhali
T
his circuit reminds you of all the important tasks that are due on a specific day every week. So be it returning your library book, switching on your favourite TV programme, putting the dustbin out or cleaning the car, it automatically flashes an LED that very day to alert you of something to be done. The LED keeps flashing until you press the reset button. The circuit consumes very little power. The circuit can count the days. The rising of the Sun is detected by a light-dependent resistor (LDR1). When the sun rises, the ambient light level reduces the resistance of LDR1. The voltage level at pin 13 of gate N4 goes low. Since the other input (pin 12) of gate N4 is high, its output also goes high. This is inverted by gate N10 whose output goes low to make counter IC3 advance by one count. This way each day is counted. Similarly, the counter advances by one every morning until it counts seven days.
9 8 • J u n e 2 0 1 0 • e l e c t ro n i c s f o r yo u
In the morning of the seventh day, all the inputs of gate N9 become high, making its output low. The low output of gate N9 is inverted by gate N11 to trigger the pulse generator built around gates N5 and N7, and it produces a short-duration pulse to trigger the flip-flop built around gates N6 and N8. As a result, the output of the flip-flop goes high to enable the astable multivibrator built around gate N3. The astable multivibrator produces 2Hz frequency to flash LED1 as a reminder. LED1 flashes until you press reset switch S2 momentarily. When the enable input (pin 8) of gate N3 is low, the output of the astable multivibrator remains high. Gate N2 inverts this high level into low and the transistor does not conduct. So LED1 doesn’t flash when the astable multivibrator is disabled. Counter IC3 also resets when the pulse generator triggers because its reset pin 11 is connected to the output of gate N11. When the counter IC3 resets, its output becomes low and it’s ready
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to begin day counting for the next week. Suppose you require a reminder for four days. Then first cover the sensor and press increment switch S1 thrice momentarily and leave it. Now your reminder (flashing of LED1) starts after four days. Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. After assembling the circuit, proper setting is required. First of all, switch on the power. LED1 flashes. Press switch S2 to stop it from flashing. Cover LDR1 and press S1 several times until LED1 flashes again. The counter is now set at a count of 0 and is ready to start weekdays counting. Press S2 to stop flashing. Do not uncover the sensor immediately after pressing S2. Else, the counter will register arrival of the next day and LED1 will flash after six days. To make it flash weekly on a particular day, keep the LDR1 in dark until night.
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CIRCUIT
IDEAS
INFRARED BUG
T.K. HAREENDRAN
T
his circuit can be used to detect the presence of modulated infrared signals in its vicinity from any electronic source, for instance, an IR handheld remote controller. It can also be used for testing IR burglar alarm systems. Fig. 1 shows the circuit of the infrared bug. Besides the power supply (one 9V PP3/6F22 compact battery pack), it consists of an infrared signal detector-cum-preamplifier followed by a melody generator and a tiny audio amplifier. The cir-
Fig. 1: Infrared bug
IVEDI S.C. DW
cuit, in principle, converts the IR signal pulse trains into noticeable aural notes. S1 is used to switch on/off mains power and LED1 indicates power‘on.’ Resistor R4 and zener diode ZD2 form a low-current voltage Fig. 2: Pin configurations of LM386, BC547/337 and UM66 stabiliser for providing T2. The amplified signal is fed to the steady 5.1V DC to the small signalmelody generator via resistor R5. The preamplifier circuit. IR LED1 is the output of the melody generator is fed main sensing element. to LM386 low-power audio amplifier The IR signal detected by IR LED1 (IC2) via variable resistor VR1, which is amplified by npn transistors T1 and works as the volume control. The loudspeaker sounds to indicate the presence of IR signal near the circuit. IC LM386 is wired as a minimum-parts amplifier with a voltage gain of ‘20,’ which is sufficient for this application. Capacitor C3 is used for decoupling of the positive rail and the R-C combination network comprising C4 and R7 bypasses high frequency to ground. The circuit can be easily wired on a small veroboard or any general-purpose PCB. Pin configurations of IC LM386, transistor BC547 and melody generator UM66 are shown in Fig. 2. A miniature metallic cabinet may be used for enclosing the gadget. z
100 • MARCH 2006 • ELECTRONICS FOR YOU
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CMYK
circuit
ideas
Triple Power Supply
Sandip Trivedi and P.D. Lele
T
his low-cost, multipurpose power supply fulfils the requirements of almost all laboratory experiments. Nonetheless, it can be easily fabricated by hobbyists. A single transformer is used to build this triple power supply. Regulator IC LM317 generates variable power supply of 1.25 to 20V, 1A. The dual ±12V, 1A power supply is generated by regulators 7812 and 7912. Similarly, dual ±5V, 1A power supply is generated by regulators 7805 and 7905. ‘On’/‘off’ switches (S2 through S4) select the required power supply. Variable power supply is used to study the characteristics of devices. Fixed +5V power supply is used for all digital, microprocessor and microcontroller experiments. Dual ±12V power supply
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is used for op-amp-based analogue circuit experiments. Fig. 1 shows the circuit of the triple power supply, while Fig. 2 shows the pin configuration of the regulators used in the circuit. Transformer X1 steps down the mains power to deliver the secondary output of 18V-0-18V. The transformer output is rectified by full-wave bridge rectifier BR1, filtered by capacitors C1, C2, C3, C7 and C8, and regulated by IC1 through IC5. Regulator IC1 (LM317) provides variable voltages (1.25 to 20V), while IC2 and IC4 provide regulated +12V and –12V, respectively. The output of IC2 is fed to regulator IC3 (7805), which provides fixed +5V. Similarly, the output of IC4 is fed to regulator IC5 (7905), which provides fixed –5V. Capacitors C4 through C6, and C9 through C11, are used for further filtering of ripples
in positive and negative regulated power supplies. LED1 glows to indicate that +5V is available, while LED2 indicates that –5V is available. Switch S1 is used for mains ‘on’/ ‘off’. Using switches S2 through S4, any of the three supplies can be independently turned off when not required in a particular experiment. This reduces unnecessary power dissipation and increases the life and reliability of the power supply. Since the circuit uses three terminal regulators, only capacitors are required at the input and output. The use of few components makes the circuit very simple. The three terminal regulators have heat-sink provision to directly deliver 1A output current. To ensure the maximum output, do not forget to
HEAT SINK IN
S2 = FOR VARIABLE VOLTAGE
3
IC1 LM317 2
S2
S3 = FOR +12V AND +5V
1
R1 120
ADJ.
S4 = FOR –12V AND –5V C3 0.1µ
S1-S4 = ON/OFF SWITCH
+1.25 TO 20V
GND
VR1 2.2K POT
S1 ON/OFF SWITCH
+12V
HEAT SINK IN
F1 1.5A FUSE
OUT
X1
L 230V AC 50Hz N
1
S3
IC2 7812
3
HEAT SINK OUT
IN
1
IC3 7805
C1 1000µ 35V
OUT
2
2
GND
BR1 W04
3
GND
C2 0.1µ
R2 330
C5 10µ 16V
C4 100µ 25V
+5V
C6 0.1µ
BR1 W04
LED1
GND X1 = 230V AC PRIMARY TO 18V-0-18V, 1.5A SECONDARY TRANSFORMER
C7 1000µ 35V
C10 10µ 16V
C9 100µ 25V
C8 0.1µ GND
S4
IN
2
–5V
GND 1
BR1-W04 1.5A, BRIDGE RECTIFIER
C11 0.1µ
R3 330
IC4 7912
LED2
1 3
HEAT SINK
OUT
IN
2
IC5 7905
3
OUT
HEAT SINK
–12V GND
Fig. 1: Tripple power supply w w w. e f y m ag . co m
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circuit
ideas
Fig. 2: Pin configurations of regulators
use heat-sinks for the regulators. The three-terminal regulators are almost non-destructible. These have inbuilt protection circuits including the thermal shutdown protection. Even if there is overload or shorting of the output, the inbuilt overload protection circuit will limit the current and slowly reduce the output voltage to zero. Similarly, if the temperature increases beyond a certain value due to excessive load and heat dissipation, the in-built thermal shutdown circuit will reduce the output current and the output voltage (gradually) to zero. Thus complete protection is provided to the circuitry. Assemble the circuit on a generalpurpose PCB and enclose in a box as shown in Fig. 3. The step-by-step procedure to build the triple power supply for the laboratory follows:
8 2 • M a r c h 2 0 0 9 • e l e c t ro n i c s f o r yo u
Fig. 3: Proposed cabinet for power supply
1. Collect all the components shown in the circuit diagram. 2. Connect switch S1, fuse, transformer and mains cord to the assembled PCB as well as the box. 3. Keep the multimeter in DC voltage range (more than 25V DC) and measure the DC voltage across capacitors C1 and C7 (1000 µF, 35V). This voltage should be around 18V×1.41=25 to 26V DC. Check both positive and negative voltages with respect to ground. 4. It is advisable to use three-wire mains cable and plug. If you are using any metallic box, earthing wire/pin of the mains plug should be soldered to the body of the metallic box using an
earthing tag. 5. If the 18V-0-18V transformer is replaced with 15V-0-15V transformer, the output voltage of the variable supply using LM317 will be correspondingly lower. 6. If proper voltages are available, go to step 7. Otherwise, check the connections. 7. Connect variable regulator LM317 to the circuit and check 1.25V to 20V output by varying the 2.2-kiloohm linear potentiometers. 8. Now connect ICs 7812, 7912, 7805 and 7905 to the circuit and check their output voltage. 9. Connect terminals, potmeter, switches and indicator LED on the front panel of the box and complete the connections. Close the box by using screws. Precaution. At the primary side of the transformer, 230V AC could give lethal shocks. So be careful not to touch this part. EFY will not be responsible for any resulting loss or harm to the user.
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circuit
ideas
Antisleep Alarm for Students
Suresh Kumar K.B.
T
his circuit saves both time and electricity for students. It helps to prevent them from dozing off while studying, by sounding a beep at a fixed time interval, say, 30 minutes. If the student is awake during the beep, he can reset the circuit to beep in the next 30 minutes. If the timer is not reset during this time, it means the student is in deep sleep or not in the room, and the circuit switches off the light and fan in the room, thus preventing the wastage of electricity. The circuit is built around Schmitttrigger NAND gate IC CD4093 (IC1), timer IC CD4020 (IC2), transistors
9 8 • M a r c h 2 0 1 0 • e l e c t ro n i c s f o r yo u
BC547, relay RL1 and buzzer. The Schmitt-trigger NAND gate (IC1) is configured as an astable multivibrator to generate clock for the timer (IC2). The time period can be calculated as T=1.38×R×C. If R=R1+VR1=15 kilo-ohms and C=C2=10 µF, you’ll get ‘T’ as 0.21 second. Timer IC CD4020 (IC2) is a 14-stage ripple counter. Around half an hour after the reset of IC1, transistors T1, T2 and T3 drive the buzzer to sound an intermediate beep. If IC2 is not reset through S1 at that time, around one minute later the output of gate N4 goes high and transistor T4 conducts. As the output of gate N4 is connected to the clock input (pin 10) of IC2 through diode
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D3, further counting stops and relay RL1 energises to deactivate all the appliances. This state changes only when IC1 is reset by pressing switch S1. Assemble the circuit on a generalpurpose PCB and enclose it in a suitable cabinet. Mount switch S1 and the buzzer on the front panel and the relay at the back side of the box. Place the 12V battery in the cabinet for powering the circuit. In place of the battery, you can also use a 12V DC adaptor.
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IDEAS
REMOTE CONTROL FOR HOME APPLIANCES
S. MOHAN
C
onnect this circuit to any of your home appliances (lamp, fan, radio, etc) to make the appliance turn on/off from a TV, VCD or DVD remote control. The circuit can be activated from up to 10 metres. The 38kHz infrared (IR) rays generated by the remote control are received by IR receiver module TSOP1738 of the circuit. Pin 1 of TSOP1738 is connected to ground, pin 2 is connected to the power supply through resistor R5 and the output is taken from pin 3. The output signal is amplified by transistor T1 (BC558). The amplified sig-
nal is fed to clock pin 14 of decade counter IC CD4017 (IC1). Pin 8 of IC1 is grounded, pin 16 is connected to Vcc and pin 3 is connected to LED1 (red), which glows to indicate that the appliance is ‘off.’ The output of IC1 is taken from its pin 2. LED2 (green) connected to pin 2 is used to indicate the ‘on’ state of the
82 • MAY 2005 • ELECTRONICS FOR YOU
EO SANI TH
appliance. Transistor T2 (BC548) connected to pin 2 of IC1 drives relay RL1. Diode 1N4007 (D1) acts as a freewheeling diode. The appliance to be controlled is connected between the pole of the relay and neutral terminal of mains. It gets connected to live terminal of AC mains via normally opened (N/O) contact when the relay energises. z
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CIRCUIT
IDEAS
SCHOOL/COLLEGE QUIZ BUZZER
GOVINDA RAJU TEKUMUDI
M
anual buzzers used for quiz competitions in schools and colleges create a lot of confusion in identifying the first respondent. Although there are circuits using PCs and discrete ICs, they are either too expensive or limited to only a few number of players. The quiz buzzer circuit given here can be used for up to eight players, which is maximum in any quiz com-
Fig. 1: Power supply
IVEDI S.C. DW
petition. The circuit uses IC 74LS373 and a few passive components that are readily available in the market. The circuit can be divided into two sections: power supply and quiz buzzer. Fig. 1 shows the power supply section. The regulated 5V power supply for the quiz buzzer section is derived from AC mains. The 230V AC mains is stepped down to 7.5V AC by transformer X1, rectified by bridge rectifier BR1, filtered by C1 and regulated by regulator IC1. Capacitor C2 bypasses ripples in the regulator output. Fig. 2 shows the quiz buzzer section. At the heart of this
section is IC 74LS373, an octal latch that is used to transfer the logic state at data input pins D0 through D7 to the corresponding Q0 through Q7 outputs. Data pins D0 through D7 are normally pulled low by resistors R1 through R8, respectively. One terminal of push-to-on switches S1 through S8 is connected to +5V, while the other terminal is connected to the respective data input pins. The switches are to be extended to the players through cord wire. The torch bulbs BL1 through BL8 can be housed in boxes with the front side of the boxes covered with a white paper having the name or number of the contestant written over it for easy identification. Place the boxes above the head level so that these can be seen by the audience also. When the power is switched on using switch S9 (provided terminals ‘A’
Fig. 2: Circuit of school/college quiz buzzer
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ELECTRONICS FOR YOU • MAY 2006 • 87
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and ‘B’ of both the power supply and quiz buzzer sections are interconnected), the circuit is ready to use. Now all the switches (S1 through S8) are open and Q0 through Q7 outputs of IC 74LS373 are low. As a result, the gates of silicon-controlled rectifiers SCR1 through SCR8 are also low. As soon as a contestant momen-
tarily presses his respective switch, the corresponding output data pin goes high. This triggers the corresponding SCR and the respective bulb glows. At the same time, the piezobuzzer (PZ1) sounds as transistor T1 conducts. Simultaneously, the base of transistor T2 becomes high to make it conduct. Latch-enable (LE) pin 11 of IC2
88 • MAY 2006 • ELECTRONICS FOR YOU
is tied to ground to latch all the Q0 through Q7 outputs. This restricts further change in the output state due to any change in the state of switches S1 through S8 by any other contestant. Only one of the eight torch bulbs glows until the circuit is reset by on/ off switch S9. Note. The complete kit is available at Kits ‘n’ Spares outlet.
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IDEAS
5-BAND GRAPHIC EQUALISER
SOMEN GHOSH
T
his equaliser uses low-cost op-amps. Good-quality opamps powered by a single voltage supply are readily available in the market. The op-amp should have a noise density of less than 24nV/√Hz, slew rate of more than 5V/µs and gain-
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bandwidth product greater than 3 MHz. The NE5532 or LM833 used in this circuit meets these requirements. Equaliser circuits typically divide the audio spectrum into separate frequency bands and have independent gain control for each band. The output of each band is mixed at IC4(A) and then fed to an audio power am-
A R. SUNDAR
KUMAR
plifier. Proper quality factor (Q) needs to be selected to avoid overlap in adjacent bands as this introduces colouration into the audio signal. We have used the multiple-feedback bandpass filter topology shown in left-most corner at the bottom of the figure. This is a circuit for single-channel bandpass filter. If the capacitors are
ELECTRONICS FOR YOU • MAY 2007 • 87
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IDEAS
Component Values for 5-band Equaliser Centre frequency fo (Hz)
C (μF)
Ra (kiloohms)
Rb (kiloohms)
60
C4=C5=0.1
R9=11
R11=27
R10=91
4.1
1.7
250
C7=C8=0.1
R14=2.7
R15=6.3
R13=22
4.1
1.7
1000
C10=C11=0.047
R18=1.5
R19=3.3
R17=11
3.7
1.6
4000
C13=C14=0.0022
R22=7.5
R23=18
R21=63
4.2
1.7
16000
C16=C17=0.0022
R26=2
R27=4.3
R25=15
4.2
1.7
of the same value, the calculations are fairly simple. For calculating the component values, use the following formulae: Centre frequency Bandwidth Quality factor Gain
(fo) (B) (Q) (A)
: : : :
1/2πC√(Ra||Rb)Rc 1/πCRc fo/B = πfoCRc –Rc/2Ra
These can be combined to give the following formulae: Ra = Q/2πfoAC Rb = Q/2πfoC (2Q2–A) Rc = Q/πfoC
Begin the calculations by choosing a large value of capacitance (~0.1F) and smaller value of resistances. Increasing the capacitance decreases resistances (Ra, Rb and Rc). Care must be taken to avoid overloading on the input buffer op-amp. Note that stray capacitances on the board reduces the value of ‘C.’ The bandwidth and gain do not depend on Rb. Hence, Rb can be used to modify the mid-frequency without affecting the bandwidth and gain. For equalisers, there are standard mid-frequencies that are normally used. The exact frequencies depend on
88 • MAY 2007 • ELECTRONICS FOR YOU
Rc (kiloohms)
Gain (A) Quality (Q)
the octave division, application and some degree of manufacturers’ preference, but nearly all share the basic octave boundaries that are based on a centre frequency of 1000 Hz. A balance between the number of filters and bandwidth need to be observed. It is possible to use a wider bandwidth and fewer filters, or narrower bandwidth and more filters. Anything narrower than 1/3 octave is rare, since the complexity of the filters increases for higher values of ‘Q.’ This can get rather expensive and in reality is of limited use for most applications in audio systems. National Semiconductor lists the following mid-frequencies for a 10band graphic equaliser: 32, 64, 125, 250, 500, 1k, 2k, 4k, 8k and 16k. It also recommends a ‘Q’ of 1.7 for equalisers. The table lists the component values for different centre frequencies of the equaliser. We used ‘Q’ of 1.7 and gain (A) of 4. The circuit for the 5-band equaliser uses IC1 (A) LM833 as the buffer stage for the equaliser. It is a non-inverting
amplifier with a gain of ‘2.’ The input signal is divided by ‘2’ by the resistive network comprising R3 and R4. Hence the net gain of this amplifier is unity. Two 100k resistors (R1 and R2) are used as a voltage divider and the junction voltage is fed to its positive input through R6. This divider has enough power to feed all other op-amps directly. Resistor Ro (R8=R12=R16=R20=R24=R28=R30=100Ω) has the dual function of noise reduction and resistive isolation of capacitive load. It may be varied between 50 and 150 ohms depending on the noise in the circuit. The potmeters (VR1 through VR5) are in the signal path and hence should be of the best quality possible. Wrap the body of the pots with bare copper wire and solder the other end of the wire to ground. Since the filters are very sensitive, all resistances should be metal-film type and the capacitors should be polyester type. Each stage of the op-amp needs to be capacitively coupled to the next stage so that the DC does not get propagated and amplified. For a good low-frequency response, this coupling capacitor should be greater than 1 µF. A 10µF, 16V capacitor is used in each stage of the circuit here. The circuit is powered by a 12V DC regulated supply. A well-regulated supply using 7812 is recommended. Ground the Vcc pin of each op-amp with a 0.1µF ceramic disk capacitor to bypass the noise.
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ideas
Solar Panel based Charger and Small LED Lamp
P.V. Vinod Kumar Thekkumuri
Y
ou can save on your electricity bills by switching to alternative sources of power. The photovoltaic module or solar panel
types of batteries: lead acid, Ni-Cd and Li-ion. The lead-acid batteries are commonly used in emergency lamps and UPS. The working of the circuit is simple. The output of the solar panel is
Fig. 1: Circuit of solar panel based charger
Fig. 2: LED lamp circuit
described here is capable of delivering a power of 5 watts. At full sunlight, the solar panel outputs 16.5V. It can deliver a current of 300-350 mA. Using it you can charge three 8 0 • M ay 2 0 0 9 • e l e c t ro n i c s f o r yo u
fed via diode 1N5402 (D1), which acts as a polarity guard and protects the solar panel. An ammeter is connected in series between diode D1 and fuse to measure the current flowing during charging of the batteries. As shown in Fig. 1, we have used an analogue multimeter in 500mA range. Diode D2 is used for protection against reverse polarity in case of wrong connection of the lead-acid battery. When you connect wrong polarity, the fuse will blow up. For charging a lead-acid battery, shift switch S1 to ‘on’ position and use connector ‘A.’ After you connect the battery, charging starts from the solar panel via diode D1, multimeter and fuse. Note that pulsating DC is the best for charging lead-acid batteries. If you use this cir-
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cuit for charging a lead-acid battery, replace it with a normal pulsating DC charger once a week. Keep checking the water level of the leadacid battery. Pure DC voltage normally leads to deposition of sulphur on the plates of lead-acid batteries. For charging Ni-Cd cells, shift switches S1 and S3 to ‘on’ position and use connector ‘B.’ Regulator IC 7806 (IC1) is wired as a constantcurrent source and its output is taken from the middle terminal (normally grounded). Using this circuit, a constant current goes to Ni-Cd cell for charging. A total of four 1.2V cells are used here. Resistor R2 limits the charging current. For charging Li-ion battery (used in mobile phones), shift switches S1 and S2 to ‘on’ position and use connector ‘C.’ Regulator IC 7805 (IC2) provides 5V for charging the Li-ion battery. Using this circuit, you can charge a 3.6V Li-ion cell very easily. Resistor R3 limits the charging current. Fig. 2 shows the circuit for a small LED-based lamp. It is simple and lowcost. Six 10mm white LEDs (LED2 through LED7) are used here. Just connect them in parallel and drive directly by a 3.6V DC source. You can use either pencil-type Ni-Cd batteries or rechargeable batteries as the power source. Assemble the circuit on a generalpurpose PCB and enclose in a small box. Mount RCA socket on the front panel of the box and wire RCA plug with cable for connecting the battery and LEDbased lamp to the charger. w w w. e f y m ag . co m
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Digital Timer Enhancement
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his simple circuit automatically activates or deactivates an electronic device at the time of alarm preset in a clock. When the alarm rings, the tone burst generated at the terminal of the buzzer triggers the circuit and the relay energises or de-energises to switch on or switch off the load. The circuit is built around ICs CD40106 (IC1) and CD4017 (IC2) and a few discrete components. IC1 is a hex Schmitt trigger, while IC2 is a decade counter. The circuit works off regulated 6V power supply, while the alarm clock runs off its own 1.5V battery. The tone burst generated at the
circuit can be used: 1. You want an appliance or gadget to switch on automatically at a preset time 2. You switch on an appliance or gadget manually at a particular time and want it to switch off automatically at a preset time Let us see how it works when you want your appliance to switch on at a preset time, say, 3 pm. Set the alarm in your clock to 3 pm and slide switch S3 towards Q1. When the alarm sounds at 3 pm, Q0 output of IC2 advances to Q1 and relay RL1 energises to connect the load (appliance) to mains power supply through its contacts. The load remains ‘on’ until you reset IC2 by
and relay RL1 de-energises to disconnect the load from mains power supply through its contacts. At this time, you need to pause the alarm using pause switch of the clock. When you press reset switch S1, LED1 glows to indicate that the circuit is ready to work. When you press start switch S2, LED2 glows to indicate start mode. Glowing of LED3 means that the counter has stopped counting and needs to be reset before use. When the counter is in stop mode, Q2 output of IC2 remains high. As this pin is connected to the clock-enable input (pin 13) of IC2, the clock input is inhibited. In this condition, any tone
piezobuzzer is tapped from its connection points. The positive terminal of the clock buzzer is connected to the base of transistor T1 and the negative terminal is connected to ground of the circuit. When the alarm clock sounds, the signal from the clock buzzer makes transistor T1 conduct. As a result, pin 1 of gate N1 goes low and it outputs high at pin 2. This low-to-high transition clocks the counter (IC2) at pin 14 through diode D1 and gate N2. In this way, IC2 advances by one at each clock produced due to the sounding alarm. There are two situations where this
momentarily pressing S1. At this time, you need to pause the alarm using pause switch of the clock. Now suppose you manually start the load at 3 pm and want it to stop automatically at 6 pm. First, reset IC2 by momentarily pressing S1 and slide switch S3 towards Q2. Set the alarm in your clock to 6 pm. To start the load, press switch S2 momentarily at 3 pm. The Q0 output of IC2 advances to Q1 and relay RL1 energises to connect the load to mains power supply through its contacts. When the alarm sounds at 6 pm, Q1 output of IC2 advances to Q2
burst signal arriving from the clock has no effect on IC2 and therefore the circuit remains in stop mode. You can now set the alarm time in the clock. Assemble the circuit on a generalpurpose PCB and enclose in a small cabinet. Connect the base of transistor (T1) to positive terminal of the alarm clock and negative terminal to ground of the circuit. Put the alarm clock at a convenient place. If you do not want to use a 6V battery, replace it with a 6V adaptor to power the circuit. Mount the LEDs and the pushbutton on the front panel of the cabinet.
Raj K. Gorkhali
T
9 8 • M ay 2 0 1 0 • e l e c t ro n i c s f o r yo u
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Traffic Controller
N.R. Paranjape
T
his simple traffic controller can be used to teach children rudiments of traffic rules. The circuit (shown in Fig. 1) uses readily available components. It mainly comprises rectifier diodes (1N4001), a 5V regulator 7805, two timers IC 555, two relays (5V, single-changeover), three 15W, 230V bulbs and some discrete components. Mains power is stepped down by transformer X1 to deliver a secondary output of 9V, 300 mA. The transformer output is rectified by a full-wave bridge rectifier comprising diodes D1 through D4, filtered by capacitor C1 and regulated by IC 7805 (IC1). IC2 is wired as a multivibrator with ‘on’ and ‘off’ periods of approximately 30 seconds each with the component values selected. As soon as mains power is switched on, pin 3 of IC2 goes high for 30 seconds. This, in turn, energises relay RL1 through transistor T1 and the red lamp (B1) glows through its normally-open (N/O) contact. At the same time, mains power is disconnected from the pole of relay RL2. As the ‘on’ time of IC2 ends, a
high-to-low pulse at its pin 3 triggers IC3 through C5. IC3 is configured as a monostable with ‘on’ time of about 4 seconds, which means pin 3 of IC3 will remain high for this period and energise relay RL2 through driver transistor T2. The amber lamp (B2) thus lights up for 4 seconds. As soon as 4-second time period of timer IC3 at pin 3 lapses, relay RL2 de-energises and the green lamp (B3) lights up for the rest of ‘off’ period of IC2, which is about 26 seconds. The green lamp is activated through the normally closed (N/C) contacts of relay RL2. So when mains power is switched on, red light glows for 30 seconds, amber for 4 seconds and green for 26 seconds. You can assemble this circuit on a general-purpose PCB and enclose in an insulated box. The box should have enough space for mounting transformer X1 and two relays. It can be fixed near 230V
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Fig. 2: Construction details of traffic controller unit
Fig. 1: Circuit of traffic controller 8 0 • N o v e m b e r 2 0 0 8 • e l e c t ro n i c s f o r yo u
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AC, 50Hz power supply or mounted on the PVC tube used in construction of the traffic light container. Construction of the traffic light container box is shown in Fig. 2. A stout cardboard box of 30x15x10cm3 is required for housing the lamps. To ensure strength, use a 10x45cm2 plywood plate having 1.5cm thickness and secure onto it three light sockets and the
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box using nuts and bolts or screws. Make three tubes of thin aluminium sheet, which is readily available in hardware shops. The inner diameter of aluminium tubes should be such that these can snugly fit on the light sockets. Using a sharp knife, make holes opposite the sockets carefully. Wire the sockets at the back and take the wires out through the PVC tube.
First, fix three 15W bulbs (B1 through B3) and then press on the tubes. Support the other ends of the tubes in the holes made on the front panel of cardboard box. Sandwich gelatine papers of the three colours between two sheets of cardboard and fix over the tubes. The visibility of red, amber and green lights improves with their mounting on the tubular shape.
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DuoPhone
Raj K. Gorkhali
T
his simple circuit of a duophone allows you to access two telephone lines through one telephone set. Each telephone conversation will remain entirely separate unless you choose to combine the two lines through a conference switch. Its unique feature is a three-party conversation/conference facility. The entire circuit is divided into three main sections—the ringer, hold and conferencing. The telephone set is connected to line 1 under normal conditions. The ringer is used for in-
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dicating a call on line 2 that is not connected to the telephone receiver. When you have a call on line 2, the ringer will buzz. The telephone receiver can then be connected to line 2 through the telephone changeover switch S4 to receive the call. The ringer section is built around IC3 and its associated components. Its circuit uses IC 1240 to detect the ring signal and keeps the buzzer ringing for an incoming call on line 2. The supply voltage for the ringer is obtained from the phone line’s AC ring (80V AC RMS) signal and is regulated inside the IC so that the noise on the line does not
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affect operation of the IC. The two-tone frequencies generated are switched by an internal oscillator in a fast sequence, which appear at the output amplifier and drive the piezo buzzer element directly. The hold section is built around IC1 and IC2 . Switch S1 is used to hold line 1 and S2 is used to put line 2 on hold. Since one telephone set is used for two separate lines, provision is thus made to hold the first call while the telephone set is connected to make or receive the second call. The circuit comprises two identical hold circuits, each with its own flashing LED to maintain the holding current. Each hold circuit has a timer LM555 (IC1 or IC2) connected as a free-running oscillator operating at a frequency of 2 Hz. The output pin 3 of each timer is used for driving an LED that flashes twice in a second. The hold circuit is powered by the telephone lines through manuallyoperated hold switches (S1 and S2). Resistors R2 and R6 are placed in the hold circuits to ensure that sufficient current is drawn from the telephone line to prevent a disconnection. The conferencing section is built around the audio coupling transformer X1. Switch S3 enables threeway conversation through both the telephone lines. The transformer couples the audio signals from one telephone line to the other. At the same time, complete DC isolation is maintained between both the telephone lines. Capacitors C1 and C3 are used for preventing any DC from flowing into the transformer windings. Resistor R1 provides a holding current on line 1 when the telephone set is connected to line 2 during a conference call. Once the three-way conversation is established through the double-pole single-throw (DPST) switch S3, the hold circuits and flashing LED indicators are turned off. LED3, which gets illuminated by the holding current through R1, provides a visual w w w. e f y m ag . co m
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indication of the conferencing. The working of the circuit is simple. To check if the wiring of switch S4 is correct, connect the telephone set to line 1. Now lift up the handset and dial the number of line 2. the ringer would sound. Now disconnect line 1 and connect line 2 through switch S4. You would get the dial tone from line 2. To check a conference call, you would need the help of two friends. First connect switch S4 to line 1 and make a call to friend 1. Now flip the DPST switch S3 to the ‘on’ position. This puts on hold friend 1 on line 1 and the conference LED3 lights up. Connect switch S4 to line 2 and dial friend 2. When the call on line 2 is answered, a three-way conversation can be made. When the duophone is not in use,
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connect switch S4 to line 1. All other switches should be in the ‘off’ mode and all LEDs should be unlit. This permits the telephone ringer to be activated if a call comes on line 2. For making calls using line 1 or line 2, you can simply connect switch S4 to the desired line. Assemble the circuit on a general purpose PCB and enclose it in a suitable cabinet. Fix the switches S1 through S4 on the front side of the cabinet. Also fix the LEDs on the front of the cabinet and the buzzer at the back of the cabinet. It would be better if you use telephone sockets for the telephone lines. Sockets are relatively inexpensive and save time when troubleshooting needs to be done. Use modular plugs to connect the circuit and the two telephone lines. By using such ‘quick discon-
nect’ plugs, you can easily remove the unit from the telephone lines. Check the polarity of the telephone lines with a multimeter and connect it to the circuit accordingly. To check the circuit after completing the wiring, connect a 6V regulated power supply to line 1. When you switch S1 to the ‘on’ position, LED1 blinks at a rate of 2 Hz. If you flip switch S1 to the ‘off’ position and switch S3 to the ‘on’ position, LED1 stops blinking and LED3 starts glowing, indicating that the conferencing facility is being used. Now disconnect line 1 from the 6V power supply, connect it to line 2 and flip switch S2 to the ‘on’ position. Now LED2 blinks at a rate of 2 Hz. Before connecting the circuit to the telephone lines, flip each hold switch to the ‘off’ position. Now your circuit is ready to be used.
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Smart Laptop Docking Station
T.K. HAREENDRAN
Y
ou can easily convert your ordinary docking station into a smart electronic laptop docking station with antitheft alarm. The add-on sensor circuit required for this is built around IC CNY70 (IC1) and IC CD4060 (IC2) as shown in Fig. 1. IC CNY70 is an integrated reflectivetype opto-sensor that contains a phototransistor and an infrared LED. The LED emits infrared light and the transistor works as a receiver. The current flowing through the phototransistor depends on the intensity of the light detected. IC CD4060 is a 14-stage ripple-carry binary counter. The counter is reset to zero by a gating positive voltage at
the reset input independent of clock. Power supply to the circuit is derived from AC mains by using stepdown transformer X1. The transformer output is rectified by a full-wave bridge rectifier comprising diodes D1 through D4 and smoothed by capacitor C1. When power switch S1 is in ‘on’ position, the circuit gets power supply and power-on indicator LED1 lights up. At the same time, the mains socket also gets the AC mains supply. This mains socket can be used to connect the laptop charger and/or a desktop lamp, etc. Working of the circuit is simple. When the laptop is in the docking station, the phototransistor inside IC1 receives the IR light from the LED,
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reflected by the laptop surface. The phototransistor conducts to make reset pin 12 of IC2 high, so IC2 does not oscillate. When someone lifts up the laptop from the docking station, the phototransistor cuts off and pin 12 of IC2 goes low. As a result, IC2 starts oscillating. After a few seconds, delay pin 3 of IC2 goes high to drive transistor T1. The piezobuzzer starts beeping to raise an alert and the LED2 glows to indicate that someone has stolen the laptop from the dockyard. The simplicity of the circuit makes
Fig. 1: Circuit for laptop docking station with antitheft alarm
Fig. 2: Proposed assembly for docking station 1 1 2 • N o v e m b e r 2 0 1 0 • e l e c t ro n i c s f o r yo u
it ideal for construction on a small PCB. After completion of wiring, check the circuit for proper functioning of all the sections and enclose the unit in a suitable ABS case. Mount the finished unit beneath the docking station using small screws/double-sided glue pads so that the opto-sensor is exactly at the centre of the docking-station base plate. Refer Fig. 2 for the arrangement. If your laptop computer is black in colour, it will reflect far less IR light. You can overcome this drawback by w w w. e f y m ag . co m
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attaching a white sticker suitably at the bottom of the laptop. Calibrate the circuit before first use. Set preset VR1 at the centre and place the laptop in the docking station. Now turn VR1 slowly until IC2 goes to standby (no-oscillation) mode. Then
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remove the laptop from the docking station, ensure that IC2 is enabled (pin 12 is low) and wait for the alarm sound. Repeat the process and adjust VR1 until you get the correct result. Note that the LED in the opto-sensor is permanently powered via resistor R2.
Similarly, you are free to experiment with the values of IC2 timing components C5, R3 and R4 for increasing or decreasing the delay time. EFY note. During testing at EFY Lab, we used CX sensor from OMRON in place of CNY70.
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TELEPHONE-OPERATED CALLING SYSTEM
YOGESH KATARIA (VU3PYF)
D
ual-tone multiple-frequency (DTMF) receiver IC is commonly used in telephone equipment. One common DTMF receiver is Holtek HT9170 used in electronic communication circuits. The Holtek HT9170 series comprises DTMF receivers integrated with digital decoder and bandsplit filter functions. All HT9170 series ICs use digital counting techniques to detect and decode all the 16 DTMF tone pairs into a 4-bit code output. This telephone-operated calling circuit is very helpful for doctors in calling the patients, in banks and in various other situations where persons have to be called or signalled. When you need to call a person amongst many standing outside your cabin, just lift the telephone handset off the cradle and press the respective number. The number of the person called will be displayed and a bell will sound to inform the person that it is his turn.
The circuit can also be used in quiz contests and by visually- or hearingimpaired people. It can be used to call a maximum of nine different persons. The circuit is built around DTMF receiver IC HT9170, BCD-to-7-segment decoder/driver 7447, quad 2-input OR gate and common-anode display. Simple melody generator IC UM66 is used to produce melody sound in the loudspeaker through Darlington-pair transistors (T1 and T2). The tone pair DTMF generated by pressing the telephone key is converted into binary values internally in the IC. The binary values are indicated by the glowing of LEDs at the output of IC1. The output of IC1 is connected to: 1. LEDs connected via resistors R15 through R18 at pins 11 through 14, respectively. LED1 indicates the LSB and LED4 indicates the MSB. 2. BCD-to-7-segment decoder/ driver 7447, whose outputs are connected to the common-anode display for displaying the pressed number on
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the telephone connected in parallel to the circuit. 3. Gates N1 and N2 to activate the call bell. Here is how the circuit works: Connect the telephone and the circuit in parallel to the telephone line. Connect 6V to the circuit. When you press switch S1, DIS1 shows ‘0.’ Lift the handset off the cradle and dial a number, say, ‘1.’ The output of IC1 becomes A3A2A1A0 = 0001. LED1 glows, the display shows ‘1’ and the call bell sounds. To stop the call bell, put the receiver on the cradle and press switch S1 momentarily. Now DIS1 shows ‘0’ and LED1 stops glowing. For calling other numbers, follow the same procedure: Lift the handset off the cradle and press the desired number (0 through 9). The respective LED will glow, the number will be displayed on DIS1 and the call bell will sound. Now put the handset on the cradle and press S1 momentarily to stop the call bell.
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generation of spark from the spark plug. Usually, there is a wire running from the alternator to the ignition coil, which has to be routed through one of the N/ C1 contacts of relay RL1 as shown in turned on. When you turn ignition off Fig. 1. Fig. 2 shows the pin configurausing key S2, you have approximately tions of SCR BT169, MOSFET BS170 15 seconds to get off the bike; this funcand transistor BC548. tion is performed by resistor R6 to disAlso, on disconnection of the coil, charge capacitor C3. Thereafter, if anysound generator IC UM3561 (IC1) gets one attempts to get on the bike or move power supply through N/O2 contact it, the alarm sounds for approximately of relay RL1. This drives the darlington 15 seconds and also disconnects the pair built around T3 and T4 to proignition circuit. duce the siren sound through loudDuring parking, hidden switch S1 speaker LS1. is normally open and does not allow To start the vehicle, both hidden triggering of MOSFET T1. But when switch S1 and ignition key S2 should someone starts the motorbike through be switched on. Otherwise, the alarm ignition switch S2, MOSFET T2 trigwill start sounding. Switching on S1 gers through diode D1 and triggers SCR1, which, in resistor R5. Relay RL1 (12V, turn, triggers MOSFET T1. 2C/O) energises to activate MOSFET T1 is configured the alarm (built around to disable MOSFET T2 IC1) as well as to disconfrom functioning. As a renect the ignition coil from sult, MOSFET T2 does not the circuit. Disconnection of Fig. 2: Pin configurations of trigger and relay RL1 rethe ignition coil prevents BT169, BS170 and BC548 mains de-energised, alarm
MOTORBIKE ALARM
T.A. BABU
T
his simple-to-build alarm can be fitted in bikes to protect them from being stolen. The tiny circuit can be hidden anywhere, without any complicated wiring. Virtually, it suits all bikes as long as they have a battery. It doesn’t drain out the battery though as the standby current is zero. The hidden switch S1 can be a small push-to-on switch, or a reed switch with magnet, or any other similar simple arrangement. The circuit is designed around a couple of lowvoltage MOSFETs configured as monostable timers. Motorbike key S2 is an ignition switch, while switch S3 is a tilt switch. Motorbike key S2 provides power supply to the gate of MOSFET T2, when
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Fig. 1: Cheap motorbike alarm
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deactivated and ignition coil connected to the circuit. Connection to the ignition coil helps in generation of spark from the spark plug. Keeping hidden switch S1 accessible only to the owner prevents the bike from pillaging. Tilt switch S3 prevents attempt to move the vehicle without starting it. Glass- and metal-bodied versions of the switch offer bounce-free switching and quick break action even when tilted slowly. Unless otherwise stated,
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the angle by which the switch must be tilted to ensure the contact operation (operating angle), must be approximately 1.5 to 2 times the stated differential angle. The differential angle is the measure of the ‘just closed’ position to the ‘just open’ position. The tilt switch has characteristics like contacts make and break with vibration, return to the open state at rest, non-position sensitivity, inert gas and hermetic sealing for protection of con-
tacts and tin-plated steel housing. If you find difficulty in getting the tilt switch, you may replace it with a reed switch (N/O) and a piece of magnet. The magnet and the reed switch should be mounted such that the contacts of the switch close when the bike stand is lifted up from rest. EFY note. Make sure that while driving, the two internal contacts of the Tilt switch don’t touch each other.
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Muscular Stimulator
Ashok K. Doctor
H
ere is a circuit that stimulates nerves of that part of your body where electrodes are
attached. It is useful to relieve headache and muscular pain and revive frozen muscles that impair movement. Though it provides muscles stimulation and invigoration, it’s mainly an
Fig. 1: Muscular stimulator circuit
Fig. 2: Timer circuit 9 6 • o c to b e r 2 0 0 8 • e l e c t ro n i c s f o r yo u
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aid in removing cellulitis. The system comprises two units: muscular stimulator and timer. Fig. 1 shows the circuit of the muscular stimulator. IC 7555 is wired as an astable multivibrator to generate about 80Hz pulses. The output of IC1 is fed to transistor T1, whose emitter is further connected to the base of transistor T2 through R3 and VR1. The collector of transistor T2 is connected to one end of the secondary winding of transformer X1. The other end of the secondary winding of the transformer is connected to ground. When IC1 oscillates, transformer X1 is driven by the pulse frequencies generated to produce high voltage at its primary terminals. Separate electrodes are connected to each end of the primary winding of transformer X1. Diode 1N4007 (D1) protects transistor T2 against high-voltage pulses generated by the transformer. Using potmeter VR1 you can control the intensity of current sensing at the electrodes. The brightness level of LED1 indicates the amplitude of the pulses. If you want to increase the intensity level, replace the 1.8-kilo-ohm resistor with 5.6 kilo-ohms or higher value up to 10 kilo-ohms. X1 is a small mains transformer with 220V primary to 12V, 100/150mA secondary. It must be reverse connected, i.e., connect the secondary winding to the collector of T2 and ground, and primary winding to the output electrodes. The output voltage is about 60V but the output current is so small that there is no threat of electric shock. Electrodes are made of small, thinguage metallic plates measuring about 2.5×2.5 cm2 in size. Use flexible wires to solder electrodes and connect to the w w w. e f y m ag . co m
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output of the device. Before attaching metal electrodes to the body, wipe them with a damp cloth. After attaching the electrodes to the body (with the help of elastic bands on velcro straps), flip switch S1 to activate the circuit and rotate the knob of intensity-control preset VR1 very slowly until you feel a slight tingling sensation. Fig. 2 shows the timer circuit. It uses IC NE555 wired in monostable mode. Initially, when you press switch
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S2, the monostable triggers and its output goes high for 10 minutes. Thereafter, its output goes low to give a beep sound from the piezobuzzer and lights up the red LED (LED2) indicating that stimulation time is over. Assemble the timer with a separate switch and a 9V DC battery in the same cabinet as the stimulator. Tape the electrodes to the skin at opposite ends of the chosen muscle and rotate VR1 knob slowly until you sense light itching
when the muscular stimulation circuit is powered on. At the same time, flip switch S2 to start the timer for counting the time. At the end of the timing cycle, the piezobuzzer beeps. Each session should last about 10 minutes. Caution: Heart patients and pregnant women should not use this device. Also, do not attach electrodes to burns, cuts, wounds or any injury. Consult your physician before using this circuit.
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Electronic Metronome
o
sani the
metronome is used by musicians for practice in maintaining a consistent tempo, or rubato, around a fixed beat. This circuit produces a regular beat at the rate of 40 to 200 beats per minute. It accentuates every second, third, fourth, fifth, sixth or eighth beat, which is adjustable as per your liking and requirement. Every beat is indicated by the glowing of an LED. The accented beat is indicated by another LED.
IC1 drives the pulse generator. The length of the pulse is about 10 ms, and it appears at pin 1 of IC3 (NOR gate N3). At each pulse, the red LED (LED1) flashes to indicate occurrence of the beat. The pulse passes through NAND gates N6 and N7 of IC4. The pulse output from pin 6 of N7 is fed to NAND gate N8. The audio signal output generated by another multivibrator (IC6) is also fed to gate N8. The audio signal can be adjusted to obtain a note of suitable pitch. The output from IC1 also goes
is longer (about 40ms) and is used to mark the accented beat. The result is a ‘tick’ sound lasting about 40 ms, which sounds every second, third, fourth, fifth, sixth or eighth beat, depending on the setting of S1. The accent pulse makes the yellow LED (LED2) flash. It is important that the base ‘tick’ note or beat is not heard on the accented beat. This is achieved by gates N5 through N7 of IC4. The final audio signal appears at pin 3 of IC5 (NAND gate N10). This
The beat is derived from an astable multivibrator (IC1) running between 0.67 Hz (40 beats per minute) and 3.47 Hz (208 beats per minute), and a pulse generator built around NOR gates N1 and N3, resistor R3 and capacitor C2. The beat covers all the musical tempi from adagio to presto. The results are a very short burst of sound, reminiscent of the ‘tick’ of a mechanical metronome. If you prefer a beep rather than a tick sound, the pulses should be lengthened by reducing the value of R3 to, say, 5.6 or 6.8 kilo-ohms.
to IC2 (CD4022), which is a divideby-eight counter/divider with eight decoded outputs. Rotary switch S1 allows the counter to be reset every two, three, four, five or six counts, or cycle through eight counts without resetting. Output Q0 of IC2 drives the second pulse generator built around NOR gates N2 and N4, resistor R4 and capacitor C3. The output is an accented beat pulse, which is fed to NAND gates N5 and N9 and the base of transistor T2. Since C3 has a higher capacitance than C2, this pulse
signal can be fed to the audio power amplifier stage. When you supply 6V DC to the circuit, you can hear the base or tempo beats and accented beats from the speaker of your power amplifier. The red LED (LED1) flashes with the beat and the yellow LED (LED2) flashes on the accented beat. Construction and testing is simple. Assemble the circuit on a breadboard or general-purpose PCB. Mount all the components, except S1, and temporarily connect pin 15 of IC2 to ground rail. IC1 produces an audible tick sound
Raj K. Gorkhali
A
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(tempo beat) at a fixed rate that varies as VR1 is adjusted. IC6 produces a tone that varies in pitch from about 250 Hz (about an octave below middle C) to about 2 kHz (about two octaves above middle C) as VR2 is adjusted. The counter goes through its normal eightstage cycle and the yellow LED (LED2)
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flashes once for every eight flashes of the red LED (LED1). Now connect a loudspeaker to pin 3 of NAND gate N10 through a 10µF capacitor. The circuit should produce a series of tick sound with a double-tick sound at every eighth tick sound. If this works well, remove pin 15 of IC2
from the ground rail and connect to six-way rotary switch S1. Remove the speaker and 10µF capacitor from pin 3 of N10 and connect pin 3 to an audio power amplifier. Use presets VR1 and VR2 such that turning their knobs clockwise increases the tempo and the pitch, respectively.
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BELL-CUM LIGHT CONTROLLER
SURESH KUMAR K.B.
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his bell-cum-light controller circuit is equipped with four switches labelled S1 through S4. While S4 is the mains ‘on’/‘off’ toggle switch for powering the timer circuit for lighting up a bulb for a specific duration (mainly during night), the functions and placement of the other three switches (which are push switches) follow. Switch S1 (labelled ‘call-bell and timer-on’) is located at the outer entry gate of the house for use by a visitor. This switch activates bell circuit for as long as the switch is kept pressed. On its release, a timer is initialised, which, in turn, switches on a bulb to light up the path between the outer gate and the house door for a specific duration (3 minutes). Switch S2 (labelled ‘timer on’) is situated inside the house for use by
the inhabitants for activating the above-mentioned timer for switching on the light for three minutes from inside the house. S2 is meant to be used during darkness with S4 ‘on.’ Like S1, switch S3 (labelled ‘callbell’) is located outside the entry gate. It is meant to be used during day, when mains switch S4 is ‘off.’ When switch S3 is pressed, it activates only the bell circuit for as long as the switch is kept pressed. Since the bell circuit is powered by a 3V battery, this circuit can be activated even if mains switch is off. With switch S4 ‘on,’ the supply to bulb B1 is routed via N/O contacts of relay RL1. Simultaneously, the AC mains stepped down by transformer X1 is rectified by diodes D1 and D2 followed by filter capacitor C1. The DC supply thus becomes available for timer circuit comprising CD4060 (IC1) and relay driver circuit comprising
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transistors T1 and T2. IC1 is a 14-stage binary counter and oscillator IC. In its quiescent state, the Q13 output (pin 3) is high, which results in conduction of transistor T1 to cut off transistor T2. Thus the relay is in de-energised state and the bulb is ‘off.’ When the master reset is activated by pressing of switch S1 and/or S2, all the output pins of IC1 including Q13 output go low. Thus transistor T1 is cut off, while T2 conducts to energise relay RL1 as also the bulb. Once S1 and S2 are released, the timer starts counting. Pressing of switch S1 additionally results in forward biasing of transistor T3, which conducts to extend 3V battery supply to melody generator UM66 (IC2). The output of the melody generator drives transistor T3 to output the tune via loudspeaker LS1. Thus, when a visitor presses switch
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S1 at the gate, the calling bell sounds and the timer resets. The Q13 output of IC1 goes low to cut off transistor T1. Transistor T2 conducts to energise the relay and turn the light ‘on.’ When switch S1 is released, the timer starts counting. After three minutes (determined by resistor R4 (100kilo-ohm) and capacitor C3 (0.1µF), the Q13 output goes high, i.e., tran-
sistor T1 conducts and T2 cuts off. The relay de-energises to turn the light ‘off.’ Since diode D5 is connected from Q13 to clock input terminal (pin 11), the terminal is always high when Q13 is high, disabling the counting of IC1. So the state is latched until the next resetting takes place. ‘On’ time period can be varied according to the distance
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between the gate and the house. It is decided by the values of resistor R4 and capacitor C3 as follows: ‘On’ period = 300xR4xC3 minutes The light controller circuit will work at night, provided mains switch S4 is ‘on.’ During night, only switches S1 and S2 will be used, while switch S3 is used in the day for the calling bell only.
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Inverter for Soldering Iron
Lovely T.P.
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ere is a simple but inexpensive inverter for using a small soldering iron (25W, 35W, etc) in the absence of mains supply. It uses eight transistors and a few resistors and capacitors. Transistors T1 and T2 (each BC547) form an astable multivibrator that produces 50Hz signal. The complementary outputs from the collectors of transistors T1 and T2 are fed to pnp Darlington driver stages formed by transistor
pairs T3-T5 and T4-T6 (utilising BC558 and BD140). The outputs from the drivers are fed to transistors T7 and T8 (each 2N3055) connected for push-pull operation. Use suitable heat-sinks for transistors T5 through T8. A 230V AC primary to 12V-0-12V, 4.5A secondary transformer (X1) is used. The centre-tapped terminal of the secondary of the transformer is connected to the battery (12V, 7Ah), while the other two terminals of the secondary are connected to the collectors of power transistors T7 and T8,
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respectively. When you power the circuit using switch S1, transformer X1 produces 230V AC at its primary terminal. This voltage can be used to heat your soldering iron. Assemble the circuit on a generalpurpose PCB and house in a suitable cabinet. Connect the battery and transformer with suitable current-carrying wires. On the front panel of the box, fit power switch S1 and a 3-pin socket for connecting the soldering iron. Note that the ratings of the battery, transistors T7 and T8, and transformer may vary as these all depend on the load (soldering iron).
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CCI IRRC UCIUT IITD EIADS E A S
AUTOMATIC NIGHT LAMP WITH MORNING ALARM D. MOHAN KUMAR
SAN
I THE
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his circuit automatically turns on a night lamp when bedroom light is switched off. The lamp remains ‘on’ until the light sensor senses daylight in the morning. A super-bright white LED is used as the night lamp. It gives bright and cool light in the room. When the sensor detects the daylight in the morning, a melodious morning alarm sounds.
sistors (LDRs) for sensing darkness and light in the room. The resistance of LDR is very high in darkness, which reduces to minimum when LDR is fully illuminated. LDR1 detects darkness, while LDR2 detects light in the morning. The circuit is designed around the popular timer IC NE555 (IC2), which is configured as a monostable. IC2 is activated by a low pulse applied to its trigger pin 2. Once triggered, output pin 3 of IC2 goes high and remains in that position un-
Low-value capacitor C2 maintains the monostable for continuous operation, eliminating the timer effect. By increasing the value of C2, the ‘on’ time of the white LED can be adjusted to a predetermined time. LDR2 and associated components generate the morning alarm at dawn. LDR2 detects the ambient light in the room at sunrise and its resistance gradually falls and transistor T1 starts conducting. When T1 conducts, melody-generator IC UM66
The circuit is powered from a standard 0-9V transformer. Diodes D1 through D4 rectify the AC voltage and the resulting DC voltage is smoothed by C1. Regulator IC 7806 gives regulated 6V DC to the circuit. A battery backup is provided to power the circuit when mains fails. When mains supply is available, the 9V rechargeable battery charges via diode D5 and resistor R1 with a reasonably constant current. In the event of mains failure, the battery automatically takes up the load without any delay. Diode D5 prevents the battery from discharging backwards following the mains failure and diode D6 provides current path from the battery. The circuit utilises light-dependant re-
til IC2 is triggered again at its pin 2. When LDR1 is illuminated with ambient light in the room, its resistance remains low, which keeps trigger pin 2 of IC2 at a positive potential. As a result, output pin 3 of IC2 goes low and the white LED remains off. As the illumination of LDR1’s sensitive window reduces, the resistance of the device increases. In total darkness, the specified LDR has a resistance in excess of 280 kiloohms. When the resistance of LDR1 increases, a short pulse is applied to trigger pin 2 of IC2 via resistor R2 (150 kiloohms). This activates the monostable and its output goes high, causing the white LED to glow.
(IC3) gets supply voltage from the emitter of T1 and it starts producing the melody. The musical tone generated by IC3 is amplified by single-transistor amplifier T2. Resistor R7 limits the current to IC3 and zener diode ZD limits the voltage to a safer level of 3.3 volts. The circuit can be easily assembled on a general-purpose PCB. Enclose it in a good-quality plastic case with provisions for LDR and LED. Use a reflective holder for white LED to get a spotlight effect for reading. Place LDRs away from the white LED, preferably on the backside of the case, to avoid unnecessary illumination. The speaker should be small so as to make the gadget compact.
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ELECTRONICS FOR YOU
DECEMBER 2003
C I R CC UI RICTU I IT DI ED EAASS
PROGRAMMABLE TIMER FOR APPLIANCES MITESH P. PARIKH
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his programmable timer is useful for domestic, commercial as well as industrial applications. It automatically turns the appliance on/off after a preset time. The time period can be varied from 8 seconds to 2 hours with
I VED DWI S.C.
the help of rotary switches S2 and S3. The circuit works in two modes: off mode and cyclic mode. Slide switch S4 is used for mode selection. In the off mode, the appliance turns on after a preset time (set by rotary switch S2), remains on for another preset time (set by rotary switch S3) and then turns
off. In the cyclic mode, this process repeats again and again. The circuit is built around three quad two-input NAND gate ICs CD4011 (IC1, IC3 and IC5), two 14-bit binary ripple counters CD4020 (IC2 and IC4) and a relay driver transistor (T1). It works off a 12V DC, 500mA power supply. You can
FEBRUARY 2004
ELECTRONICS FOR YOU
CIRCUIT IDEAS also power the circuit from mains by using a 12V DC, 500mA adaptor in place of the 12V DC power supply. Let’s assume that you want an appliance to turn on after two minutes and keep it on for another two minutes. For this set the rotary switches S2 and S3 to positions as shown in the figure. Initially, when power switch S1 is closed, a small charging current pulse through capacitors C2 and C3 resets both the counters (IC2 and IC4) to make all their outputs (Q4 through Q14) low. The high output at pin 10 of NAND gate N3 starts the first oscillator comprising NAND gates N1 and N2, which provides clock pulses to IC2 at the rate of one pulse per second. The glowing of red LED (LED1) indicates that this oscillator is working well and timer is ‘on.’ During the first 2 minutes, relay RL1 remains de-energised by the control circuit formed by NAND gates N7, N8 and N9 and LED2 is off, which indicates that the appliance is in ‘off’ codition. The second oscillator built around NAND gates N4 and N5 (which provides clock pulses to IC4 at the rate of one pulse per sec-
ELECTRONICS FOR YOU
FEBRUARY 2004
ond) is inhibited by the timing control circuit formed by NAND gates N6, N10 and N11. After 128 pulses (approximately two minutes), the Q8 output of IC2 goes high to perform the following three functions: 1. Make the output at pin 10 of NAND gate N3 low via rotary switch S2, which inhibits the first oscillator 2. Energise relay RL1 via NAND gates N8 and N9 and relay driver transistor T1 to make appliance ‘on’ 3. Make the output at pin 10 of NAND gate N10 low, which is connected to the inputs of NAND gate N11 to make its output at pin 11 high. This high output is further connected to the input (pin 1) of NAND gate N4. Now the second oscillator starts oscillating and provides clock pulses to pin 10 of IC4 at the rate of one pulse per second. Now, after 128 pulses (approximately two minutes), the Q8 output of IC4 goes high. This de-energises the relay via NAND gates N7 and N9 and relay driver transistor T1, provided the mode-selec-
tion slide switch S4 is towards off position. The high Q8 output will inhibit the second oscillator via NAND gates N6, N10 and N11 to stop clock pulses to pin 10 of IC4. Thus the relay is energised only once (for 2 minutes) since clock pulses to both IC2 and IC4 are stopped altogether and their outputs get latched. In case the mode-selector switch S4 is towards ‘cycle on’ side, clock pulses to IC4 would continue and the relay is alternately energised and de-energised for two minutes each. This continues until the circuit is switched off and started again, or the mode-selector switch is slided towards ‘cycle off’ side. Rotary switch S2 is used for start time selection and rotary switch S3 is used for hold time selection. The start and hold time can be increased up to 24 hours by changing the values of R and C components of the oscillator circuit of first and second oscillator. For heavier load, use a relay of a higher current rating. The circuit can be made on a multipurpose PCB and put in a plastic or metal cabinet with proper ventilation.
CCI IRRC UCIU T II TD E IADS E A S
LOW-COST ELECTRONIC QUIZ TABLE VINOD C.M.
H
ere is a simple, low-cost quiz table for four game participants. It determines the contestant who first presses the switch (S1 through S4) to answer a question and locks out the remaining three entries. Simultaneously, the
be ‘on’ after a particular competitor has pressed the pushbutton. These timings can be set by presets VR1 through VR4 as required. The circuit works off 12V, 1.5A power supply. The current rating of the power supply should be according to the load (wattage of bulbs). For higher-wattage
I VED DWI S.C.
connected in parallel to bulb BL1 sounds for the preset time. At the same time, capacitor C1 charges up to 12V, which then discharges through preset VR1. The discharging time of capacitor C1 is decided by preset VR1. For example, if preset VR1 is set for a resistance of 4.7k, it will give a delay of approximately 4 seconds, mean-
Fig. 1: Schematic of low-cost electronic quiz table
Fig. 2: Set-up for electronic quiz table
respective audio alarm sounds and the bulb glows. The quiz table can be used for more number of contestants simply by adding buzzers, bulbs, MOSFETs and diodes. Besides, it provides an option for varying the time for which an individual buzzer and the corresponding bulb should
ELECTRONICS FOR YOU
APRIL 2004
bulbs, use power supply of a higher current rating. If participant A presses switch S1, MOSFET T1 is triggered and the corresponding bulb BL1 (connected between drain of the MOSFET and 12V supply) glows and simultaneously piezobuzzer PZ1
ing that buzzer PZ1 and bulb BL1 will be ‘on’ for 4 seconds. It also indicates that participant A is the first to press his switch. Even if any other participant, say, participant B, presses switch S2 after participant A has already pressed switch S1, buzzer PZ2 and bulb BL2 will not function since MOSFET T2 has no gate voltage to trigger because it is grounded through R2 and D1. The same principle applies for other contestants as well. Instead of bulbs, you can also use a group of LEDs. Fig. 2 shows the set-up for electronic quiz table.
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Remote-operated Master Switch
D. Mohan Kumar
enerally, a bedside master switch is used to switch on lamps both indoors and outdoors when there is a threat of intruder. This circuit can be used to activate the master switch from the bed without searching for the switch in darkness. It can be activated by the TV remote handset. The security lamps
tial divider comprising resistors R4 and R5 maintains half of 5.1V at pin 2 of IC1. In brief, the voltage at pin 2 of IC1 is higher than at pin 3 and its output remains low. LED2 remains ‘off’ and transistor T2 does not conduct. Relay RL1 remains de-energised and, as a result, security lamps (both indoors and outdoors) remain switched off. When you press any key of the remote TV handset, IR rays fall on the
glows to indicate activation of the relay as well as switching ‘on’ of the security lights. Connect a single-pole, single-throw ‘on’/‘off’ switch (MS) to activate the security lamps manually
glow for three minutes and then turn off. The circuit is sensitive and can be activated from a distance of up to 25 metres. IR receiver module TSOP 1738 (IRX1) is used to sense the pulsed 38kHz IR rays from the TV remote handset. The IR receiver module has a PIN photodiode and a preamplifier enclosed in an IR filter epoxy case. Its open-collector output is 5 volts at 5mA current in the standby mode. In the standby mode, no IR rays from the remote handset fall on the IR receiver, so its output pin 3 remains high and LED1 doesn’t glow. Through resistor R2, the base of transistor T1 remains high and it does not conduct. As a result, the voltage at pin 3 of IC CA3130 (IC1) remains low. The poten-
receiver (IRX1) and its output goes low. LED1 flashes in sync with pulsation of the IR rays. At the same time, transistor T1 (BC558) conducts to take pin 3 of IC1 high. IC1 is used as a comparator with timer action. When transistor T1 conducts, pin 3 of IC1 gets a higher voltage than pin 2 making the output of IC1 high. Meanwhile, capacitor C4 charges to full voltage and keeps pin 3 high for a few minutes even after T1 is non-conducting. Resistor R3 provides discharge path for capacitor C4, which decides the time period for which the output of comparator IC1 should remain high. The high output of IC1 energises relay RL1 through relay-driver transistor T2. Thus the load, i.e., security lamps, turn on for three to four minutes. LED2
when required. Zener diode ZD1 provides 5.1V DC for safe operation of the IR receiver and associated circuit. Power for the circuit is derived from a step-down transformer (X1) and a bridge rectifier comprising diodes D1 through D4. Smoothing capacitor C1 removes ripples, if any, from the power supply. Assemble the circuit on a generalpurpose PCB and enclose in a suitable cabinet. Drill holes on the front panel for mounting the IR sensor and LEDs. Connect the master switch between the normally-open (N/O) contact and pole of relay RL1 so that the master switch can be used when needed. The relay contacts rating should be more than 4A. Mount the unit near the master switch using minimal wiring.
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Automatic Light Controller Using 7806
M.K. Chandra Mouleeswaran
V
oltage regulator ICs (78xx series) provide a steady output voltage, as against a widely fluctuating input supply, when the common terminal is grounded. Any voltage about zero volt (ground) connected in the common terminal is added to the output voltage. That means the increase in the common terminal voltage is reflected at the output. On the other hand, if the common terminal is disconnected from the ground, the full input voltage is available at the output. This characteristic is utilised in the present circuit. When the common terminal is connected to the ground, the regulator output is equivalent to the rated voltage, and as soon as the terminal is disconnected from the ground, the output increases up to the input voltage. The common terminal is controlled by a transistor, which works as a switch on the terminal. For automatic control of light, a light-dependent resistor (LDR1) is connected to the base
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of the transistor. In this way, the voltage regulator is able to operate a light bulb automatically as per the ambient light. To derive the power supply for the circuit, the 50Hz, 230V AC mains is stepped down by transformer X1 to deliver a secondary output of 12V, 250 mA. The secondary output of the transformer is applied to a bridge rectifier comprising diodes D1 through D4, filtered by capacitor C1 and fed to the input terminal of the regulator (IC1). The common terminal (pin 2) of IC1 is connected to the ground line of the circuit through transistor BC557 (T1). The transistor is biased by R2, R3, VR1 and LDR1. The grounding of IC1 is controlled by transistor T1, while light is sensed by LDR1. Using preset VR1, you can adjust the light-sensing level of transistor T1. The output of IC1 is fed to the base of transistor T2 (through resistor R4 and zener diode ZD1) and relay RL1. LED1 connected across the positive and ground supply lines acts as a power-‘on’ indicator.
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Normally, the resistance of LDR1 is low during daytime and high during nighttime. During daytime, when light falls on LDR1, pnp transistor T1 conducts. The common terminal of IC1 connects to the ground and IC1 outputs 6V. As a result, transistor T2 does not conduct and the relay remains de-energised. The light bulb remains ‘off’ as the mains connection is not completed through the relay contacts. During nighttime, when no light falls on LDR1, it offers a high resistance at the base junction of transistor T1. So the bias is greatly reduced and T1 doesn’t conduct. Effectively, this removes the common terminal of IC1 from ground and it directs the full input DC to the output. Transistor T2 conducts and the relay energises to light up the bulb as mains connection completes through the relay contacts. As LDR1 is in parallel to VR1+R3 combination, it effectively applies only half of the total resistance of the network formed by R3, VR1 and LDR1 to the junction at T1 in total darkness. In bright light, it greatly reduces the total effective resistance at the junction. The circuit is simple and can be assembled on a small general-purpose PCB. Use a heat-sink for IC1. Make sure that LDR1 and the light bulb are well separated. The circuit can be used for streetlights, tubelights or any other home electrical lighting system that needs to be automated.
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TELECONFERENCING SYSTEM
PRINCE PHILLIPS
H
ere is a low-cost teleconferencing system that lets you talk to two persons at a time in any part of the world over two telephone lines. The circuit makes use of a coupling transformer and some passive components. The circuit is connected between the two telephone lines. It works like this: When ‘X’ calls ‘A’ on the first telephone line, ‘A’ puts this call on hold, dials ‘Y’ on the other telephone line (which is free) and keeps this call too on hold, and slides switches S1 and S2 to ‘on’ position. Now ‘X,’ ‘A’ and ‘Y’ can talk to one another simultaneously over the two telephone lines.
IVEDI S.C. DW
Both the primary and secondary coils of the coupling transformer consist of 500 turns of 40SWG insulated copper wire. At the secondary side, a small circuit is used for DC holding. This cir-
line in operation even though no telephone on that line is present. Here, transistor T1 acts like a resistor to DC and as high impedance for audio signals. The high impedance
cuit is built around transistor T1 (BC547), resistors R2 and R3 (15 kiloohms and 100 ohms, respectively), condenser C3 (22µF, 63V) and two LEDs as indicators for both the primary and secondary sides. It provides proper DC characteristic to hold second telephone
of the circuit is provided by condenser C3, which prevents any audio signal from appearing at the base of T1. Thus any audio voltage appearing across telephone line No. 2 will not cause a corresponding current in the transistor. z
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ELECTRONICS FOR YOU • AUGUST 2005 • 81
CMYK
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BIPOLAR transistor TESTER
Raju R. Baddi
T
his tester is primarily meant to test bipolar transistors. It can indicate the type of the transistor as well as identify its base, collector and emitter pins. The circuit is very simple. The direction of current flow from the terminals of the transistor under test (TUT) is indicated by a pair of LEDs (green-red). An npn transistor produces a red-green-red glow, while a pnp tran-
sistor produces a green-red-green glow, depending on the test point that connects to the terminal of the transistor. Emitter and collector are differentiated by pressing pushbutton switch S1 that actually increases the supply voltage of the circuit by about 5.1V. At the heart of the circuit is IC CD4069 (IC3), which oscillates and produces pulses required to test a pair of transistor leads for conduction in both the directions. Different combina-
Fig. 1: Circuit of bipolar transistor tester
Fig. 2: Author’s prototype of bipolar transistor tester
tions are selected by an arrangement of counter CD4040 (IC1) and bilateral switch CD4016 (IC2). Fig. 1 shows the circuit of the bipolar transistor tester. A pair of LEDs is connected to each test point through which current flows in both the directions. Each LED corresponds to a particular direction. In this manner, both junctions of the transistor can be tested. The LEDs are arranged to indicate the type of the semiconductor across the p-n junction. The counter is clocked by the AC generator built around gates N5 and N6. This makes the LEDs glow
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continuously for easy observation, revealing the direction of current flow between different test points. So if the red LED connected to certain point glows, it means that n-type of the junction is connected to that test point, and vice versa. Thus a red-green-red glow indicates npn type of the transistor, while a green-red-green glow indicates a pnp transistor. From this observation, you can easily detect the base. Collector and emitter are differentiated based on the principle that the base-emitter junction breaks down under reverse bias much more easily than the base-collector junction. Thus under increased AC voltage, you can easily see that the emitter conducts more in the reverse direction (associated LED glows significantly) than the collector. Use of transparent or semi-transparent LEDs is recommended. Adjust preset VR1 (2-megaohm) to get equal glow when any two test points are shorted. Unregulated 15V-18V is regulated by the zener-transistor combination to power the circuit. The testing procedure is simple. Normally, the transistors can be plugged in any orientation as they come in a variety of possible arrangements of base, collector and emitter pins, such as CEB, BEC and CBE. Simply plug the TUT in the possible combinations of three points. A red-green-red glow means that it is npn transistor and the pin associated with green LED is base. To identify the emitter and collector, simply press switch S1 and observe green LEDs adjacent to already glowing red LEDs. The green LED glowing with a high intensity indicates the emitter side, while the low-intensity LED indicates w w w. e f y m ag . co m
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the collector side. Similarly, a green-red-green glow means that the transistor is pnp type and the pin associated with the red LED is the base. To identify the emitter and collector, simply press switch S1 and
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observe red LEDs associated with the already glowing green LEDs on the sides. The LED glowing with a high intensity indicates the emitter side, while the lowintensity LED indicates the collector side. Assemble the circuit on a general-
purpose PCB and enclose in a small box. Keep the preset knob in the middle. In order to make it easy to plug the TUT, you can increase the number of test points as shown in the author’s prototype in Fig. 2.
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AUTOMATIC-OFF TIMER FOR CD PLAYERS
SURESH KUMAR K.B.
A
re you in the habit of falling asleep while listening to music? If yes, you’ll love this circuit. It will automatically start functioning when you switch off your bedroom light and shall turn your CD player ‘off’ after a predetermined time. In the presence of ambient light, or when you switch on light of the room in the morning, the CD player will again start playing. Unlike the usual timers, you don’t have to set this timer before sleeping.
The circuit derives its power directly from the bridge rectifiers. When ‘on’/‘off’ switch S1 is closed, LED1 glows to indicate that the circuit is powered ‘on.’ In the presence of light, the resistance of the light-dependent resistor (LDR1) is low, so transistor T1 conducts to drive transistor T2 into cutoff state and the timer circuit remains inactive. The collector of transistor T2 is connected to reset pin 12 of IC CD4060 (IC1) via signal diode D5. IC CD4060 is a 14-stage ripple counter with a built-in oscillator. The time period of oscillations (t) is determined by capacitor C3 and resistor R8 connected to
EO SANI TH
pins 9 and 10 of IC1, respectively, as follows: t=2.3RC where ‘R’ is the value of resistor R8 and ‘C’ is the value of capacitor C3. When transistor T2 is cut-off, its collector voltage is high. So pin 12 of IC1 is high and IC1 is in reset condition. When light is switched off, the resistance of LDR1 increases, driving transistor T1 into cut-off state. The collector voltage of transistor T1 goes high to light up LED2 (indicating that the timer circuit is
During counting, in case the power fails momentarily, capacitor C2 (1000µF) will provide the necessary power backup for IC1. That is, during the period, pin 3 of IC1 is low. When output pin 3 of IC1 goes high, the relay is energised through transistors T3 and T4 and, at the same time, counting is disabled by the feedback from pins 3 through 11 (clock input) of IC1 via signal diode D7. That is, due to the feedback, output pin 3 remains high unless another high-to-low pulse is received at its reset pin 12.
LDR1 Timer LED2 Reset pin 12 Count LED3
After the relay is energised, there will be no AC power in the socket. The glowing of LED5 indicates that your CD player has been switched off. The desired ‘off’ time period for the timer circuit can be set by choosing proper values of resistor R8 and capacitor C3. If R8 is 680 kilo-ohms and C3 is 0.22 µF, the ‘off’ time period is around 45 minutes. The glowing of LED4 gives the warning that your CD player is going to be switched off shortly. In case you want to extend the timer setting for another round, just press reset switch S2 momentarily. LED4 stops glowing and counting starts again from the initial stage. z
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enabled) and transistor T2 starts conducting. As the collector voltage of transistor T2 goes low to around 0.2V, ground potential becomes available at reset pin 12 of IC1. The low state at pin 12 enables the oscillator and it starts counting. LED3 at pin 7 of IC1 starts blinking. Its blinking frequency depends on the R-C components connected between its pins 9 and 10. The status of LED2 and LED3 in the circuit with light falling and not falling on LDR1 is given below:
90 • DECEMBER 2005 • ELECTRONICS FOR YOU
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Telephone call recorder
Alizishaan Khatri
T
oday telephone has become an integral part of our lives. It is the most widely used communication device in the world. Owing to its immense popularity and
edge of standard telephone wiring and a stereo plug. In India, landline telephones primarily use RJ11 wiring, which has two wires—tip and ring. While tip is the positive wire, ring is the negative one. And together they complete the
Fig. 1: Call recorder circuit
Fig. 2: Pin configuration of stereo jack
Fig. 3: RJ connector
widespread use, there arises a need for call recording devices, which find application in call centres, stock broking firms, police, offices, homes, etc. Here we are describing a call recorder that uses very few components. But in order to understand its working, one must first have the basic knowl-
telephone circuit. In a telephone line, voltage between tip and ring is around 48V DC when handset is on the cradle (idle line). In order to ring the phone for an incoming call, a 20Hz AC current of around 90V is superimposed over the DC voltage already present in the idle line. The negative wire from the phone line goes to IN1, while the positive wire goes to IN2. Further, the negative wire from OUT1 and the positive wire from OUT2 are connected to the phone. All the resistors used are 0.25W carbon film resistors and all the capacitors used are rated for 250V or more. The negative terminal of ‘To AUX IN’ is connected to pin 1 of the stereo jack while the positive terminal is connected to pins 2 and 3 of the stereo jack. This stereo jack, in turn, is connected to the AUX IN of any recording device, such as computer, audio cas-
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sette player, CD player, DVD player, etc. Here we shall be connecting it to a computer. When a call comes in, around 90V AC current at 20Hz is superimposed over the DC voltage already present in the idle line. This current is converted into DC by the diodes and fed to resistor R1, which reduces its magnitude and feeds it to LED1. The current is further reduced in magnitude by the resistor R2 and fed to the right and left channels of the stereo jack, which are connected to the AUX IN port of a computer. Any audio recording software, such as AVS audio recorder (available at: http://www.avs4you.com/AVSAudio-Recorder.aspx), Audacity audio recorder (http://audacity.sourceforge. net/), or audio recorder (http://www. audio-tool.net/audio_recorder_for _free.html), can be used to record the call. When a call comes in, one needs to launch the audio recording software and start recording. For phone recording, simply connect the stereo jack to the AUX IN port of the PC. Install the audacity audio recorder (different versions are available for free for different operating systems at http://audacity. sourceforge.net/) on your PC. Run the executable audacity file. In the main window, you will find a dropdown box in the top right corner. From this box, select the AUX option. Now you are ready to record any call. As soon as a call comes in, press the record button found in the audacity main window and then pick up the telephone receiver and answer the call. Press the stop button once the call ends. Now go to the file menu and select the ‘Export as WAV’ option and save the file in a desired location. You may change the value of resisw w w. e f y m ag . co m
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tor R2 if you want to change the output volume. you can use a variable resistor in series with R2 to vary the volume of the output. The recorded audio clip can be edited using different options in the audacity software. You can assemble the circuit on a
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general-purpose PCB and enclose it in a small cabinet. Use an RJ11 connector and stereo jack for connecting the telephone set and computer (for call recording). Telephone cords can be used to connect to the phone line and the circuit. Use of a shielded cable is
recommended to reduce disturbances in the recording. These can also be reduced by increasing the value of R2 to about 15 kilo-ohms. EFY note. Audacity recording software is included in this month’s EFY-CD under ‘utilities’ section.
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ELECTRONIC HORN
ASHOK K. DOCTOR
IVEDI S.C. DW
ere’s a simple circuit of an electronic horn that is built around quadruple op-amp
IC LM3900 (IC1). IC LM3900 has four independent op-amps (A1 through A4) with a large output voltage swing. It can work at up to 32V DC. The first op-amp (A1) is wired as
a low-frequency squarewave generator. Op-amp A2 works as an integrator, while op-amp A3 works as a
comparator. A2 and A3 together work as a ‘wandering voltage generator’ op-amp. Op-amp A4 is wired as a buffer and its output provides base current to npn transistor T2. npn transistor T2 and audio output transformer X1 form a voltage-controlled oscillator.
When power is switched on, a basic tone is generated by transistor T2 and transformer X1, which is frequency-modulated by the wandering voltage generator, which, in turn, is influenced by the low-frequency squarewave generator. The circuit works off regulated
9V. To generate several different tones, connect its point A1 to pins 1, 3, 4, 5, 8, 9, 10, 11, 12 and 13 of IC1 and point A2 to pins 1, 2, 3, 6, 8, 11 and 13. The circuit can be used as an automobile horn by using about 10W audio amplifier.
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ELECTRONICS FOR YOU • JANUARY 2007 • 109
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liquid level Alarm
Lovely T.P.
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ere is a simple circuit for liquid level alarm. It is built around two BC547 transistors (T1 and T2) and two timer 555 ICs (IC1 and IC2). Both IC1 and IC2 are wired in astable multivibrator mode. Timer IC1 produces low frequency, while timer IC2 produces high frequency. As a result, a beeping tone is generated when the liquid tank is full. Initially, when the tank is empty, transistor T1 does not conduct. Consequently, transistor T2 conducts and
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pin 4 of IC1 is low. This low voltage disables IC1 and it does not oscillate. The low output of IC1 disables IC2 and it does not oscillate. As a result, no sound is heard from the speaker. But when the tank gets filled up, transistor T1 conducts. Consequently, transistor T2 is cut off and pin 4 of IC1 becomes high. This high voltage enables IC1 and it oscillates to produce low frequencies at pin 3. This low-frequency output enables IC2 and it also oscillates to produce high frequencies. As a result, sound is produced from the speaker. Using preset VR1 you can
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control the volume of the sound from the speaker. The circuit can be powered from a 9V battery or from mains by using a 9V power adaptor. Assemble the circuit on a generalpurpose PCB and enclose in a suitable cabinet. Install two water-level probes using metal strips such that one touches the bottom of the tank and the other touches the maximum level of the water in the tank. Interconnect the sensor and the circuit using a flexible wire.
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MULTI-MELODY GENERATOR WITH INSTRUMENTAL EFFECT
EFY LAB
T
his melody generator can generate various English and Hindi tunes as also instrumental effects. Various modes of melodies can
programmed data. Its inbuilt preamplifier provides a simple interface to the driver circuit. The IC can be replaced with other UM348XXX series, WR630173 or WE4822 melody generator ICs. A WR630173 preprogrammed
as Hindi melody generator can be used here. There are 16 tunes stored in S3 S4 S5 Mode of tone output WR630173 including Opened Opened Closed The same tone keeps repeating. Press mera joota hai japani, S2 for the next tone. mera naam chin chin Opened Closed Closed Play a new tone for every pressing chu, hare rama hare of S2. krishna, raghu pati Closed Opened Closed Play all the tones one by one and raghav raja ram and then repeat the cycle without S2 input. Closed Closed Closed Play all the tones and stop (no repetition). ramaiya vasta vaiya. The circuit is powbe selected through DIP switches. ered by a 3V battery. Switch S2 is the Other advantages are high volume and main input-select switch for producvolume control. ing different tones in the loudspeaker. IC UM3481A is a 16-pin multi-inVarious modes of operation are sestrument melody generator. It is a lected through DIP switches S3, S4 and mask-ROM-programmed IC designed S5 connected to pins 3, 5 and 7 of IC1, to play the melody according to the respectively. Pin 7 is the envelope cir-
Positions of DIP Switches for Various Modes
88 • JUNE 2006 • ELECTRONICS FOR YOU
EO SANI TH
cuit terminal through which instrumental effects are produced. The preamplifier outputs are available at pins 10 and 11, which are fed to loudspeaker-driver transistors T1 (SK100) and T2 (SL100), respectively. When you switch on the circuit by closing switch S1, LED1 glows. If DIP switches S3 and S5 are closed and S4 open, pressing input switch S2 will generate a melody tone from the loudspeaker. Vary VR1 to adjust its volume. Pressing S2 again will generate a new melody tone. If switches S3 and S4 are opened while S5 is closed, the same tone keeps repeating for every pressing of S2. The positions of DIP switches and the various modes of melodies are summarised in the table. When switch S5 is open, it will generate an instrumental effect from the loudspeaker. This effect is produced by the enveloping circuit consisting of capacitor C1 and resistor R2 connected to pin 7 of IC1. In fact, by hit and trial you can choose the values of these components as per your taste by listening to the output sound. Only C1 or R2 or its parallel combination can be used to generate a distinct instrument effect. To select any of these options, two jumper terminals J1 and J2 are provided in the circuit at C1 and R2, respectively. For example, if you want to use only C1, you can join J1 terminals using hookup wire or jumper cap and keep J2 open. The repetition of the musical effect depends on the status of switches S3 and S4. The oscillation frequency is produced by the resistor and capacitor connected at pins 14 and 13 of IC1. This frequency is used as a time base for the tone, rhythm and tempo generators. The quality of the melody tones depends on this frequency. Resistor R6 (100-kilo-ohm) connected to pin 15 makes the circuit insensitive to variations in the power supply. WWW.EFYMAG.COM
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PICNIC LAMP
D. MOHAN KUMAR
Y
ou can take this white LEDbased night lamp on your picnic outings. The lamp has sound trigger and push-to-on facilities and gives ample light during a walk at night. It will also prove useful in locating the door of your tent in the darkness. A click of the fingers will switch on the lamp for three minutes to help you in a strange place. The circuit uses low-power ICs to save the battery power. JFET op-amp TL071 (IC1) amplifies the sound picked up by the condenser microphone. Resistor R1 and low-value capacitor C1 (0.22µF) make the amplifier insensitive to very lowfrequency sounds, eliminating the chance of false triggering. VR1 is used to adjust the sensitivity of the microphone and VR2 adjusts the gain of IC1. The amplified output from IC1 is coupled to trigger pin 2 of IC2, which
IVEDI S.C. DW
is a monostable multivibrator built around low-power CMOS timer IC 7555. Resistor R4 keeps trigger input pin 2 of the monostable normally high in the absence of the trigger input. Timing elements R6 and C4 give a time
white LED (LED1) through ballast resistor R7. The circuit can be easily assembled on a perforated board. Make the circuit assembly as compact as possible to enclose in a small case. Use three
delay of three minutes. Reset pin 4 of IC2 is connected to the positive rail through R5 and to the negative rail through C2 to provide power-on-reset function. The output of IC2 powers the
1.5V pen-light cells to power the circuit. Adjust VR1 and VR2 suitably to get sufficient sensitivity of IC1. Toggle switch S1 can be used to switch on the lamp like a torch. z
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ELECTRONICS FOR YOU • MARCH 2006 • 101
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VISUAL AC MAINS VOLTAGE INDICATOR
RAJ K. GORKHALI
Y
ou should not be surprised if someone tells you that the mains voltage fluctuation could be anywhere from 160 volts to 270 volts. Although majority of our electrical and electronics appliances have some kind of voltage stabilisation internally built-in, more than 90 per cent of the faults in these appliances occur due to these power fluctuations. This simple test gadget gives visual indication of AC mains voltage from 160 volts to 270 volts in steps of 10 volts. There are twelve LEDs numbered LED1 to LED12 to indicate the voltage level. For input AC mains voltage of less than 160 volts, all the LEDs remain off. LED1 glows when the voltage
reaches 160 volts, LED2 glows when the voltage reaches 170 volts and so on. The number of LEDs that glow keeps increasing with every additional 10 volts. When the input voltage reaches 270 volts, all the LEDs glow. The circuit basically comprises three LM339 comparators (IC1, IC2 and IC3) and a 12V regulator (IC4). It is powered by regulated 12V DC. For power supply, mains 230V AC is stepped down to 15V AC by stepdown transformer X1, rectified by a bridge rectifier comprising diodes D1 through D4, filtered by capacitor C4 and regulated by IC4. The input voltage of the regulator is also fed to the inverting inputs of gates N1 through N12 for controlling the level of the AC. The LED-based display circuit is
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built around quad op-amp comparators IC1 through IC3. The inverting input of all the comparators is fed with the unregulated DC voltage, which is proportional to mains input, whereas the non-inverting inputs are derived from regulated output of IC4 through a series network of precision resistors to serve as reference DC voltages. Resistors R13 to R25 are chosen such that the reference voltage at points 1 to 12 is 0.93V, 1.87V, 2.80V, 3.73V, 4.67V, 5.60V, 6.53V, 7.46V, 8.40V, 9.33V, 10.27V and 11.20V, respectively. When the input voltage varies from 160V AC to 270V AC, the DC voltage at the anode of ZD1 also varies accordingly. With input voltage varying from 160V to 270V, the output across filter capacitors C1 and C2 varies from 14.3V to 24.1V approxi-
ELECTRONICS FOR YOU • MAY 2006 • 89
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mately. Zener ZD1 is used to drop fixed 12V and apply proportional voltages to all comparator stages (inverting pins). Whenever the voltage at the non-inverting input of the comparators goes high, the LED connected at the output glows. Assemble the circuit on a general-
purpose PCB such that all the LEDs make a bargraph. In the bargraph, mark LED1 for minimum level of 160V, then LED2 for 170V and so on. Finally, mark LED12 for maximum level of 270V. Now your test gadget is ready to use. For measuring the AC voltage,
90 • MAY 2006 • ELECTRONICS FOR YOU
simply plug the gadget into the mains AC measuring point, press switch S1 and observe the bargraph built around LEDs. Let’s assume that LED1 through LED6 glow. The measured voltage in this case is 220V. Similarly, if all the LEDs glow, it means that the voltage is more than 270V.
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HEAT-SENSITIVE SWITCH M.K. CHANDRA MOULEESWARAN AND MISS KALAI PRIYA
IVEDI S.C. DW
t the heart of this heat-sensitive switch is IC LM35 (IC1), which is a linear temperature sensor and linear temperature-to-volt-
A
rails via a voltage divider network formed by potmeter VR1. Since the wiper of potmeter VR1 is connected to the inverting input of IC2, the voltage presented to this pin is linearly variable. This voltage is used as the reference level for the comparator against the output supplied by IC1.
at the inverting input of IC2. So the load is turned on as soon as the ambient temperature rises above the set level. Capacitor C3 at this pin helps iron out any ripple that passes through the positive supply rail to avoid errors in the circuit operation. By adjusting potmeter VR1 and
age converter circuit. The converter provides accurately linear and directly proportional output signal in millivolts over the temperature range of 0°C to 155°C. It develops an output voltage of 10 mV per degree centigrade change in the ambient temperature. Therefore the output voltage varies from 0 mV at 0°C to 1V at 100°C and any voltage measurement circuit connected across the output pins can read the temperature directly. The input and ground pins of this heat-to-voltage converter IC are connected across the regulated power supply rails and decoupled by R1 and C1. Its temperature-tracking output is applied to the non-inverting input (pin 3) of the comparator built around IC2. The inverting input (pin 2) of IC2 is connected across the positive supply
So if the non-inverting input of IC2 receives a voltage lower than the set level, its output goes low (approximately 650 mV). This low level is applied to the input of the load-relay driver comprising npn transistors T1 and T2. The low level presented at the base of transistor T1 keeps it nonconductive. Since T2 receives the forward bias voltage via the emitter of T1, it is also kept non-conductive. Hence, relay RL1 is in de-energised state, keeping mains supply to the load ‘off’ as long as the temperature at the sensor is low. Conversely, if the non-inverting input receives a voltage higher than the set level, its output goes high (approximately 2200 mV) and the load is turned ‘on.’ This happens when IC1 is at a higher temperature and its output voltage is also higher than the set level
thereby varying the reference voltage level at the inverting input pin of IC1, the temperature threshold at which energisation of the relay is required can be set. As this setting is linear, the knob of potmeter VR1 can be provided with a linear dial caliberated in degrees centigrade. Therefore any temperature level can be selected and constantly monitored for external actions like turning on a room heater in winter or a room cooler in summer. The circuit can also be used to activate emergency fire extinguishers, if positioned at the probable fire accident site. The circuit can be modified to operate any electrical appliance. In that case, relay RL1 must be a heavy-duty type with appropriately rated contacts to match the power demands of the load to be operated. z
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ELECTRONICS FOR YOU • NOVEMBER 2005 • 99
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VARIABLE POWER SUPPLY USING A FIXED-VOLTAGE REGULATOR IC
DR MAHESH N. JIVANI AND DR NIKESH A. SHAH
A
voltage regulator (also called a ‘regulator’) with only three terminals appears to be a simple device, but it is in fact a very complex integrated circuit. It converts a varying input voltage into a constant ‘regulated’ output voltage. Voltage regulators are available in a variety of outputs like 5V, 6V, 9V, 12V and 15V. The LM78XX series of voltage regulators are designed for positive input. For applications requiring negative input, the LM79XX series is used. Fig. 1 shows the pin configuration of a 5V 7805 regulator. The output voltage of a regulator circuit can be increased
Fig. 1: Pin configuration of 7805 regulator
Fig. 2: Circuit for increasing
by using a pair of ‘voltage-divider’ resistors. It is not possible to obtain a voltage lower than the stated rating. You cannot use a 12V regulator to make a 5V power supply, but you can use a 5V regulator to make a 12V supply. Voltage regulators are very robust. These can withstand over-current draw due to short circuits and also over-heating. In both cases, the regulator will cut off before any damage occurs. The only way to destroy a regulator is to apply reverse voltage to its input. Reverse polarity destroys the regulator almost instantly. Fig. 2 shows the circuit for increasing the output voltage of a regulator circuit using a pair of voltage-divider resistors. Let’s assume the value of R1 as 470 ohms, which means that a constant current of 10.6 mA will be available between terminals 2 and 3 of 7805. This constant current plus the regulator standby current of about 2.5 mA will flow through R2 to ground regardless of its value. the output voltage
Common Resistor Combinations for the 7805 Regulator Vout (approx.)
R1 (ohms)
R2 (ohms)
5V 6V 8V 9V 12V
470 470 470 470 470
0 100 220 330 510
Because of this constant 13.1mA current, R2 can now be set to a value that will give constant 7 volts across resistor R2. A resistor value of 533 or 510 ohms (standard value) will give the necessary 7 volts. With 5 volts across R1 and 7 volts across R2, a total of about 12 volts (regulated) will appear across terminal 2 and ground. If a variable resistor is used as R2, the output voltage can be easily fine-tuned to any value greater than 5 volts. The standby current will vary slightly in the regulator 7805, but 2.5 mA will yield good results in the calculations. If an exact voltage (within 0.3 volt) is needed, R2 must be a variable resistor. To make any fixed regulator adjustable, use the following formula: Vout= Vfixed+R2
Fig. 3: Circuit of variable power supply using a 5V regulator
96 • NOVEMBER 2006 • ELECTRONICS FOR YOU
IVEDI S.C. DW
Vfixed R1
+Istandby
where Vout is the desired output voltage, Vfixed is the fixed voltage of the IC regulator (5 volts) and Istandby is the standby current of the regulator (2.5 mA). For resistor R1, use any value from 470 ohms to 1 kilo-ohm for best results. For variable resistor R2, put any value from the table given here for desired voltage operation. Fig. 3 shows the circuit of a 6V12V variable power supply using a 5V regulator. The 220V AC mains voltage is stepped down by transformer X1 to 9 volts, rectified by WWW.EFYMAG.COM
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the bridge rectifier comprising 1N4007 diodes D1 through D4, filtered by smoothing capacitors C1 and C2, and regulated by IC 7805 (IC1). Capacitors C1 and C2 help to maintain a constant input to the regulator. Capacitor C1 should be rated at a minimum of 1000 µF for each ampere
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of current drawn and at least twice the input voltage. Wire the 270nF or greater disk (ceramic) capacitor close to the input terminal of the IC, and a 10µF or greater electrolytic capacitor across the output. The regulator ICs typically give 60 dB of ripple rejection, so 1V of input ripple appears as
a mere 1 mV of ripple in the regulated output. Attach the 5-way rotary switch to resistors of different values to get the regulated output as shown in the table. Or, you can use a 1-kilo-ohm potmeter as a variable resistor to get the regulated 5V-12V output.
ELECTRONICS FOR YOU • NOVEMBER 2006 • 97
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Mini UPS System
edi
s.c. dwiv
his circuit provides an uninterrupted power supply (UPS) to operate 12V, 9V and 5V DC-powered instruments at up to 1A current. The backup battery takes up the load without spikes or delay when the mains power gets interrupted. It can also be used as a workbench power supply that provides 12V, 9V and 5V operating voltages. The circuit im-
When the mains power fails, diode D3 gets reverse biased and D4 gets forward biased so that the battery can automatically take up the load without any delay. When the battery voltage or input voltage falls below 10.5V, a cut-off circuit is used to prevent deep discharging of the battery. Resistor R3, zener diode ZD1 (10.5V) and transistor T2 form the cut-off circuit. When the voltage level is above 10.5V, transistor
vary between 10.5V and 12V, when the UPS system is in battery mode. Outputs at points B and C provide 9V and 5V, respectively, through regulator ICs (IC1 and IC2), while output A provides 12V through the zener diode. The emergency lamp uses two ultra-bright white LEDs (LED2 and LED3) with current limiting resistors R5 and R6. The lamp can be manually
mediately disconnects the load when the battery voltage reduces to 10.5V to prevent deep discharge of the battery. LED1 indication is provided to show the full charge voltage level of the battery. miniature white LEDs (LED2 and LED3) are used as emergency lamps during power failure at night. A standard step-down transformer provides 12V of AC, which is rectified by diodes D1 and D2. Capacitor C1 provides ripple-free DC to charge the battery and to the remaining circuit. When the mains power is on, diode D3 gets forward biased to charge the battery. Resistor R1 limits the charging current. Potentiometer VR1 (10k) with transistor T1 acts as the voltage comparator to indicate the voltage level. VR1 is so adjusted that LED1 is in the ‘off’ mode. when the battery is fully charged, LED1 glows indicating a full voltage level of 12V.
T2 conducts and its base becomes negative (as set by R3, VR2 and ZD1). But when the voltage reduces below 10.5V, the zener diode stops conduction and the base voltage of transistor T2 becomes positive. It goes into the ‘cut-off’ mode and prevents the current in the output stage. Preset VR2 (22k) adjusts the voltage below 0.6V to make T2 work if the voltage is above 10.5V. When power from the mains is available, all output voltages—12V, 9V and 5V—are ready to run the load. On the other hand, when the mains power is down, output voltages can run the load only when the battery is fully charged (as indicated by LED1). For the partially charged battery, only 9V and 5V are available. Also, no output is available when the voltage goes below 10.5V. If battery voltage varies between 10.5V and 13V, output at terminal A may also
switched ‘on’ and ‘off’ by S1. the circuit is assembled on a general-purpose PCB. There is adequate space between the components to avoid overlapping. heat sinks for transistor T2 and regulator ICs (7809 and 7805) to dissipate heat are used. The positive and negative rails should be strong enough to handle high current. Before connecting the circuit to the battery and transformer, connect it to a variable power supply. Provide 12V DC and adjust VR1 till LED1 glows. After setting the high voltage level, reduce the voltage to 10.5V and adjust VR2 till the output trips off. After the settings are complete, remove the variable power supply and connect a fully-charged battery to the terminals and see that LED1 is on. After making all the adjustments connect the circuit to the battery and transformer. The battery used in the circuit is a 12V, 4.5Ah UPS battery.
D. Mohan Kumar
T
9 4 • N o v e m b e r 2 0 0 9 • e l e c t ro n i c s f o r yo u
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DIGITAL DICE
SAGAR G. YADAV
T
he digital dice presented here acts just like a normal dice. It has six faces (refer Fig. 2) like the normal dice and uses four different logic gate combinations to bring out the six faces of the dice. At the heart of the circuit is a 14stage ripple-carry binary counter IC CD4060BC (IC1) with built-in oscilla-
IVEDI S.C. DW
The circuit is divided into three sections: counter, logic and display. The counter section is built around binary counter IC CD4060BC (IC1). The counter frequency (f) is decided by the in-built oscillator formed by resistor R1 and capacitor C1 as follows: f=1/2.2R1C1. Here, the frequency is fixed at around 2056 Hz. Only the first three outputs of the
LED2 and LED5 always glow at the same count, as do LED1 and LED6, and LED3 and LED4. Using these three pairs of LEDs and LED7, four logical combinations have been made in the circuit. LED1 and LED6 glow at all counts, except ‘0’ and ‘1.’ Further, it can be noticed that they glow when ‘A’ or ‘B’ is high, hence a NOR gate whose output is A+B according to Boolean algebra will perform the job
counter (designated as A, B and C, respectively) have been used in the circuit. The counter is designed to reset at the sixth count (110) as only six counts are required for operation. This is done with the help of diodes D1 and D2 and resistor R3, which are connected such that they generate an AND logic. From the table it can be noticed that at the sixth count, the counter outputs A and B hold logic 1 simultaneously for the first time, so by ANDing A and B outputs you can give logic 1 to the reset terminal of the counter at the sixth count, thereby resetting the counter.
of operating these LEDs. LED3 and LED4 glow at all counts, except for the first three counts, i.e., they glow when either A, or B and C are high. This logic function can be obtained by using an OR gate and an AND gate, but since we are using only NAND and NOR gates in the circuit we make use of two NAND gates and a NOR gate (with A+BC output) to perform this function. LED2 and LED5 glow only at the first and fifth counts. In other words, they glow only when the complement of B and C outputs goes high. This function can be obtained by using two NAND gates such that their output corresponds to the Boolean expression BC or B+C according to De Morgan’s theorem.
Fig. 1: Digital dice
Fig. 2: Different faces of dice
tor. The logic section is designed around CMOS quad 2-input NOR gate IC CD4001BC (IC2) and quad 2-input NAND gate IC CD4011BC (IC3). The display section is formed by a group of seven LEDs.
100 • OCTOBER 2005 • ELECTRONICS FOR YOU
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Dice Score and LEDs Lit at Different Counts Count Dice score 0 1 1 2 2 3 3 4 4 5 5 6 6 —
A
B
C
0 0 0 0 1 1 1
0 0 1 1 0 0 1
0 1 0 1 0 1 0
LEDs lit — — LED1 LED1 LED1 LED1 —
— — — LED3 LED3 LED3 —
LED7 glows at even counts like 0, 2 and 4. In other words, it glows when the C output is low. This function can be achieved easily by inverting the C output twice using the remaining two NAND gates. The output will also be buffered by these two inverter gates. The display section comprises seven LEDs. LED1 and LED6 have
— LED2 — — — LED2 —
LE D7 — LED7 — LED7 — —
LED5 — — — LED5 —
— LED4 LED4 LED4 —
LED6 LED6 LED6 LED6 —
common cathodes, as do LED2 and LED5, and LED3 and LED4. The anodes of all the LEDs are tied together to the positive terminal of the battery via resistors R4 through R10, respectively. When you place your finger on the touch pad, the oscillator starts oscillating. The counter will start counting at the rate of 2056 Hz and all the
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LEDs of the display section will appear to glow simultaneously due to the high counter frequency. This high-frequency counting will make the dice foolproof. When you remove your finger from the touch pad, the counter will stop counting and the display section will show any one of the six possible faces with a probability of 1/6. The entire circuit can be powered by a 9V battery as the inbuilt oscillator of the counter IC will not work properly below 7V. Use of CMOS ICs means less power consumption. The circuit can be constructed on a general-purpose PCB and housed inside a plastic case with the LEDs array mounted on the top as shown in Fig. 2. The touch pad can be mounted beside the array. z
ELECTRONICS FOR YOU • OCTOBER 2005 • 101
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Mobile Car Stereo Player
T.K. Hareendran
U
sing a mobile phone while driving is dangerous. It is also against the law. However, you can use your mobile phone as a
Fig. 1: Circuit of mobile car stereo player
Fig. 2: (a) 3.5mm stereo socket and (b) 3.5mm stereo jack
Fig. 3: Proposed enclosure
powerful music player with the help of a stereo power amplifier. This does away with the need of a sophisticated in-dash car music system. Most mobile phones have a music player that offers a number of features including preset/manual sound equalisers. They have standard 3.5mm stereo sockets that allow music to be played through standard stereo headphones/ sound amplifiers. Nokia 2700 classic is an example. A car audio amplifier with 3.5mm socket can be designed and simply connected to the mobile phone output via a shielded cable with suitable connectors/ jacks (readymade 3.5mm male-tomale connector cable is a good alternative). Fig. 1 shows the circuit of car stereo player. It is built around popular single-chip audio power amplifier TDA1554Q (IC1). The TDA1554Q is an integrated class-B power amplifier in a 17-lead single-in-line (SIL) plastic power package. IC TDA1554Q contains four 11W identical amplifiers with differential input stages (two inverting and two
1 1 8 • O c to b e r 2 0 1 0 • e l e c t ro n i c s f o r yo u
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non-inverting) and can be used for single-ended or bridge applications. The gain of each amplifier is fixed at 20 dB. Here it is configured as two 22W stereo bridge amplifiers. The amplifier is powered from the 12V car battery through RCA socket J2. Diode D1 protects against wrongpolarity connection. LED1 indicates the power status. Connect stereo sound signal from the 3.5mm headset socket of the mobile phone to audio input socket J1. When you play the music from your mobile, IC1 amplifies the input. The output of IC1 is fed to speakers LS1 and LS2 fitted at a suitable place in your car. Electrolytic capacitor C5 connected between pin 4 of IC1 and GND improves the supply-voltage ripple rejection. Components R2 and C4 connected at mute/standby pin (pin 14) of IC1 eliminate the switch on/off plop. The circuit is quite compact. A good-quality heat-sink assembly is crucial for IC1. Fig. 2 shows the stereo socket and stereo jack. Assemble the circuit on a generalpurpose PCB and enclose in a suitable cabinet. Small dimensions of the power amplifier make it suitable for being enclosed in a plastic (ABS) case with vent holes. Signal input socket, speaker output terminals, on/off switch, indicator, fuse holder and power supply socket are best located on the front panel of the enclosure as shown in Fig. 3.
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FRIENDLY CHARGER FOR MOBILE PHONES
D. MOHAN KUMAR
ost mobile chargers do not have current/voltage regulation or short-circuit protection. These chargers provide raw 6-12V DC for charging the battery pack. Most of the mobile phone battery packs have a rating of 3.6V, 650
and the mobile phone. It has features like voltage and current regulation, over-current protection, and high- and low-voltage cut-off. An added speciality of the circuit is that it incorporates a short delay of ten seconds to switch on when mains resumes following a power failure. This protects the mobile phone from instant volt-
mAh. For increasing the life of the battery, slow charging at low current is advisable. Six to ten hours of charging at 150-200mA current is a suitable option. This will prevent heating up of the battery and extend its life. The circuit described here provides around 180mA current at 5.6V and protects the mobile phone from unexpected voltage fluctuations that develop on the mains line. So the charger can be left ‘on’ over night to replenish the battery charge. The circuit protects the mobile phone as well as the charger by immediately disconnecting the output when it senses a voltage surge or a short circuit in the battery pack or connector. It can be called a ‘middle man’ between the existing charger
age spikes. The circuit is designed for use in conjunction with a 12V, 500mA adaptor (battery eliminator). Op-amp IC CA3130 is used as a voltage comparator. It is a BiMOS operational amplifier with MOSFET input and CMOS output. Inbuilt gate-protected p-channel MOSFETs are used in the input to provide very high input impedance. The output voltage can swing to either positive or negative (here, ground) side. The inverting input (pin 2) of IC1 is provided with a variable voltage obtained through the wiper of potmeter VR1. The non-inverting input (pin 3) of IC1 is connected to 12V stabilised DC voltage developed across zener ZD1. This makes the output of IC1 high.
M
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After a power resumption, capacitor C1 provides delay of a few seconds to charge to a potential higher than of inverting pin 2 of CA3130, thus the output of IC1 goes high only after the delay. In the case of a heavy power line surge, zener diode ZD1 (12V, 1W) will breakdown and short pin 3 of IC1 to ground and the output of IC1 drops to ground level. The output of IC1 is fed to the base of npn Darlington transistor BD677 (T2) for charging the battery. Transistor T2 conducts only when the output of IC1 is high. During conduction the emitter voltage of T2 is around 10V, which passes through R6 to restrict the charging current to around 180 mA. Zener diode ZD2 regulates the charging voltage to around 5.6V. When a short-circuit occurs at the battery terminal, resistor R8 senses the over-current, allowing transistor T1 to conduct and light up LED1. Glowing of LED2 indicates the charging mode, while LED1 indicates shortcircuit or over-current status. The value of resistor R8 is important to get the desired current level to operate the cut-off. With the given value of R8 (3.3 ohms), it is 350 mA. Charging current can also be changed by increasing or decreasing the value of R7 using the ‘I=V/R’ rule. Construct the circuit on a common PCB and house in a small plastic case. Connect the circuit between the output lines of the charger and the input pins of the mobile phone with correct polarity.
ELECTRONICS FOR YOU • SEPTEMBER 2006 • 97
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Motion Sensor for Security Light
George Varkey
H
ere is a system based on PIR motion detector module BS1600 (or BS1700) that
The working of the circuit is simple. When you power-on the circuit after assembling all the components including the CFL, the CFL will glow for 10 seconds, turn off for 30 seconds,
Fig. 1: Circuit of motion sensor for security light
can be used for security or corridor lighting in powersaving mode. Fig. 2: PIR motion detector The 12V DC module (BS1600 or BS1700) power supply required for the motion detector and the relay driver is derived from 230V, 50Hz mains using a transformerless circuit as shown in Fig. 1.
glow for 10 seconds and then turn off. Now the circuit is ready to work. When any movement is detected, around 3.3V appears on the base of relay-driver transistor T1 and it conducts to energise relay RL1. As a result, Triac1 (BT136) fires to provide full 230V and light up the CFL. Another normally-opened contact of the relay (N/O2) is used here to hold the output until reset. If the switch is not in ‘hold’ position, the light
edi
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will remain ‘on’ for about ten seconds (as programmed in the motion sensor). In short, when there is a movement near the sensor, the CFL glows for about ten seconds. It will remain ‘on’ if switch S1 is in ‘hold’ position. Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. Use a three-pin connector for connecting the PIR sensor in the circuit with correct polarity. The motion detector is embedded onto the transparent cover of the light assembly as shown in Fig. 2 An arrangment of CFL assembly in the author’s prototype (Fig. 3) is shown in Fig. 4. In this arrangement, a PIR sensor and 23W, 230V AC CFL are used. Seal all four sides with Blue Tac for water-tightness. Insulate the track side of the PCB using an insulating foam and glue to the base. Waterjet cutting 23×23 mm
watertight fitting using blue tac
TERMINAL BLOCK
PCB INSULATED FROM LAMP FITTING
Fig. 3: Author’s prototype
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Fig. 4: CFL assembly
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CIRCUIT IDEAS
ANTI-BAG-SNATCHING ALARM D. MOHAN KUMAR
SAN
I THE
O
ere is a simple alarm circuit to thwart snatching of your valuables while travelling. The circuit kept in your bag or suitcase sounds a loud alarm, simulating a police horn, if some-
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the voltage at the non-inverting input is higher than at the inverting input and the output of IC1 is high. The output from pin 6 of IC1 is fed to trigger pin 2 of IC NE555 (IC2) via coupling capacitor C1 (0.0047 µF). IC2 is configured as a monostable. Its trigger pin 2 is held
(IC3) gets the supply voltage at its pin 5. IC UM3561 is a complex ROM with an inbuilt oscillator. Resistor R8 forms the oscillator component. Its output is fed to the base of single-stage transistor amplifier BD139 (T1) through resistor R9 (1 kilo-ohm).
one attempts to snatch your bag or suitcase. This will draw the attention of other passengers and the burglar can be caught red handed. In the standby mode, the circuit is locked by a plug and socket arrangement (a mono plug with shorted leads plugged into the mono-jack socket of the unit). When the burglar tries to snatch the bag, the plug detaches from the unit’s socket to activate the alarm. The circuit is designed around op-amp IC CA3140 (IC1), which is configured as a comparator. The non-inverting input (pin 3) of IC1 is kept at half the supply voltage (around 4.5V) by the potential divider comprising resistors R2 and R3 of 100 kilo-ohms each. The inverting input (pin 2) of IC1 is kept low through the shorted plug at the socket. As a result,
high by resistor R4 (10 kilo-ohms). Normally, the output of IC2 remains low and the alarm is off. Resistor R6, along with capacitor C3 connected to reset pin 4 of IC2, prevents any false triggering. Resistor R5 (10 mega-ohms), preset VR (10 megaohms) and capacitor C2 (4.7 µF, 16V) are timing components. With these values, the output at pin 3 of IC2 is about one minute, which can be increased by increasing either the value of capacitor C2 or preset VR. When there is an attempt at snatching, the plug connected to the circuit detaches. At that moment, the voltage at the inverting input of IC1 exceeds the voltage at the non-inverting input and subsequently its output goes low. This sends a low pulse to trigger pin 2 of IC2 to make its output pin 3 high. Consequently, the alarm circuit built around IC UM3561
The alarm tone generated from IC3 is amplified by transistor T1. A loudspeaker is connected to the collector of T1 to produce the alarm. The alarm can be put off if the plug is inserted into the socket again. Transistor T1 requires a heat-sink. Resistor R7 (330 ohms) limits the current to IC3 and zener diode ZD1 limits the supply voltage to IC3 to a safe level of 3.3 volts. Resistor R9 limits the current to the base of T1. The circuit can be easily constructed on a vero board or general-purpose PCB. Use a small case for housing the circuit and 9V battery. The speaker should be small so as to make the gadget handy. Connect a thin plastic wire to the plug and secure it in your hand or tie up somewhere else so that when the bag is pulled, the plug detaches from the socket easily.
FEBRUARY 2004
ELECTRONICS FOR YOU
CIRCUIT IDEAS
DC MOTOR CONTROL USING A SINGLE SWITCH V. DAVID
EDI DWIV S.C.
his simple circuit lets you run a DC motor in clockwise or anti-clockwise direction and stop it using a single switch. It provides a constant voltage for proper operation of the motor. The glowing of LED1 through LED3 indicates that the motor is in stop, forward rotation and
When you momentarily press switch S1, timer 555 (IC1) provides a pulse to decade counter CD4017 (IC2), which advances its output by one and its high state shifts from Q0 to Q1. When Q1 goes high, the output of IC3 at pin 3 goes low, so the motor starts running in clockwise (forward) direction. LED2 glows to indicate that the motor is running in forward direction.
If you press S1 again, the high output of IC2 shifts from Q3 to Q4. Since Q4 is connected to reset pin 15, it resets decade counter CD4017 and its Q0 output goes high, so the motor does not rotate. LED1 glows via diode D1 to indicate that the motor is in stop condition. Thereafter, the cycle repeats. If you don’t want to operate the motor
reverse conditions, respectively. Here, timer IC1 is wired as a monostable multivibrator to avoid false triggering of the motor while pressing switch S1. Its time period is approximately 500 milliseconds (ms). Suppose, initially, the circuit is in reset condition with Q0 output of IC2 being high. Since Q1 and Q3 outputs of IC2 are low, the outputs of IC3 and IC4 are high and the motor doesn’t rotate. LED1 glows to indicate that the motor is in stop condition.
Now if you press S1 again, the high output of IC2 shifts from Q1 to Q2. The low Q1 output of IC2 makes pin 3 of IC3 high and the motor doesn’t rotate. LED1 glows (via diode D2) to indicate that the motor is in stop condition. Pressing switch S1 once again shifts the high output of IC2 from Q2 to Q3. The high Q3 output of IC2 makes pin 3 of IC4 low and the motor starts running in anti-clockwise (reverse) direction. LED3 glows to indicate that the motor is running in reverse direction.
in reverse direction, remove timer IC4 along with resistors R5 and R7 and LED3. And connect ‘b’ terminal of the motor to +Vcc. Similarly, if you don’t want to run the motor in forward direction, remove timer IC3 along with resistors R4 and R6 and LED2. And connect ‘a’ terminal of the motor to +Vcc. The circuit works off a 9V regulated power supply for a 9V DC motor. Use a 6V regulated power supply for a 6V DC motor.
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ELECTRONICS FOR YOU
SEPTEMBER 2004
CIRCUIT IDEAS
AUTOMATIC SCHOOL BELL RAJ KUMAR MONDAL
I VED DWI . C . S
onsider that a school has a total of eight periods with a lunch break after the fourth period. Each period is 45 minutes long, while the duration of the lunch break is 30 minutes. To ring this automatic school bell to start the first period, the peon needs to momentarily press switch S1. Thereafter, the bell sounds every 45 minutes to indicate the end of consecutive periods, except immediately after the fourth period,
(IC2 and IC3) and AND gate CD4081 (IC4). Timer IC1 is wired as an astable multivibrator, whose clock output pulses are fed to IC2. IC2 increases the time periods of IC1 (4.5 and 3 minutes) by ten times to provide a clock pulse to IC3 every 45 minutes or after 30 minutes, respectively. When the class periods are going on, the outputs of IC3 switch on transistors T1 and T2 via diodes D4 through D12. Resistors R4 and R5 connected in series to the emitter of npn transistor T2
gate. When SCR1 is fired, it provides ground path to operate the circuit after resetting both decade counters IC2 and IC3. At the same time, LED1 glows to indicate that school bell is now active. When switch S2 is pressed momentarily, the anode of SCR1 is again grounded and the circuit stops operating. In this condition, both LED1 and LED2 don’t glow. When the eighth period is over, Q9 output of IC3 goes high. At this time, transistors T1 and T2 don’t get any voltage
when it sounds after 30 minutes to indicate that the lunch break is over. When the last period is over, LED2 glows to indicate that the bell circuit should now be switched off manually. In case the peon has been late to start the school bell, the delay in minutes can be adjusted by advancing the time using switch S3. Each pushing of switch S3 advances the time by 4.5 minutes. If the school is closed early, the peon can turn the bell circuit off by momentarily pressing switch S2. The bell circuit contains timer IC NE555 (IC1), two CD4017 decade counters
decide the 4.5-minute time period of IC1. The output of IC1 is further connected to pin 14 of IC2 to provide a period with a duration of 45 minutes. Similarly, resistors R2 and R3 connected in series to the emitter of npn transistor T1 decide the 3minute time period of IC1, which is further given to IC2 to provide the lunchbreak duration of 30 minutes. Initially, the circuit does not ground to perform its operation when 12V power supply is given to the circuit. When switch S1 is pressed momentarily, a high enough voltage to fire silicon-controlled resistor SCR1 appears at its
through the outputs of IC2. As a result, the astable multivibrator (IC1) stops working. The school bell sounds for around 8 seconds at the end of each period. One can increase/decrease the ringing time of the bell by adding/removing diodes connected in series across pins 6 and 7 of IC1. The terminals of the 230V AC electric bell are connected to the normally-open (N/O) contact of relay RL1. The circuit works off a 12V regulated power supply. However, a battery source for back-up in case the power fails is also recommended.
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ELECTRONICS FOR YOU
OCTOBER 2004
circuit
ideas
USB Power Socket
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T.K. Hareendran
T
oday, almost all computers contain logic blocks for implementing a USB port. A USB port, in practice, is capable of delivering more than 100 mA of continuous current at 5V to the peripherals that are connected to the bus. So a USB port can be used, without any trouble, for powering 5V DC operated tiny electronic gadgets. Nowadays, many handheld devices (for instance, portable reading lamps) utilise this facility of the USB port to recharge their built-in battery pack with the help of an internal circuitry. Usually 5V DC, 100mA current is required to satisfy the input power demand. Fig. 1 shows the circuit of a versatile USB power socket that safely converts the 12V battery voltage into stable 5V. This circuit makes it possible to power/ recharge any USB power-operated device, using in-dash board cigar lighter socket of your car. The DC supply available from the cigar lighter socket is fed to an adjustable, three-pin regulator LM317L (IC1).
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Fig. 1: Circuit of USB power socket
CIGAR PLUG
COIL CORD
Assemble the circuit on a general-purpose PCB and enclose in a slim plastic cabinet along with the Fig. 3: Pin indicator and USB configuration of LM317L (To- socket. While wir92 package) ing the USB outlet, ensure correct polarity of the supply. For interconnection between the cigar plug pin and the device, use a long coil cord as shown in Fig. 2. Pin configuration of LM317L is shown in Fig. 3.
USB POWER SOCKET WITH INDICATOR
Fig. 2: Interconnection of cigar plug and USB power socket using a coil cord
Capacitor C1 buffers any disorder in the input supply. Resistors R1 and R2 regulate the output of IC1 to steady 5V, which is available at the ‘A’ type female USB socket. Red LED1 indicates the output status and zener diode ZD1 acts as a protector against high voltage.
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circuit
ideas
SPY EAR
Fig. 1: Circuit for spy ear
is designed for operation with power supplies in the 4-15V DC range. It is housed in a standard 8-pin DIL package, coned components amplify the sumes very small quiessound signals picked up by cent current and is ideal the condenser microphone for battery-powered (MIC). The amplified signal portable applications. from the preamplifier stage The processed outis fed to input pin 3 of IC put signal from capaciLM386N (IC1) through Fig. 2: Compact unit of spy ear tor C2 goes to one end capacitor C2 (100nF) and of volume control VR1. volume control VR1 (10-kilo-ohm log). The wiper is taken to pin 3 of LM386N A decoupling network comprising reaudio output amplifier. Note that the sistor R5 and capacitor C3 provides the R6-C4 network is used to RF-decouple preamplifier block with a clean supply positive-supply pin 6 and R8-C7 is an voltage. optional Zobel network that ensures Audio amplifier IC LM386N (IC1) high frequency stability when feeding an inductive headphone load. Capacitor C6 (22µF, 16V) wired between pin 7 and ground gives additional ripple rejection. The output of LM386N power amplifier can safely drive a standard 32-ohm monophonic headphone/earphone. Assemble the circuit on a small general-purpose PCB and house in a suitable metallic enclosure with an integrated battery holder and headphone/earphone socket as shown in Fig. 2. Fit the on/off switch (S1), volume control (VR1) and power indicator (LED1) on the enclosure. Finally, fit the condenser microphone (MIC) on the front side of the enclosure and link it to the input of the preamplifier via a short length of the shielded wire.
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e l e c t ro n i c s f o r yo u • A p r i l 2 0 1 0 • 9 3
T.K. HAREENDRAN
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hat binoculars do to improve your vision, this personal sound enhancer circuit does for listening. This lightweight gadget produces an adjustable gain on sounds picked up from the built-in high-sensitivity condenser microphone. So you can hear what you have been missing. With a 6V (4×1.5V) battery, it produces good results. As shown in Fig. 1, a small signal amplifier is built around transistor BC547 (T1). Transistor T1 and the relat-
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CIRCUIT
IDEAS
CURRENT SENSOR
D. MOHAN KUMAR
IVEDI S.C. DW
igh-wattage appliances like electric irons, ovens and heaters result in unnecessary power loss if left ‘on’ for hours unnoticed. Here is a circuit that senses the flow of current through the appliances and gives audible beeps every fifteen minutes to remind you of power-’on’ status. This is a non-contact version of current monitor and can sense the flow of current in high-current appliances from a distance of up to 30 cm . It uses
transistors in the input to provide very high input impedance (1.5 T-ohms), very low input current (10 pA) and high-speed switching performance. The inverting input of IC1 is preset with VR1. In the standby mode, the primary of the transformer accepts e.m.f. from the instrument or surrounding atmosphere, which results in low-voltage input to IC1. This low voltage at the non-inverting input keeps the output of IC1 low. Thus transistor T1 doesn’t conduct and pin 12 of IC2 goes high to disable IC2. As a result, the remaining part of the cir-
by feeding Q9 output to the piezobuzzer for aural alarm through the intermediate circuitry. Resistors R5 and R6 along with capacitor C1 maintain the oscillations in IC2 as indicated by blinking LED1. The high output from IC2 is used to activate a simple oscillator comprising transistors T2 and T3, resistors R8 and R10, and capacitor C2. When the Q9 output of IC2 becomes high, zener diode ZD1 provides 3.1 volts to the base of transitor T2.
a standard step-down transformer (09V, 500mA) as the current sensor. Its secondary winding is left open, while the primary winding ends are used to detect the current. The primary ends of the transformer are connected to a full-wave bridge rectifier comprising diodes D1 through D4. The rectified output is connected to the non-inverting input of IC CA3140 (IC1). IC CA3140 is a 4.5MHz BIMOS operational amplifier with MOSFET input and bipolar transistor output. It has gate-protected MOSFET (PMOS)
cuit gets inactivated. When a high-current appliance is switched on, there will be a current drain in the primary of the transformer to the negative rail due to an increase in the e.m.f. caused by the flow of current through the appliance. This results in voltage rise at the non-inverting input and the output of IC1 becomes high. This high output drives transistor T1 into conduction and the reset pin of IC2 becomes low, which enables IC2. IC CD4060 (IC2) is a 14-stage ripple counter. It is used as a 15-minute timer
Since transistor T2 is biased by a highvalue resistor (R8), it will not conduct immediately. Capacitor C2 slowly charges and when the voltage at the base of T2 increases above 0.6 volt, it conducts. When T2 conducts, the base of T3 turns low and it also conducts. The piezobuzzer connected to the collector of T3 gives a short beep as capacitor C2 discharges. This sequence of IC2 output at Q9 becoming high and conduction of transistors T2 and T3 resulting in beep sound repeats at short intervals.
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94 • DECEMBER 2007 • ELECTRONICS FOR YOU
WWW.EFYMAG.COM
circuit
ideas
Infrared Illuminator
T.K. Hareendran
I
nfrared (IR) illuminators are widely used to improve the imagecapturing quality of security cameras fitted in dark zones. Just like our eyes, cameras also can’t record move-
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dark gray, blue or even black. They come in various configurations and radiation patterns, but 5mm types with 15- to 40-degree patterns are the most popular. Typically, IR LEDs run at around 1.3 to 1.7 volts, depending on the
to actuate the relay through driver transistor T3 can be varied by adjusting VR1. Diode 1N4001 eliminates any back voltage when the relay de-energises. Switch S1 is the mains
LED current (typically 10 to 30 mA). However, this may vary with the type and manufacturer. Practically, IR illuminators may have 6 or 60 to 100 or more LEDs, depending on the output needed. The circuit (refer Fig. 1) can be divided into three parts: ambient light sensor, relay driver and IR LEDs. The ambient light sensor is built around multiturn linear potmeter VR1 and light-dependent resistor LDR1. The relay driver section is built around transistors T1 through T3. The IR LEDs section is built around LED1 through LED40. The light sensor circuit is a simple transistor switch with the base of the Darlington pair (formed by T1 and T2) connected to a voltage divider. Variable resistor VR1 and the 10mm encapsulated LDR are used to sense the ambient light. As light falls on the surface of LDR1, its resistance changes. The amount of minimum light needed
power on/off switch and switch S2 is added to bypass the ambient light detection function. Relay RL1 energises only when the ambient light level falls below a threshold value set by VR1, i.e., when it’s dark. Normally-opened (N/O) contacts of the relay ground path to the IR LEDs (LED1 through LED40) to make them glow. The blue LED (LED41) indicates the circuit activity. When there is ambient light and you want to use the illuminator, switch S2 ‘on.’ All the LEDs (LED1 through LED40) glow to fulfil your requirement. Assemble the circuit on a generalpurpose PCB and enclose in a suitable cabinet. The IR LEDs assembly is very important. A set of 40 (5×8) 5mm infrared LEDs (IR LED1 through IR LED40) with independent current-limiting resistors (R3 through R10) per string is used. This section is powered by the input DC supply through the relay contacts.
Fig. 1: Circuit for infrared illuminator
ments in dark. However, unlike our eyes, most of the latest cameras can capture infrared light. In an IR Fig. 2: Infrared illuminator Illuminator, many infrared IR LEDs are grouped together to throw good amount of IR light. Typically, LEDs output at 470 nm (blue region), 525 nm (green region) and 625 nm (red region). IR LEDs produce longer wavelengths, 880 nm and 940 nm being the common ones. Most CCD cameras are a little more sensitive to 880 nm, although when these LEDs are used for security applications, some individuals can detect a very dim red glow from them. The 940nm LED radiations are completely invisible to the eye. Some of these LEDs are clear, while others are tinted with pale shades of
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Mount IR LEDs on the general-purpose PCB board such that these make three circles. After soldering, carefully cut the outside of the circuit board in a round shape and fit it in a suitable metal/plastic cabinet. If available, add
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a suitable reflector sheet for the IR LED bank. Finally, fit the LDR bank on top of the enclosure with switches, indicator-sensitivity-control pot and power input socket. Fig. 2 shows the infrared illuminator unit.
To make the circuit actuate the relay when the intensity of ambient light is less than the preset light level, throw light on LDR1 and then slowly adjust the potentiometer until LED1 lights up and the relay energises.
e l e c t ro n i c s f o r yo u • D e c e m b e r 2 0 1 0 • 1 1 3
circuit
ideas
Infrared Illuminator
T.K. Hareendran
I
nfrared (IR) illuminators are widely used to improve the imagecapturing quality of security cameras fitted in dark zones. Just like our eyes, cameras also can’t record move-
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dark gray, blue or even black. They come in various configurations and radiation patterns, but 5mm types with 15- to 40-degree patterns are the most popular. Typically, IR LEDs run at around 1.3 to 1.7 volts, depending on the
to actuate the relay through driver transistor T3 can be varied by adjusting VR1. Diode 1N4001 eliminates any back voltage when the relay de-energises. Switch S1 is the mains
LED current (typically 10 to 30 mA). However, this may vary with the type and manufacturer. Practically, IR illuminators may have 6 or 60 to 100 or more LEDs, depending on the output needed. The circuit (refer Fig. 1) can be divided into three parts: ambient light sensor, relay driver and IR LEDs. The ambient light sensor is built around multiturn linear potmeter VR1 and light-dependent resistor LDR1. The relay driver section is built around transistors T1 through T3. The IR LEDs section is built around LED1 through LED40. The light sensor circuit is a simple transistor switch with the base of the Darlington pair (formed by T1 and T2) connected to a voltage divider. Variable resistor VR1 and the 10mm encapsulated LDR are used to sense the ambient light. As light falls on the surface of LDR1, its resistance changes. The amount of minimum light needed
power on/off switch and switch S2 is added to bypass the ambient light detection function. Relay RL1 energises only when the ambient light level falls below a threshold value set by VR1, i.e., when it’s dark. Normally-opened (N/O) contacts of the relay ground path to the IR LEDs (LED1 through LED40) to make them glow. The blue LED (LED41) indicates the circuit activity. When there is ambient light and you want to use the illuminator, switch S2 ‘on.’ All the LEDs (LED1 through LED40) glow to fulfil your requirement. Assemble the circuit on a generalpurpose PCB and enclose in a suitable cabinet. The IR LEDs assembly is very important. A set of 40 (5×8) 5mm infrared LEDs (IR LED1 through IR LED40) with independent current-limiting resistors (R3 through R10) per string is used. This section is powered by the input DC supply through the relay contacts.
Fig. 1: Circuit for infrared illuminator
ments in dark. However, unlike our eyes, most of the latest cameras can capture infrared light. In an IR Fig. 2: Infrared illuminator Illuminator, many infrared IR LEDs are grouped together to throw good amount of IR light. Typically, LEDs output at 470 nm (blue region), 525 nm (green region) and 625 nm (red region). IR LEDs produce longer wavelengths, 880 nm and 940 nm being the common ones. Most CCD cameras are a little more sensitive to 880 nm, although when these LEDs are used for security applications, some individuals can detect a very dim red glow from them. The 940nm LED radiations are completely invisible to the eye. Some of these LEDs are clear, while others are tinted with pale shades of
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Mount IR LEDs on the general-purpose PCB board such that these make three circles. After soldering, carefully cut the outside of the circuit board in a round shape and fit it in a suitable metal/plastic cabinet. If available, add
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a suitable reflector sheet for the IR LED bank. Finally, fit the LDR bank on top of the enclosure with switches, indicator-sensitivity-control pot and power input socket. Fig. 2 shows the infrared illuminator unit.
To make the circuit actuate the relay when the intensity of ambient light is less than the preset light level, throw light on LDR1 and then slowly adjust the potentiometer until LED1 lights up and the relay energises.
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circuit
ideas
Tester For Remote Control
T.A. Babu
H
ere is a simple tester for checking the basic operations of an infrared remote control unit. It is low-cost and easy to construct. The tester is built around infrared receiver module TSOP1738. Operation of the remote control is acknowledged by a tone from the buzzer. The circuit is sensitive and has a range of approxi-
Fig. 1: Circuit diagram of remote tester
mately five metres. The integrated IR receiver detects, amplifies and demodulates IR signals from the remote control unit. The piezobuzzer connected at its output sounds to indicate the presence of signal from the remote control unit. As shown in Fig. 1, output pin 3 of IR receiver module TSOP1738 (IRX1) normally remains high and the piezobuzzer is in silent mode. When the IR module IRX1 receives an infrared signal, its output goes low and, as a result, the piezobuzzer sounds to indicate the reception of sig-
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nal from the remote (such as TV remote control unit). P o w e r supply for the circuit is derived from Fig. 2: Pin configuration of the mains usTL431 and TSOP 1738 ing a capacitive potential dropper, a half-wave rectifier, a shunt regulator and associated components. Make sure that capacitor C1 is of X2 type. Use a suitably small enclosure to make the unit handy. Assemble the circuit on a generalpurpose PCB and enclose in a cabinet. Make sure that the IR receiver module is placed on the front panel of the cabinet so that it can receive the IR signals easily. Before soldering/connecting the shunt regulator and IR module, refer Fig. 2 for the pin configuration.
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circuit
ideas
NUMERIC WATER-LEVEL INDICATOR Daniyal Syed
ost water-level indicators for water tanks are based upon the number of LEDs that glow to indicate the corresponding level of water in the container. Here we present a digital version of the water-level indicator. It uses a 7-seg-
terminal of the sensor must be kept at the bottom of the container (tank). IC 74HC147 has nine active-low inputs and converts the active input into active-low BCD output. The input L-9 has the highest priority. The outputs of IC1 (A, B, C and D) are fed to IC2 via transistors T1 through T4. This logic inverter is used
when the water level reaches L-1 position, the display shows ‘1,’ and when the water level reaches L-8 position, the display shows ‘8.’ Finally, when
ment display to show the water level in numeric form from ‘0’ to ‘9.’ The circuit works off 5V regulated power supply. It is built around priority encoder IC 74HC147 (IC1), BCD-to7-segment decoder IC CD4511 (IC2), 7-segment display LTS543 (DIS1) and a few discrete components. Due to high input impedance, IC1 senses water in the container from its nine input terminals. The inputs are connected to +5V via 560-kilo-ohm resistors. The ground
to convert the active-low output of IC1 into active-high for IC2. The BCD code received by IC2 is shown on 7-segment display LTS543. Resistors R18 through R24 limit the current through the display. When the tank is empty, all the inputs of IC1 remain high. As a result, its output also remains high, making all the inputs of IC2 low. Display LTS543 at this stage shows ‘0,’ which means the tank is empty. Similarly,
the tank is full, all the inputs of IC1 become low and its output goes low to make all the inputs of IC2 high. Display LTS543 now shows ‘9,’ which means the tank is full. Assemble the circuit on a general-purpose PCB and enclose in a box. Mount 7-segment LTS543 on the front panel of the box. For sensors L-1 though L-9 and ground, use corrosionfree conductive-metal (stainless-steel) strips.
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circuit
ideas
Bicycle Indicator
T.K. Hareendran
T
he electronic bicycle signaling unit described here uses lowcost components and is a good
Fig. 1: Circuit of bi-cycle indicator
Fig. 2: Suggested enclosure (master unit)
substitute to many commercially available versions. It works in an extremely different manner and is convenient to operate. The circuit works off a 9V PP3 (alkaline-type) battery and is basically a set of two independent free-running oscillators (astable multivibrators) built around four low-power transistors and a few passive components. Both
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the square-wave oscillators (one built around T1 and T2 and the other built around T3 and T4) drive four red LEDs (LED1 and LED2, and LED5 and LED6, respectively), which blink to indicate
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determined by timing capacitors C1 and C2. Resistors R2 and R3 limit the operating current of LEDs (LED1 and LED2). At the same time, the green LED (LED3) starts glowing to indicate the present direction status. Similar action happens in the next oscillator circuit built around transistors T3 and T4 when switch S2 is flipped to ‘on’ position. Indicators at the front right (FR) and rear right (RR) start blinking, and at the same time the green LED (LED4) glows to indicate the direction status. Switch S3 is used for emergency indication. When it is flipped to ‘on’ position, both the oscillators get power supply through diodes D1 and D2. As a result, LED1 through LED6 start working simultaneously. In this condition, all the LEDs blink, except LED3 and LED4, which glow steadily. After assembling the circuit on a generalpurpose PCB, enclose it in a suitable cabinet as shown in Fig. 2 and mount on the handle bar of the bicycle, prefFig. 3: Suggested enclosure (indicators) erably at the mechanical centre point. Connect switch S1 at the left-hand side, S2 the direction of turn. Adat the right-hand side and emergency ditional steady-glow LEDs switch S3 in the middle of the master (LED3 and LED4) are inunit. Now place this master unit at corporated to indicate the the top of the handle bar and do the working status. essential interconnections using flexThe working of the cirible wires. Connect the front indicacuit is straightforward. When tors (LED1 and LED5) to the left and switch S1 is flipped to ‘on’ poright side of the handle and similarly sition, DC supply from rear indicators (LED2 and LED6) can the battery is extended to the oscilbe mounted in the carrier frame of lator circuit formed by transistors the bicycle. For the direction indicaT1 and T2. Now the left-side osciltor, you can use the symbol shown in lator starts oscillating and the visual Fig. 3 and place it at the centre of the indicators at the front left (FL) and handle. rear left (RL) start blinking at a rate
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circuit
ideas
Digital Thermometer
Raj K. Gorkhali
T
his digital thermometer can measure temperatures up to 150°C with an accuracy of ±1°C. The temperature is read on a 1V full scale-deflection (FSD) moving-coil voltmeter or digital voltmeter. Operational amplifier IC 741 (IC3) provides a constant flow of current
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through the base-emitter junction of npn transistor BC108 (T1). The voltage across the base-emitter junction of the transistor is proportional to its temperature. The transistor used this way makes a low-cost sensor. You can use silicon diode instead of transistor. The small variation in voltage across the base-emitter junction is amplified by second operational amplifier (IC4),
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before the temperature is displayed on the meter. Preset VR1 is used to set the zero-reading on the meter and preset VR2 is used to set the range of temperature measurement. Operational amplifiers IC3 and IC4 operate off regulated ±5V power supply, which is derived from 3-terminal positive voltage regulator IC 7805 (IC1) and negative low-dropout regulator IC 7660 (IC2). The entire circuit works off a 9V battery. Assemble the circuit on a general-purpose PCB and enclose in a small plastic box. Calibrate the thermometer using presets VR1 and VR2. After calibration, keep the box in the vicinity of the object whose temperature is to be measured.
e l e c t ro n i c s f o r yo u • J u ly 2 0 1 0 • 9 9
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IDEAS
BRAKE FAILURE INDICATOR
D. MOHAN KUMAR
o you want to get an early warning of brake failure while driving? Here is a brake failure indicator circuit that constantly monitors the condition of the brake and gives an audio-visual indi-
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monitoring the brake switch and reminds you of the condition of the brake every time the brake is applied. The circuit uses an op-amp IC CA3140 (IC2) as voltage comparator and timer NE555 (IC3) in monostable configuration for alarm. Voltage comparator IC2 senses the voltage level
cation. When the brake is applied, the green LED blinks and the piezobuzzer beeps for around one second if the brake system is intact. If the brake fails, the red LED glows and the buzzer stops beeping. The circuit will work only in vehicles with negative grounding. It also gives an indication of brake switch failure. In hydraulic brake systems of vehicles, a brake switch is mounted on the brake cylinder to operate the rear brake lamps. The brake switch is fluidoperated and doesn’t function if the fluid pressure drops due to leakage. The fluid leakage cannot be detected easily unless there is a severe pressure drop in the brake pedal. This circuit senses the chance of a brake failure by
across the brake switch. Its non-inverting input (pin 3) gets half the supply voltage through potential divider resistors R3 and R4 of 10 kilo-ohms each. The inverting input (pin 2) of IC2 is connected to the brake switch through diode D1, IC 7812 (IC1) and resistor R2. It receives a higher voltage when the brake is applied. Normally, when the brake is not applied, the output of IC2 remains high and the red LED (LED1) glows. The output of IC2 is fed to trigger pin 2 of the monostable through coupling capacitor C2. Resistor R1 is used for the input stability of IC2. IC1 and C1 provide a ripple-free regulated supply to the inverting input of IC2. IC3 is wired as a monostable to give pulse output of one second. Tim-
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ing elements R7 and C4 make the output high for one second to activate the buzzer and LED2. Usually, the trigger pin of IC3 is high due to R6 and the buzzer and LED2 remain ‘off.’ When the brake pedal is pressed, pin 2 of IC2 gets a higher voltage from the brake switch and its output goes low to switch off the red LED. The low output of IC2 gives a short negative pulse to the monostable through C2 to trigger it. This activates the buzzer and LED2 to indicate that the brake system is working. When there is pressure drop in the brake system due to leakage, LED1 remains ‘on’ and the buzzer does not sound when the brake is applied. The circuit can be assembled on any general-purpose PCB or perforated board. Connect point A to that terminal of the brake switch which goes to the brake lamps. The circuit can be powered from the vehicle’s battery. The circuit requires well-regulated power supply to avoid unwanted triggering while the battery is charging from the dynamo. IC4, C6 and C7 provide regulated 12V to the circuit. The power supply should be taken from the ignition switch and the circuit ground should be clamped to the vehicle’s body. A bicolour LED can be used in place of LED1 and LED2 if desired. z
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CIRCUIT
IDEAS
SEISMIC SENSOR
D. MOHAN KUMAR
IVEDI S.C. DW
his circuit simulates a seismic sensor to detect vibrations/ sounds. It is very sensitive and
both its inverting and non-inverting inputs tied to the negative rail through a resistive network comprising R1, R2 and R3. Under idle conditions (as adjusted by VR1), both the inputs receive
riod of IC2 is determined by R7 and C5. With the shown values, it will be around two minutes. The high output from IC2 activates T2 and the buzzer
can detect vibrations caused by the movement of animals or human beings. So it can be used to monitor protected areas to restrict entry of unwanted persons or animals. The circuit uses readily available components and the design is straightforward. A standard piezo sensor is used to detect vibrations/sounds due to pressure changes. The piezo element acts as a small capacitor having a capacitance of a few nanofarads. Like a capacitor, it can store charge when a potential is applied to its terminals. It discharges through VR1, when it is disturbed. In the circuit, IC TLO71 (IC1) is wired as a differential amplifier with
almost equal voltages, which keeps the output low. TLO71 is a low-noise JFET input op-amp with low input bias and offset current. The BIFET technology provides fast slew rates. Capacitor C1 is provided in the circuit to keep the differential input of IC1 for better performance. When the piezo element is disturbed (by even a slight movement), it discharges the stored charge. This alters the voltage level at the inputs of IC1 and the output momentarily swings high as indicated by green LED1. This high output is used to trigger switching transistor T1, which triggers monostable IC2. The timing pe-
starts beeping along with red light indication from LED2. Assemble the circuit on a common PCB and enclose in a suitable cabinet. Connect the piezo element to the PCB using single-core shielded wire. Enclose the piezo element inside a rustproof, small aluminium box. The piezo element should be firmly glued to the enclosure facing the fine side towards the case. Fix the sensor assembly on the back side of a ceramic tile or granite tile with good adhesive. Fix the tile (or bury it in the earth) near the entrance with the sensor assembly facing downwards. Whenever a pressure change develops near the sensor, the circuit will be activated.
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circuit
ideas
Flasher For Deepawali
Sunil Kumar
H
ere is the circuit for a portable electric lamp-cum-LED flasher. It uses a 25W, 230V AC bulb and nine LEDs. When the bulb glows all the LEDs remain ‘off,’ and when the LEDs glow the bulb remains ‘off.’ The circuit is built around timer IC 555 (IC1), which is wired as an astable multivibrator generating square wave.
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The output of IC1 drives transistor T1. Working of the circuit is simple. When output pin 3 of IC1 goes high, transistor T1 conducts to fire triac1 and the bulb glows. Bulb L1 turns off when output pin 3 of IC1 goes low. The collector of transistor T1 is connected to anodes of all the LEDs (LED1 through LED9). So when T1 is cut-off the LEDs glow, and when T1 conducts the LEDs go off. Current-limiting resistor R4 protects the LEDs from higher
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currents. In brief, the bulb and the LEDs flash alternately depending on the frequency of IC1. Flashing rates of the bulb as well as LEDs can be varied by adjusting potmeter VR1. Connect the power supply line (L) of mains to bulb L1 via switch S1 and neutral (N) to MT1 terminal of triac1. A 12V, 200mA AC adaptor is used to power the circuit. Using switch S1, you can switch off the bulb permanently if you do not want it to flash. Assemble the circuit on a general-purpose PCB and enclose in a circular plastic cabinet keeping the bulb at the centre and LEDs at the circumference. Drill holes for mounting the ‘on’/‘off’ switch. Use a bulb holder for bulb L1 and LED holders for the LEDs. Also use an IC socket for timer IC 555. Warning. While assembling, testing or repairing, take care to avoid the lethal electric shock.
e l e c t ro n i c s f o r yo u • N o v e m b e r 2 0 1 0 • 1 1 5
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IDEAS
FRONT DOOR GUARD
D. MOHAN KUMAR
H
ere is a circuit to thwart the attempt of burglary in your home. When an intruder tries to open the door of your house, it sounds a loud alarm and switches on the porch light. The alarm disables only when the door is closed again or S2 is switched off. The circuit is failproof and reliable and, unlike IRor 555 timer-based burglar alarms, overcomes the problem of false triggering. The circuit is based on the triggering action of the low-power monostable/astable multivibrator IC CD4047 (IC1). It is wired in the
IVEDI S.C. DW
monostable mode to set and reset IC CD4027 (IC2). The output pulse width of IC1 depends on the external R3-C2 network connected to its pins 1, 2 and 3. Monostable operation is achieved when IC1 is triggered by low-to-high transition of the positive trigger input (pin 8). It can be retriggered by another low-to-high transition of pin 8. When the master reset input (pin 9) is high, Q output (pin 10) goes low and Q output (pin 11) goes high. Here the monostable configuration of IC1 has R3-C2 network with values of C2 and R3 as 0.0047 µF and 470 kilo-ohms, respectively. Trigger pin 8 is connected to the emitter of T1.
Normally, with reed switch S1 closed, transistor T1 is non-conducting and its emitter current is zero. So the monostable remains in the standby mode with low output at pin 10. When T1 conducts upon opening reed switch S1, a positive pulse from the emitter of T1 triggers the monostable and a short positive pulse is available to Q output of the monostable. It resets when T1 is made non-conducting by closing reed switch S1. The negative trigger pin 6 and astable input pin 5 of IC1 are tied to the ground along with ground pin 7. The short-duration (one-second) low-to-high output from IC1 is used to set and reset IC2. It is a low-power
Q output goes low and Q output goes high. This lights up LED3 and drives transistor T2 (BC548). The output from transistor T2 is used to activate
triac1 (BT136) and alarm tone generator IC3. The triac is used to switch on porch lamp. IC3 (IC UM3561) is a tone generator that produces different tones based on its pin connections. Here it is used to generate a fire brigade alarm by connecting its pin 6 to Vcc. Resistor R10 keeps the oscillation of IC3 to the required level. Zener diode ZD1 and re-
Fig. 1: Circuit of front door guard
dual J-K master/slave flip-flop having independent J, K, set, reset and clock inputs. The flip-flops are edge-sensitive to the clock input and Q and Q outputs change states on the positivegoing transition of the clock pulses. IC2 is wired such that its Q output turns high when reset pin 4 receives a high pulse (as indicated by LED2). When set pin 7 receives a high pulse,
Fig. 2: Pin configuration of BC548/549 and BT136
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ELECTRONICS FOR YOU • OCTOBER 2006 • 103
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sistor R8 provide 3V DC to IC3. The circuit can be easily assembled on a general-purpose PCB. All the ICs are commonly available low-cost versions. A standard 9V, 500mA adaptor can be used to power the circuit. Reed switch S1 is an important part of the circuit, and it should be normally open type.
Mount the reed switch on the doorframe and the remaining circuit on the nearby wall. The magnet for controlling the reed switch should be fixed on the door so as to close the reed switch when the door closes. When the door opens, the contacts of the reed switch break to activate the alarm. Isolate the
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triac from the remaining parts of the circuit and ensure adequate spacing between its pins to avoid short circuit. Warning. Since triac is used in the circuit, some parts will be at mains potential. So exercise utmost care during testing and installation to avoid lethal shock.
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CIRCUIT IDEAS
STABILISED POWER SUPPLY WITH SHORT-CIRCUIT INDICATION D. MOHAN KUMAR
I VED DWI S.C.
ere is an efficient 4-stage stabilised power supply unit for testing electronic circuits. It provides well regulated and stabilised output, which is
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18V, 500mA step-down transformer to generate 18V AC. A rectifier diode comprising diodes D1 and D2 provides 18V DC, which is smoothed by capacitor C1 and given to the combination of regulator ICs (IC1 through IC4). The regulator ICs pro-
negative terminal is also at full supply voltage (selected). If there is a short circuit at the output, LED2 glows to activate the piezobuzzer. A fuse-failure indicator distinguishes short circuit at the output and input fail-
essential for most electronic circuits to give proper results. The circuit provides an audio-visual indication if there is a short circuit in the PCB under test, so the power supply to the circuit ‘under test’ can be cut-off immediately to save the valuable components from damage. The circuit provides four different regulated outputs (12V, 9V, 6V and 5V) and an unregulated 18V output, which are selectable through rotary switch S2. The selected output is indicated on the analogue voltmeter connected to the outputs rails. The circuit uses a standard 18V-0-
duce fixed, regulated outputs of 12V, 9V, 6V and 5V, respectively, which are connected to the rotary switch contacts. This power supply is useful for loads requiring up to 200mA current. Complementary transistors T1 and T2 conduct when the power to the circuit is switched on. Full selected supply voltage is available at the collector of transistor T2, which is used to power the load. LED3 indicates the presence of output voltage. The negative terminal of piezobuzzer PZ1 is connected to the output rail via LED2, so the piezobuzzer remains silent as its
ure. It consists of a bicolour LED (LED1) and resistors R1 and R2. When power is available and the fuse is intact, red and green halves of LED1 are effectively in parallel to output a yellowish light. When fuse fails, green LED goes off and red LED lights up to indicate fuse breakdown. The circuit can be easily constructed on a general-purpose PCB. Use small heatsinks for all ICs to dissipate heat. The output voltage can be read on a voltmeter. Enclose the circuit in a metal box with provisions for voltmeter, LEDs, rotary switch, etc.
ELECTRONICS FOR YOU
JULY 2004
CIRCUIT IDEAS
WATER-LEVEL CONTROLLER K.P. VISWANATHAN
ere is a simple, automatic waterlevel controller for overhead tanks that switches on/off the pump motor when water in the tank goes below/ above the minimum/maximum level. The water level is sensed by two floats to operate the switches for controlling the pump motor. Each sensors float is suspended from
positions. Normally, N/C contact of switch S1 is connected to ground and N/C contact of switch S2 is connected to 12V power supply. IC 555 is wired such that when its trigger pin 2 is grounded it gets triggered, and when reset pin 4 is grounded it gets reset. Threshold pin 6 and discharge pin 7 are not used in the circuit. When water in the tank goes below the minimum level, moving contacts (P1
above using an aluminium rod. This arrangement is encased in a PVC pipe and fixed vertically on the inside wall of the water tank. Such sensors are more reliable than induction-type sensors. Sensor 1 senses the minimum water level, while sensor 2 senses the maximum water level (see the figure). Leaf switches S1 and S2 (used in tape recorders) are fixed at the top of the sensor units such that when the floats are lifted, the attached 5mm dia. (approx.) aluminium rods push the moving contacts (P1 and P2) of leaf switches S1 and S2 from normally closed (N/C) position to normally open (N/O) position. Similarly, when the water level goes down, the moving contacts revert back to their original
and P2) of both leaf switches will be in N/C position. That means trigger pin 2 and reset pin 4 of IC1 are connected to ground and 12V, respectively. This triggers IC1 and its output goes high to energise relay RL1 through driver transistor SL100 (T1). The pump motor is switched on and it starts pumping water into the overhead tank if switch S3 is ‘on.’ As the water level in the tank rises, the float of sensor 1 goes up. This shifts the moving contact of switch S1 to N/O position and trigger pin 2 of IC1 gets connected to 12V. This doesn’t have any impact on IC1 and its output remains high to keep the pump motor running. As the water level rises further to reach the maximum level, the float of sensor 2
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ELECTRONICS FOR YOU
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EDI DWIV S.C.
pushes the moving contact of switch S2 to N/O position and it gets connected to ground. Now IC1 is reset and its output goes low to switch the pump off. As water is consumed, its level in the overhead tank goes down. Accordingly, the float of sensor 2 also goes down. This causes the moving contact of switch S2 to shift back to NC position and reset pin 4 of IC1 is again connected to 12V. But IC1 doesn’t get triggered because its trigger pin 2 is still clamped to 12V by switch S1. So the pump remains switched off. When water level further goes down to reach the minimum level, the moving contact of switch S1 shifts back to N/C position to connect trigger pin 2 of IC1 to ground. This triggers IC1 and the pump is switched on. The float sensor units can be assembled at home. Both the units are identical, except that their length is different. The depth of the water tank from top to the outlet water pipe can be taken as the length of the minimum-level sensing unit. The depth of the water tank from top to the level you want the tank to be filled up to is taken as the length of the maximum-level sensing unit. The leaf switches are fixed at the top of the tank as shown in the figure. Each pipe is closed at both the ends by using two caps. A 5mm dia. hole is drilled at the centre of the top cap so that the aluminium rod can pass through it easily to select the contact of leaf switches. Similarly, a hole is to be drilled at the bottom cap of the pipe so that water can enter the pipe to lift the float. When water reaches the maximum level, the floats should not go up more than the required distance for pushing the moving contact of the leaf switch to N/O position. Otherwise, the pressure on the float may break the leaf switch itself. The length of the aluminium rod is to be selected accordingly. It should be affixed on the metal/thermocole float using some glue (such as Araldite).
CIRCUIT IDEAS
HIGHWAY ALERT SIGNAL LAMP
I VED DWI S.C.
D. MOHAN KUMAR
H
ere is a signal lamp for safe highway driving. The lamp automatically emits brilliant tricolour light when a vehicle approaches the rear side of your vehicle. It emits light for 30 seconds that turns off when the approaching vehicle overtakes. The ultra-bright blue, white and red LEDs of the signal lamp emit very bright light to alert the approach- Fig. 1: Circuit diagram of highway alert signal lamp
ing vehicle’s driver even during the day, giving additional safety during night, or when you need Fig. 2: Pin configuration to stop your vehicle on side of the highway. The circuit saves considerable battery power. The circuit is built around two timer ICs NE555 (IC1 and IC2). IC1 is designed as a standard monostable, while IC2 is designed as an astable. Darlington phototransistor L14F1 (T1) is used as a photosensor to activate the monostable. The collector of phototransistor T1 is con-
ELECTRONICS FOR YOU
APRIL 2004
nected to trigger pin 2 of IC1, which is normally kept high by resistor R1. W h e n Fig. 3: Suggested headlight from arrangement of LEDs an approaching vehicle illuminates the phototransistor, it conducts to give a short pulse to IC1, and the output of IC1 goes high for a period determined by resistor R2 and capacitor C1. The output of IC1 is fed to the base of transistor T2 via resistor R3. Transistor T2 conducts to drive transistor T3 and its collector goes high to take reset pin 4 of IC2 to high
level. This activates astable IC2, which switches on and off the LED chain alternately. The intermittent flashing of LEDs gives a beautiful tricolour flashlight effect. The circuit can be easily constructed on a small piece of general-purpose PCB. Fig. 2 shows the bottom and front views of Darlington phototransistor L14F1. The proposed arrangement of LEDs, which are soldered in a circular fashion on a generalpurpose PCB, is shown in Fig. 3. Use a circular reflector for the LEDs to get brighter light. Fix the LED arrangement on the rear side of your vehicle, and the phototransistor where it is illuminated directly by the headlight of the approaching vehicle. 12V DC supply to the circuit, can be provided by your vehicle battery with proper polarity.
CIRCUIT
IDEAS
SIMPLE DIGITAL SECURITY SYSTEM
PHERDAUS ISLAM
Y
ou can use this simple and reliable security system as a watchdog by installing the sensing loops around your building. You have to stretch the loop wires two feet above the ground to sense the unauthorised entry into your premises. Wire loops 1, 2 and 4 are connected to the A, B and C inputs of 7-segment decoder 4511 (IC1), respectively, while the D input of IC1 is grounded perma-
IVEDI S.C. DW
nected as shown in Fig. 1. If you don’t want to use a buzzer, switch it off by opening switch S2. The circuit works off a 9V regulated power supply. However, battery back-up is recommended. A commoncathode, 7-segment display (LTS543) is used for displaying whether the loops are intact or not. If loop 1 is broken, the display will show ‘1’. If two or all the three loops are broken, the display will show the sum of the respective broken loop
Fig. 2: The proposed wiring diagram of loops
numbers. For example, if loops 1 and 4 are broken, the display will show 5(1+4). When all the three loops are intact, the display will show ‘0.’ All the three inputs of gate N1 remain low to give a high output. This high output is further given to gate N2 and, as a result, its output remains low. This keeps
transistor T1 in cut-off position and the piezobuzzer does not sound. When any loop is broken, the output of NOR gate N1 goes low, while the output of gate N2 goes high. Transistor T1 conducts and the buzzer sounds to alert you. You can mute the buzzer by switching off power to the circuit through switch S1. z
Fig. 1: The digital security system circuit
nently. The loops are also connected to a dual 3-input NOR gate and inverter CD4000 (IC2) to activate the alarm. Fig. 1 shows the circuit of the digital security system, while Fig. 2 shows the proposed wiring diagram for the loops around the premises. Before using this security system, make sure that loops shown in Fig. 2 are con-
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ELECTRONICS FOR YOU • MARCH 2005 • 105
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CIRCUIT IDEAS
NON-CONTACT POWER MONITOR D. MOHAN KUMAR
H
ere is a simple non-contact AC power monitor for home appliances and laboratory equipment that should remain continuously switched-on. A fuse failure or power breakdown in the equipment going unnoticed may cause irreparable loss. The monitor sounds an alarm on detecting power failure to the equipment. The circuit is built around CMOS IC CD4011 utilising only a few components. NAND gates N1 and N2 of the IC are wired as an oscillator that drives a piezobuzzer directly. Resistors R2 and R3 and capacitor C2 are the oscillator components. The amplifier comprising transistors T1 and T2 disables the oscillator when mains power is available. In the standby mode, the base of T1 picks up 50Hz mains hum during the positive half cycles of AC and T1 conducts. This provides base current to T2 and it also conducts, pulling the collector to ground potential.
ELECTRONICS FOR YOU
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As the collectors of T1 and T2 are connected to pin 2 of NAND gate N1 of the oscillator, the oscillator gets disabled when the transistors conduct. Capacitor C1 prevents rise of the collector voltage of T2 again during the negative half cycles. When the power fails, the electrical field around the equipment’s wiring ceases and T1 and T2 turn off. Capacitor C1 starts charging via R1 and preset VR and when it gets sufficiently charged, the oscillator is
MAR IL KU SUN
e n a b l e d and the piezobuzzer produces a shrill tone. Resistor R1 protects T2 from short circuit if VR is adjusted to zero resistance. The circuit can be easily assembled on a perforated/ breadboard. Use a small plastic case to enclose the circuit and a telescopic antenna as aerial. A 9V battery can be used to power the circuit. Since the circuit draws only a few microamperes current in the standby mode, the battery will last several months. After assembling the circuit, take the aerial near the mains cable and adjust VR until the alarm stops to indicate the standby mode. The circuit can be placed on the equipment to be monitored close to the mains cable.
circuit
ideas
Multitone Siren
Pradeep G.
T
his multitone siren is useful for burglar alarms, reverse horns, etc. It produces five different audio tones and is much more earcatching than a single-tone siren. The circuit is built around popular CMOS oscillator-cum-divider IC 4060
and small audio amplifier LM386. IC 4060 is used as the multitone generator. A 100µH inductor is used at the input of IC 4060. So it oscillates within the range of about 5MHz RF. IC 4060 itself divides RF signals into AF and ultrasonic ranges. Audio signals of different frequencies are available at pins 1, 2, 3, 13 and 15 of IC 4060 (IC1).
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These multifrequency signals are mixed and fed to the audio amplifier built around IC LM386. The output of IC2 is fed to the speaker through capacitor C9. If you want louder sound, use power amplifier TBA810 or TDA1010. Only five outputs of IC1 are used here as the other five outputs (pins 4 through 7 and 14) produce ultrasonic signals, which are not audible. Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. Regulated 6V-12V (or a battery) can be used to power the circuit.
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MIDNIGHT security light
D. MOHAN KUMAR
M
ost thefts happen after midnight hours when people enter the second phase of sleep called ‘paradoxical’ sleep. Here is an energy-saving circuit that causes the thieves to abort the theft attempt by lighting up the possible sites of intrusion (such as kitchen or backyard of your house) at around 1:00 am. It automatically resets in the morning. The circuit is fully automatic and uses a CMOS IC CD 4060 to get the desired time delay. Light-dependent resistor LDR1 controls reset pin 12 of IC1 for its automatic action. During day time, the low resistance of LDR1 makes pin 12 of IC1 ‘high,’ so it doesn’t oscillate. After
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sunset, the high resistance of LDR1 makes pin 12 of IC1 ‘low’ and it starts oscillating, which is indicated by the flashing of LED2 connected to pin 7 of IC1. The values of oscillator components (resistors R1 and R2 and capacitor C4) are chosen such that output pin 3 of IC1 goes ‘high’ after seven hours, i.e., around 1 am. This high output drives triac 1 (BT136) through LED1 and R3. Bulb L1 connected between the phase line and M2 terminal of triac 1 turns on when the gate of triac 1 gets the trigger voltage from pin 3 of IC1. It remains ‘on’ until pin 12 of IC1 becomes high again in the morning. Capacitors C1 and C3 act as power reserves, so IC1 keeps oscillating even if there is power interruption for a few seconds. Capacitor C2 keeps trigger
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pin 12 of IC1 high during day time, so slight changes in light intensity don’t affect the circuit. Using preset VR1 you can adjust the sensitivity of LDR1. Power supply to the circuit is derived from a step-down transformer X1 (230V AC primary to 0-9V, 300mA secondary), rectified by a full-wave rectifier comprising diodes D1 through D4 and filtered by capacitor C1. Assemble the circuit on a generalpurpose PCB with adequate spacing between the components. Sleeve the exposed leads of the components. Using switch S1 you can turn on the lamp manually. Enclose the unit in a plastic case and mount at a location that allows adequate daylight. Caution. Since the circuit uses 230V AC, many of its points are at AC mains voltage. It could give you lethal shock if you are not careful. So if you don’t know much about working with line voltages, do not attempt to construct this circuit. EFY will not be responsible for any kind of resulting loss or damage.
e l e c t ro n i c s f o r yo u • F e b r ua ry 2 0 0 9 • 9 5
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Versatile Probe
Raju R. Baddi edi
Y
ou can use this versatile probe for continuity testing and identification of transistor type and transformer windings. The n-side or p-side of a transistor can be identified quickly in one go. You can make two contacts with the probe in one hand
Fig. 1: Circuit of versatile probe
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while the other hand is free. Fig. 1 shows the circuit of the probe. The operation of the circuit is simple. It is driven by an alternating current flowing through two LEDs (LED1 and LED2). So one LED corresponds to forward direction of current flow, while the other shows reverse direction of current flow. This helps to detect orientation of the p-n junction with respect to the probes. The LEDs can be arranged near the probes to glow either for the p-side or the n-side as per your choice. The frequency is determined by capacitor C1 and preset VR1 connected between gates G1
Fig. 2: Constructional detail of versatile probe
Testing Results for Different Components Component
Probe D
Probe C
Red LED
Green LED
1st terminal 1st terminal
2nd terminal 2nd terminal
Off On
On Off
Transistors Any type pnp or npn
C E
E C
X X
X X
npn-type transistor
B B
E C
On On
Off Off
p-n junction p-n junction Result: ‘p’ is common, so npn transistor
pnp-type transistor
B B
E C
Off Off
On On
n-p junction n-p junction Result: ‘n’ is common, so pnp transistor
Primary terminal 1
Primary terminal 2
Glow with low intensity
Glow with low intensity
Both LEDs glowing with low intensity Result: Primary side
Glow with high intensity
Glow with high intensity
Both LEDs glowing with high intensity Result: Secondary side
On
Off
Diode
Step-down transformer Continuity
Secondary terminal 1 Secondary terminal 2 Connect with LEDs probe
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Result Probe D side is anode (p) and probe C side is cathode (n) Probe D side is ‘n’ and probe C side is ‘p’ Unused pin is base Unused pin is base
Indicates shorting
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and G2. The frequency can be varied using preset VR1. Higher frequency results in more sensitivity to inductive reactance. The preset is trimmed so that when the probes are shorted, both the LEDs glow equally. Fig. 2 shows the probe arrangement for testing. Most of the battery power is consumed only when the LEDs glow. The probes have been constructed to provide a good grip on the components
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under testing. One probe’s tip has been widened. (Drop the empty refill of a ball-pen from some height to remove the ball, then insert a sharp needle or something similar into the tip. Slowly push the needle inside and widen the tip so that a component lead can be inserted into it during testing.) Slightly unequal probe lengths help to make easy contacts. Assemble the circuit on a general-
purpose PCB which is as compact as possible and put it inside a glue stick tube (whose inner mechanism has been removed) at its centre. The metallic disk and metallic strips can be cut out from any tin container. For the probes, use the spring mechanism of gel ball pens. Probes C and D are the points representing the probe terminals. Two button cells (CR2032) are used to power the probe circuit.
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MUSICAL LIGHT CHASER
DEBARAJ KEOT
T
his music-operated lighting effect generator comprises five sets of 60W bulbs that are ar-
ranged in zig-zag fashion. The bulb sets glow one after another depending on the intensity of the audio signal. No electrical connection is to be made between the music system and the
IVEDI S.C. DW
lighting effect generator circuit. You just need to place the gadget near the speakers of the music system. Fig. 1 shows the complete circuit of the musical light chaser, while Fig.
Fig. 1: Circuit diagram of musical light chaser
82 • JANUARY 2005 • ELECTRONICS FOR YOU
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2 shows pin configurations of 9V regulator 7809, triac BT136 and level meter IC LB1403. The circuit is powered by regulated 9V DC. The AC mains is stepped down by transformer X1 to deliver a secondary output of 12V AC at 250 mA. The transformer output is rectified by a full-wave rectifier comprising diodes D1 and D2 and filtered by capacitors C1 and C2. Regulator IC 7809 (IC1) provides regulated 9V power supply to the circuit. Closing switch S1 provides power to the circuit and LED1 glows to indicate that the circuit is ready to work. When you put your music system in front of the condenser microphone of the light chaser, the sound pressure variation is converted into electrical signals by the condenser microphone. These weak electrical signals are amplified by op-amp µA741 (IC2), which is configured as an inverting amplifier. Using preset VR1 you can set the sensitivity of the circuit. The amplified output is fed to IC LB1403 (IC3) at its input pin 8. IC3 is a five-dot LED level meter commonly used in stereo systems for LED bargraph displays. It has a built-in am-
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Fig. 2: Pin configuration
plifier, comparators and constant current source at its output pins. Depending on the intensity of the input audio signals, all or some outputs of IC3 go low to drive transistors T1 through T5, which, in turn, fire the corresponding triacs TR1 through TR5 via their gates and multicoloured zigzag bulb sets comprising ZL1 through ZL5 glow. When the audio level is low, only triac T1 is fired and the zig-zag bulb set ZL1 turns on and off sequentially. When the audio level is high, triacs TR1 through TR5 get fired and all the bulb sets (ZL1 through ZL5) turn on and off sequentially. Pin 7 of IC3 is used for selecting the response speed of the lighting. The larger the time constant, the slower the response, and vice versa.
The time constant can be changed by changing the values of resistor R6, variable resistor VR2 and capacitor C7. Here, variable resistor VR2 is used for varying the response speed of the chaser light as desired. When VR2 is set in the minimum resistance position, the response is very fast, and when it is set at the maximum resistance, the response is slow. The complete circuit including the power supply can be constructed on any general-purpose PCB or a small Vero board. Triacs TR1 through TR5 should be kept away from the op-amp and its related components. The metallic parts of the triacs should not touch each other and the other parts of the circuit. After assembling the circuit, house it in a suitable shockproof plastic cabinet. Make some holes in the cabinet for heat dissipation. Note. 1. Some zig-zag lights have a special bulb called ‘master bulb’ for automatic flickering. It should be removed and replaced with a simple non-flickering colour bulb. 2. Never touch any naked part of the circuit when it is connected to the mains. z
ELECTRONICS FOR YOU • JANUARY 2005 • 83
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Electronic Candles
Raj K. Gorkhali
H
ere is a simple circuit that can produce the effect of candle light in a normal electric bulb. A candle light, as we all know, resembles a randomly flickering light. So, the objective of this project activity is to produce a randomly flickering light effect in an electric bulb. To achieve this, the entire circuit can be divided into three parts. The first part comprises IC1 (555), IC2 (74LS164), IC3 (74LS86), IC4 (74LS00) and the associated components. These generate a randomly changing train of pulses.
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gate trigger circuit components. It is basically half-wave AC power being supplied to the electric bulb. The third part is the power supply circuit to generate regulated 5V DC from 230V AC for random signal generator. It comprises a stepdown transformer (X1), full-wave rectifier (diodes D3 and D4), filter capacitor (C9), followed by a regulator (IC5). The random signal generator of the circuit is built around an 8-bit serial in/parallel out shift register (IC2). Different outputs of the shift register IC pass through a set of logic gates (N1 through N5) and final out-
to provide better flickering effect in the bulb. The random signal triggers the gate of SCR1. The electric bulb gets AC power only for the period for which SCR1 is fired. SCR1 is fired only during the positive half cycles. Conduction of SCR1 depends upon the gate triggering pin 3 of IC2, which is random. Thus, we see a flickering effect in the light output. Assemble the circuit on a generalpurpose PCB and enclose it in a suitable
put appearing at pin 6 of gate N5 is fed back to the inputs of pins 1 and 2 of IC2. The clock signal appears at pin 8 of IC2, which is clocked by an astable multivibrator configured around timer (IC1). The clock frequency can be set using preset VR1 and VR2. It can be set around 100 Hz
case. Fix bulb and neon bulb on the front side of the cabinet. Also, connect a power cable for giving AC mains supply to the circuit for operation. The circuit is ready to use. Warning. Since the circuit uses 230V AC, care must be taken to avoid electric shock.
Fig. 1: Circuit diagram for electronic candle
Fig. 2: Pin configurations of C106 and 7805
The second part of the circuit consists of SCR1 (C106), an electric bulb connected between anode of SCR1 and mains live wire, and
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Automatic sprinkler control system
Raj K. Gorkhali
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f you have an automatic sprinkler control system, chances are that you are currently using all of its outputs controlling sprinklers. The problem arises when you want to add another sprinkler control solenoid valve. Using this circuit, you can add an expansion module to the control-
filtered by capacitors C1 and C2. The rectified DC is fed to the timer circuit through resistor R1 and zener diode ZD1 to produce 12 volts to enable the timer circuit. IC 555 (IC1) is wired as an astable mutivibrator producing about 7Hz pulses. You can change this frequency by changing the values of timing components R2, R3 and C4. The output
conducts to forward-bias the internal LED of optocoupler IC3. Triac1 is controlled through IC3, which provides gate current through resistor R12 to turn it on. As a result, solenoid valve-1 activates to provide path for water to flow. After 20 minutes, Q14 of IC2 goes high, making transistor T1 conduct.
ler without upgrading the controller or running new wiring. The circuit is fitted in place of an existing solenoid and allows the controller to switch on two valves (hence two sprinklers) instead of one. Designed for use with standard 24V AC controller systems, the circuit uses the 24V output to power a timer which activates one of the valves for, say, 20 minutes. After 20 minutes, the circuit switches this valve ‘off’ and the other valve ‘on.’ The second valve is activated for the remainder of time that the controller is programmed to give for that station. The 24V AC from the controller is half-wave-rectified by diode D1 and
of IC1 is fed to clock pin 10 of IC2. The 7Hz frequency is further divided by IC2, which is a 14-stage binary counter. IC2 is reset through capacitor C6 and resistor R4 when power switch S1 is closed. Output pin 3 of IC1 goes low when it resets. Counter IC2 is then clocked at 7 Hz, with its final output (Q14) going high after 20 minutes. This high output of IC2 activates the solenoid valves via optocoupled triacs built around IC3 and IC4. Working of the circuit is simple. When you power-on the circuit using switch S1, IC1 is enabled and IC2 resets. Q14 of IC2 remains low, making transistor T1 cut-off. Transistor T2
Transistor T2 cuts off and the internal LED of optocoupler IC4 is forward-biased. Triac2 is controlled through IC4, which provides gate current through resistor R9 to turn it on. As a result, solenoid valve-2 activates to give path for water flow. Timer IC1 is disabled as soon as the output of IC2 goes high, making its reset pin 4 low to stop clocking to IC2 and so its output remains high. Triac2 has a snubber circuit built around R10 and C6. Triac1 too has a snubber circuit built around R13 and C7. The snubber circuit is used to suppress any voltage spike produced by switching the solenoid valve coils. Assemble the circuit on a general-
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purpose PCB and house in a waterproof plastic box with an outlet hole cut in it for the wiring. Seal the wiring hole and box lid with a generous application of silicon sealant. before installing the circuit in the sprinkler control system, test it by providing 24V AC as the input and
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using suitable light bulbs as loads on the output. Once the timer is ready for installation, remove one control valve from the controller and replace it with 24V AC input of this circuit (terminals ‘A’ and ‘B’ in the circuit diagram). Connect the ground terminal of the circuit
to the solenoid valve as shown in the circuit. The timer is set here such that valve 1 remains open for 20 minutes and after 20 minutes, valve 1 closes and valve 2 opens for the following 20 minutes. valve 2 remains open until switch S1 is closed.
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AUTOMATIC PHASE CHANGER
MUHAMMAD AJMAL P.
IVEDI S.C. DW
n three-phase applications, if low voltage is available in any one or two phases, and you want your equipment to work on normal voltage, this circuit will solve your problem. However, a proper-rating fuse needs to be used in the input lines (R, Y and B) of each phase. The circuit provides correct voltage in the same power supply lines through relays from the other phase where correct voltage is available. Using it you can operate all your equipment even when correct voltage is available on a single phase in the
The mains power supply phase R is stepped down by transformer X1 to deliver 12V, 300 mA, which is rectified by diode D1 and filtered by capacitor C1 to produce the operating voltage for the operational amplifier (IC1). The voltage at inverting pin 2 of oprational amplifier IC1 is taken from the voltage divider circuit of resistor R1 and preset resistor VR1. VR1 is used to set the reference voltage according to the requirement. The reference voltage at non-inverting pin 3 is fixed to 5.1V through zener diode ZD1. Till the supply voltage available in phase R is in the range of 200V-230V,
As soon as phase-R voltage goes below 200V, the voltage at inverting pin 2 of IC1 goes below reference voltage of 5.1V, and its output goes low. As a result, transistor T1 conducts and relay RL1 energises and load L1 is disconnected from phase ‘R’ and connected to phase ‘Y’ through relay RL2. Similarly, the auto phase-change of the remaining two phases, viz, phase ‘Y’ and phase ‘B,’ can be explained. Switch S1 is mains power ‘on’/’off’ switch.
building. The circuit is built around a transformer, comparator, transistor and relay. Three identical sets of this circuit, one each for three phases, are used. Let us now consider the working of the circuit connecting red cable (call it ‘R’ phase).
the voltage at inverting pin 2 of IC1 remains high, i.e., more than reference voltage of 5.1V, and its output pin 6 also remains high. As a result, transistor T1 does not conduct, relay RL1 remains de-energised and phase ‘R’ supplies power to load L1 via normallyclosed (N/C) contact of relay RL1.
Use relay contacts of proper rating and fuses should be able to take-on the load when transferred from other phases. While wiring, assembly and installation of the circuit, make sure that you: 1. Use good-quality, multi-strand insulated copper wire suitable for your
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ELECTRONICS FOR YOU • JULY 2007 • 93
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current requirement. 2. Use good-quality relays with proper contact and current rating. 3. Mount the transformer(s) and relays on a suitable cabinet. Use a Tag Block (TB) for incoming/outgoing con-
94 • JULY 2007 • ELECTRONICS FOR YOU
nections from mains. EFY Note: 1. During testing in the lab, we used a 12V, 200-ohm, singlephase changeover relay with 6A current rating. Similarly, ampere-rated fuses were used.
2. If the input voltage is low in two phases, loads L1 and L2 may also be connected to the third phase. In that situation, a high-rating fuse will be required at the input of the third phase which is taking the total load.
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Skin Response Meter
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D. Mohan Kumar
H
uman skin offers some resistance to current and voltage. This resistance changes with the emotional state of the body. The circuit proposed here measures changes in your skin resistance following changes in your mental state. In the relaxed state, the resistance offered by the skin is as high as 2 mega-ohms or more, which reduces to 500 kilo-ohms or less when the emotional stress is too high. The reduction in skin resistance is related to increased blood flow and permeability followed by the physiological changes during high stress. This increases the electrical conductivity of the skin. This circuit is useful to monitor the skin’s response to relaxation techniques. It is very sensitive and shows response during a sudden moment of stress. Even a deep sigh will give response in the circuit. The circuit uses a sensitive amplifier to sense variations in the skin resistance. IC CA3140 (IC1) is designed as a resistance-to-voltage converter that outputs varying voltage based on the skin’s conductivity. It is wired as an inverting amplifier to generate constant current to skin in order to measure the skin resistance. IC CA3140 is a 4.5MHz BiMOS operational amplifier with MOSFET inputs and bipolar output. The gate-protected inputs have high impedance and can sense current as low as 10 pA. This device is ideal to sense small currents in low-input-current applications. The inverting input (pin 2) of IC1 is connected to ground (through preset VR1) and one of the touch plates, while the non-inverting input (pin 3) is grounded directly. The output from IC1 passes through current-limiting resistor R1 to the second touch plate. R1 act as a feedback 8 8 • J u n e 2 0 0 9 • e l e c t ro n i c s f o r yo u
resistor along with the skin when the touch plates make contact with the skin. So the gain of IC1 depends on the feedback provided by R1 and the skin. In the inverting mode of IC1, a positive input voltage to its pin 2 through the feedback network makes its output low. If the skin offers very high resistance in the relaxed state, input voltage to pin 2 reduces and the output remains high. Thus the gain of IC1 varies depending on the current passing through the skin, which, in turn, depends on the skin response and emotional state. In the standby state, touch plates are free. As there is no feedback to IC1, it gives a high output (around 6 volts), which is indicated by shifting of the meter to right side. When the touch plates are shorted by the skin, the feedback circuit completes and the output voltage reduces to 4 volts or less depending on the resistance of the skin. Since the feedback network has a fixed resistor (R1) and VR1 is set to a fixed resistance value, the current flowing through it depends only on the resistance of the skin. The output from IC1 is displayed on a sensitive moving coil meter (VU meter). By varying preset VR2, you can adjust the sensitivity of the meter. For easy visual observation, an LED display is also included. IC LM3915 (IC2) is used to give a logarithmic display through LED indications. It can sink
current from pin 18 to pin 10 with each increment of 125 millivolts at its input pin 5. Using VR3 you can adjust the input voltage of IC2, while using VR4 you can control the brightness of the LEDs. In practice, the circuit provides both meter reading and LED indications. If the LED display is not needed, IC2 can be omitted. Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet with touch pads glued on the top, 5-10 mm apart. Touch pads can be any type of conducting plates, such as aluminium or copper plates, having dimensions of 1×1 cm2. The moving coil meter can be a small VU meter with 1-kilo-ohm coil resistance and 0-10 digit reading. After assembling the circuit, adjust the presets such that IC1 outputs around 6 volts. None of the LEDs (LED1 through LED3) glows in this position with the touch plates open. Now gently touch the touch plates with your middle finger. Maintain the finger still allowing one minute to bond with the pads and keep your body relaxed. Adjust VR3 until the green LED (LED1) lights up and the meter shows full deflection. Adjust VR2 to get maximum deflection of the meter. This indicates normal resistance of the skin, provided the body is fully relaxed. If you are stressed or have ill feeling, skin resistance decreases and the blue LED lights up followed by the red LED along with a deflection of the meter towards the lower side. In short, the red LED and zero meter reading indicate you are stressed, and the green LED and high meter reading indicate you are relaxed. Practise some relaxation technique and observe how much your body is relaxed. w w w. e f y m ag . co m
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MAINS SUPPLY FAILURE ALARM
IVEDI S.C. DW
T.K. HAREENDRAN
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henever AC mains supply fails, this circuit alerts you by sounding an alarm. It also provides a backup light to help you find your way to the torch or the generator key in the dark. The circuit is powered directly by a 9V PP3/6F22 compact battery. Pressing of switch S1 provides the 9V power supply to the circuit. A red LED (LED2), in conjunction with zener diode ZD1 (6V), is used to indicate the battery power level. Resistor R9 limits the operating current (and hence the brightness) of LED2. When the battery voltage is 9V, LED2 glows with full intensity. As the battery voltage goes below 8V, the intensity of LED2 decreases and it glows very dimly. LED2 goes off when the battery voltage goes below 7.5V. Initially, in standby state, both the LEDs are off and the buzzer does not sound. The 230V AC mains is directly fed to mains-voltage detection optocoupler IC MCT2E (IC1) via resistors R1, R2 and R3, bridge rectifier BR1 and capacitor C1. Illumination of the LED inside optocoupler IC1 activates its internal phototransistor and clock input pin 12 of IC2 (connected to 9V via N/C contact of relay RL1) is pulled low. Note that only one monostable of dual-monostable multivibrator IC
CD4538 (IC2) is used here. When mains goes off, IC2 is triggered after a short duration determined by components C1, R4 and C3. Output pin 10 of IC2 goes high to forward bias relay driver transistor T1 via resistor R7. Relay RL1 energises to activate the piezobuzzer via its N/O contact for the time-out period of the monostable multivibrator (approximately 17 minutes). At the same time, the N/C contact removes the positive supply to resistor R4. The time-out period of the monostable multivibrator is determined by R5 and C2. Simultaneously, output pin 9 of IC2
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goes low and pnp transistor T2 gets forward biased to light up the white LED (LED1). Light provided by this back-up LED is sufficient to search the torch or generator key. During the mono time-out period, the circuit can be switched off by opening switch S1. The ‘on’ period of the monostable multivibrator may be changed by changing the value of resistor R5 or capacitor C2. If mains doesn’t resume when the ‘on’ period of the monostable lapses, the timer is retriggered after a short delay determined by resistor R4 and C3. z
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MAT SWITCH
D. MOHAN KUMAR
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his simple circuit produces a warning beep when somebody crosses a protected area in your home or office. The switch, hidden below the floor mat, triggers the alarm when the person walks over it. The circuit uses a conductive foam
92 • MAY 2007 • ELECTRONICS FOR YOU
IVEDI S.C. DW
as the switch. It can be two small pieces of conductive pads usually used to pack sensitive ICs as antistatic cover. Alternatively, you can make the switch by coating conducting carbon ink on two small pieces of a copper-clad board. When the circuit is in standby mode, transistor T1 does not conduct, since its base is floating. When the person walks, the switch is pressed and current flows through R1 and the switch to provide positive bias to transistor T1. Transistor T1 conducts and its collector voltage drops, which acts as a negative trigger input for the monostable
wired around IC NE555 (IC1). IC1 outputs a pulse of fifty-seconds duration with preset values of R4 and C3. This pulse is applied to the buzzer through transistor T2. The buzzer sounds a warning beep on unauthorised entry. The pulse duration can be changed to the desired value by changing the values of R4 and C 3 . Resistor R2 in the circuit makes the trigger pin of IC1 high to prevent false triggering. Assemble the circuit on a generalpurpose PCB and enclose in a plastic case. Use a 9V battery to power the circuit. Connect the touchpad switch with the PCB and hide under the mat at the entrance. The PCB can be mounted on the nearby wall. Make the switch carefully using conducting foam or copper clad coated with conducting ink. Place the two pieces with their conducting surface facing each other. Solder carefully a thin copper electric wire and ensure that it makes contact when the two plates touch together on pressing. Provide two 1cm rubber tabs between the plates to avoid touch in the standby mode.
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Water-Level Indicator using 7-segment display
Riju Thazhathu Veettil
his water-level indicator uses a 7-segment display, instead of LEDs, to indicate the water level (low, half and full) in the tank. Moreover, a buzzer is used to alert you of water overflowing from the tank. The circuit shows the water level by displaying L, H and F for low, half and full, respectively. The circuit uses five sensors to sense the different water levels in the
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a high voltage at the input pin of the NOT gate, it outputs a low voltage. Similarly, for a low voltage at the input pin of the NOT gate, it outputs a high voltage. When the tank is empty, the input pins of IC 7404 are pulled high via a 1-mega-ohm resistor. So it outputs a low voltage. As water starts filling the tank, a low voltage is available at the input pins of the gate and it outputs a high voltage.
tank. Sensor A is connected to the negative terminal (GND) of the power supply. The other four sensors (B through E) are connected to the inputs of NOT gate IC 7404. When there is
When the water in the tank rises to touch the low level, there is a low voltage at input pin 5 of gate N3 and high output at pin 6. Pin 6 of the gate is connected to pin 10 of gate N9, so pin
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10 also goes high. Now as both pins 9 and 10 of gate N9 are high, its output pin 8 also goes high. As a result, positive supply is applied to DIS3 and it shows ‘L’ indicating low level of water in the tank. Similarly, when water in the tank touches the half level, pins 4 and 5 of AND gate N8 become high. As a result, its output also goes high and DIS2 shows ‘H’ indicating half level of water in the tank. At this time, pin 9 of gate N9 also goes low via gate N4 and DIS3 stops glowing. When the water tank becomes full, the voltage at pin 1 of gate N1 and pin 3 of gate N2 goes low. Output pin 3 of gate N7 goes high and DIS1 shows ‘F’ indicating that the water tank is full. When water starts overflowing the tank, pin 13 of gate N6 goes low to make output pin 12. The buzzer sounds to indicate that water is overflowing the tank and you need to switch off the motor pump. Assemble the circuit on a general-purpose PCB and enclose in a suitable box. Use a non-corrosive material such as steel strip for the five sensors and hang them in the water tank as shown in the circuit diagram. Use regulated 5V to power the circuit.
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APPLIANCE TIMER-CUM-CLAP SWITCH
PANKAJ D. CHOUDHURY
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hen planning for a weekend outing to return late in the evening, we are often in an ambivalence whether to leave the staircase/outside light ‘on’ or ‘off.’ We sometimes miss our favourite TV programme because we forget to switch on the TV in time. If we are in the habit of taking an afternoon nap, we either turn on the mosquito repellent earlier than required or get up being bitten by mosquitoes. The timer-cum-clap switch presented here can solve all these problems and many more. It is a simple circuit that can be programmed to turn on household appliances like lights, fans, TV sets, music systems, etc exactly at a preset time and turn off at another preset time automatically, thereby saving on electricity. You can
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turn the appliance ‘on’ or ‘off’ with the clap of your hand, if so desired, without having to touch the unit physically. The transistor-based timer circuit uses readily available components, is easy to assemble as well as inexpensive, and can be programmed to switch on/off a load from one second to 100 hours in advance. To make the circuit cost-effective as well as simple to construct, a general-purpose digital clock is incorporated as the basic timing device. The alarm output of the clock is used to toggle the output power supply for switching an appliance ‘on’ or ‘off.’ Transistors T6 and T7 are configured as a bistable flip-flop that has two stable states. Transistor T7 will be in cut-off mode corresponding to transistor T6 in conduction mode, and vice versa. When transistor T6 conducts, its
IVEDI S.C. DW
collector potential is very near to the emitter potential, i.e., ground, and therefore there is no base current to transistors T7 through R6. Thus, transistor T7 is in cut-off state. The collector of T7 is above ground potential and the current flows through resistors R7 and R13 to maintain the base current of T6. Thus, T6 remains in conduction state and T7 in cut-off state indefinitely. Now, if a voltage pulse is applied to the base of transistor T7 from some external source, a momentary base current will trigger it into conduction and its collector potential will come down to near ground potential. Thus, the
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current flowing through resistor R13 will pass through the collector of T7 and there will be no current through R7, making T6 go into cut-off state and thereby raising the collector potential of T6 to some positive value. This, in turn, will keep T7 conducting. Now the base current of T7 will pass through resistors R14 and R6. This state will sustain until some external voltage is applied to the base of T6. The external voltage pulse (for switching) is taken from two sources: the alarm output of a clock or the sound picked up by condenser microphone ‘M’ after proper amplification by transistors T1, T2 and T3. Since most of the digital clocks give out negative pulses to the buzzer (whose other end is directly connected to the positive terminal of the battery), a reverse diode (D8) and a pnp transistor (T10) are used at this stage. The negative pulses are rectified by D8 and filtered by C9 to supply a steady base current of T10. Otherwise, the output will become noisy because of the pulsating nature of the alarm. (If the clock gives out positive pulses, T10 can be replaced with an npn transistor like BC547. Diode D8 has to be reversed and R18 has to be connected between the base of T10 and ground.) The external voltage pulse is fed at the common emitter of transistors T4 and T5 through capacitor C8. When the alarm starts (sending negative voltage pulses), capacitor C9 discharges through D8 and, at the same time, charges through R19, thus triggering the base current of T10. The emitter current of T10 charges capacitor C8, which passes through the emitter of either T4 or T5 depending on their bias. When T6 is conducting, T4 is forward biased and the voltage pulse is fed at the base of T7, bringing T7 into conduction and T6 into cut-off mode. This makes T5 forward-biased and T4 reverse-biased. The next voltage pulse, either through T10, D1 or D2 corresponding to the clock alarm, clap sound or operation of the reset switch, sends a base current of T6 through the
emitter of T5 and the output changes over. When clap switch is not required, S2 can be turned off. S3 is the reset switch (push-to-on type), which is used to toggle the output between ‘on’ and ‘off’ states. R10-C7 and R8C6 are parallel paths to R7 and R6 for quick switchover of the bistable latch. Two AA-size batteries supply 3V DC to the clock and maintain a positive voltage to the collectors of T6 and T7 through diode D7. This keeps the circuit active during power failures also. A step-down transformer supplies 12V DC to the relay coil and sound amplifier section. Diodes D5 and D6 are rectifier diodes and C5 is the ripple filter capacitor. Diode D4 prevents the 3V battery from draining out into the rest of the circuit. The digital clock is a commonly available digital calendar with at least one alarm setting and one countdown timer setting. The digital calendar, being cheap, keeps the total cost of the project low and allows for precise settings of the alarm times. The alarm can be set 24 hours in advance, while a second alarm can be selected in the countdown timer mode, which allows for setting of the time 100 hours (99:59:59 hours to be precise) in advance. Availability of more than one alarm setting in the clock will give the added advantage of setting multiple switching times. Instead of the digital calendar, any other digital clock or battery-operated quartz clock (with alarm) can also be used as the basic timing device, though the alarm time setting is less precise in case of the latter. Instead of one clock, multiple clocks can be wired by connecting diodes parallel to D8. Note that once set in the clock mode, the alarm operates daily at the same time. But in the countdown mode, it operates only once. So if an appliance is to be turned on and off daily at the same time without human intervention, at least two digital clocks have to be wired (if the clock does not have two alarm settings apart from the countdown timer).
100 • NOVEMBER 2006 • ELECTRONICS FOR YOU
This simple circuit can be assembled on a general-purpose PCB. The clock, battery, switches, relay, transformer, etc are wired with the PCB (not shown in the circuit). A plastic switchboard (available in electrical shops) can be used as the cabinet for assembling the unit. Holes can be drilled easily on the plastic cabinet. House the PCB, transformer, relay, etc inside the cabinet. Fix the plug socket, switches and external connector on the rear side of the cabinet. Indicator LEDs (fixed on LED sockets) on the front panel show ‘on’ or ‘off’ condition of the output plug. Glue the condenser microphone inside the front or side wall with small holes drilled in front of it to receive external sound. The battery chamber housing two pencil cells can be fixed inside the cabinet or on the rear of the cabinet as per convenience. The clock is glued on top of the cabinet. Before fixing the clock on the cabinet, open it carefully to disconnect its piezoelectric buzzer. The terminal that shows pulsating voltage during an alarm operation (detected with a multimeter) is connected to the base of T10 through D8 and R19. The internal battery is replaced and the terminals are connected to the external battery chamber with proper polarity. The operation of the circuit can be divided into two parts: clap mode and timer mode. The timer can be put in clap mode by turning on the clap switch (S2). The connected appliance can now be turned on/off by clapping with an audible intensity. The clock timer will function as usual in this mode. While clapping, leave a gap of a few seconds between two successive claps. Thus, the gadget will show better response because it has been designed to consider two overlapping claps as one, ignoring the second one. For timer mode, switch S2 is turned off. The alarm is set at the time when switchover is required. The second switchover time can be set in the countdown timer. For that, the time difference between the present time and the time at which switching is reWWW.EFYMAG.COM
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quired is calculated and this time is set in the countdown timer. When setting is done, set the output plug as ‘on’ or ‘off’ (as desired) by pressing
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reset switch S3. While setting the alarm, ensure a delay of at least three minutes between two successive alarm times (on/
off) to allow for the first alarm to subside completely. Otherwise, the unit may malfunction (ignore the second alarm).
ELECTRONICS FOR YOU • NOVEMBER 2006 • 101
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WIRELESS SWITCH
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Navpreet Singh Tung
N
ormally, home appliances are controlled by means of switches, sensors, etc. However, physical contact with switches may be dangerous if there is any shorting. The circuit described here requires no physical contact for operating the appliance. You just need to move your hand between the infrared LED (IR LED1) and the phototransistor (T1). The infrared rays transmitted by
IR LED1 is detected by the phototransistor to activate the hidden lock, flush system, hand dryer or else. This circuit is very stable and sensitive compared to other AC appliance control circuits. It is simple, compact and cheap. Current consumption is low in milliamperes. The circuit is built around an IC CA3140, IRLED1, phototransistor and other discrete components. When regu-
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lated 5V is connected to the circuit, IR LED1 emits infrared rays, which are received by phototransistor T1 if it is properly aligned. The collector of T1 is connected to non-inverting pin 3 of IC1. Inverting pin 2 of IC1 is connected to voltage-divider preset VR1. Using preset VR1 you can vary the reference voltage at pin 2, which also affects sensitivity of the phototransistor. Op-amp IC1 amplifies the signal received from the phototransistor. Resistor R3 controls the base current of transistor BC548 (T2). The high output of IC1 at pin 6 drives transistor T2 to energise relay RL1 and switch on the appliance, say, hand dryer, through the relay contacts. The working of the circuit is simple. In order to switch on the appliance, you simply interrupt the infrared rays falling on the phototransistor through your hand. During the interruption, the appliance remains on through the relay. When you remove your hand from the infrared beam, the appliance turns off through the relay. Assemble the circuit on any general-purpose PCB. Identify the resistors through colour coding or using the multimeter. Check the polarity and pin configuration of the IC and mount it using base. After soldering the circuit, connect +5V supply to the circuit.
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CAR-REVERSING HORN WITH FLASHER
Ashok K. Doctor
H
ere is a simple circuit that starts playing the car horn whenever your car is in reverse gear. The circuit (refer Fig. 1) employs dual timer NE556 to generate the sound. One of the timers is wired
and D2 goes high for a few seconds depending on the time period developed through resistor R4 and capacitor C4. At this point, the astable multivibrator is enabled to start oscillating. The output of the astable multivibrator is fed to the speaker through capacitor C6. The speaker,
Fig. 2: Flasher circuit
Fig. 1: Car reverse horn
as an astable multivibrator to generate the tone and the other is wired as a monostable multivibrator. Working of the circuit is simple. When the car is in reverse gear, reverse-gear switch S1 of the car gets shorted and the monostable timer triggers to give a high output. As a result, the junction of diodes D1
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in turn, produces sound until the output of the monostable is high. When the junction of diodes D1 and D2 is low, the astable multivibrator is disabled to stop oscillating. The output of the astable multivibrator is fed to the speaker through capacitor C6. The speaker, in turn, does not produce sound. Assemble the circuit on a generalpurpose PCB and enclose in a suitable cabinet. Connect the circuit to the car
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reverse switch through two wires such that S1 shorts when the car gear is reversed and is open otherwise. To power the circuit, use the car battery. The flasher circuit (shown in Fig. 2) is built around timer NE555, which is wired as an astable multivibrator that outputs square wave at its pin 3. A 10W auto bulb is used for flasher. The flashing rate of the bulb is decided by preset VR1. Assemble the circuit on a generalpurpose PCB and enclose in a suitable cabinet. The flasher bulb can be mounted at the car’s rear side in a reflector or a narrow painted suitable enclosure. EFY note. A higher-wattage bulb may reduce the intensity of the headlight. You can enclose both the carreversing horn and flasher circuits together or separately in a cabinet in your car.
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SHUTTER GUARD
D. MOHAN KUMAR
IVEDI S.C. DW
his sensitive vibration sensor is exclusively made for shops to protect against burglary. It will detect any mechanical or acoustic vibration in its vicinity when somebody tries to break the shutter and immedi-
T
to enable reset pin 4 of IC2 so that it can function freely. Transistor T1 amplifies the piezo-sensor signal and triggers monostable IC2. The base of transistor T1 is biased using a standard piezo element that acts as a small capacitor and flexes freely in response to mechanical vibrations so that the
generates a tone simulating the police siren with R11 as its oscillationcontrolling resistor. Zener diode ZD1 provides stable 3.1V DC for the tonegenerating IC. Assemble the circuit on a general-
ately switch on a lamp and sound a warning alarm. A 15-minute time delay after switch-on allows sufficient time for the shop owner to close the shutter. The front end of the circuit has a timer built around the popular binary counter IC CD4060 (IC1) to provide 15-minute time delay for the remaining circuitry to turn on. Resistors R3 and R4 and capacitor C2 will make Q9 output high after 15 minutes. Diode D1 inhibits the clock input (pin 11) to keep the output high till the power is switched off. Blinking LED1 indicates the oscillation of IC1. The high output from IC1 is used
output of IC2 is high till the prefixed time period. In the standby mode, the alarm circuit built around IC3 remains dormant as it does not get current. Timing components R8 and C6 make the output of IC2 high for a period of three minutes. When any mechanical vibration (caused by even a slight movement) disturbs the piezo element, trigger pin 2 of IC2 momentarily changes its state and the output of IC2 goes high. This triggers triac 1 and the alarm circuit activates. Triac BT136 completes the lamp circuit by activating its gate through resistor R9. IC UM3561 (IC4)
purpose PCB and enclose in a suitable, shockproof case. Connect the piezo element to the circuit by using a single-core shielded wire. Glue a circular rubber washer on the fine side of the piezo element and fix it on the shutter frame with the washer facing the frame so that the piezo element is flexible to sense the vibrations. Fix the lamp and the speaker on the outer side and the remaining parts inside the case. Since triac is used in the circuit, most points in the PCB will be at mains lethal potential. So it is advised not to touch any part of the circuit while testing.
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ELECTRONICS FOR YOU • SEPTEMBER 2007 • 97
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Transistorised Logic Probe for TTL
Raju R. Baddi
A
logic probe is a handheld penlike probe used for analysing and troubleshooting the logical states ‘0’ or ‘1’ of a digital circuit. While most logic probes are powered by the circuit under test, some probes use batteries. These can be used for either TTL (transistor-transistor logic) or CMOS (complementary metallic oxide semiconductor) integrated circuit devices. The circuit described here can be used for TTL logic only, and shows the presence of either state (logic 1 or 0).
Fig. 1: Circuit of transistorised logic probe
As shown in Fig. 1, the logic probe circuit is built around four transistors and a few passive components. It uses two LEDs that show logic states 1 (green) and 0 (red). The green LED glows when the voltage at probe tip exceeds about 2.4V and the red LED glows when this voltage falls below about 1.2V, which is adequate to detect the normal TTL levels. When no LED glows, it means the probe is in suspended state, i.e., it is neither showing logic 1, nor logic 0. Working of the logic probe circuit is simple and can be divided into three states: The probe is suspended, the probe’s tip is at logic 1 and the probe’s tip is at logic 0. When the probe is in suspended condition, the junction voltage of resistors R1 and R2 is almost equal to that obtained when R1 and R2 alone form
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a voltage divider bias for the supply voltage. Under this condition, the baseemitter junction of transistor T1 is forward biased, so the voltage drop across resistor R5 (>0.6V) is enough to drive transistor T3 into conduction, which, in turn, cuts off transistor T4. As a result, the red LED (LED2) does not glow. The current flowing through R3 is nearly equal to that through R5. However, this does not produce enough voltage drop (0.3V-0.6V) across resistor R3 to make transistor T2 conduct. As a result, the green LED (LED1) does not glow. Thus neither of the LEDs glows when the probe is in suspended condition. When the probe tip is at logic 1, transistor T3 forward biases to cut-off transistor T4. But, this voltage should exceed 2.4V (normally TTL logic 1 is well above this voltage), this will cause a greater current to flow through emitter resistor R5 of T1, which also flows through collector resistor R3 causing a larger voltage drop across R3. Thus transistor T2 conducts and the green LED (LED1) glows to indicate the presence of logic 1 (high) at the probe tip. When the probe tip is at logic 0, transistor T3 cuts off to make transistor
Fig. 2: Proposed arrangement for compact logic probe
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e l e c t ro n i c s f o r yo u • S e p t e m b e r 2 0 0 9 • 1 1 9
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T4 conduct. As a result, the red LED (LED2) glows to indicate logic 0 (low). It can be easily seen from the circuit diagram that a voltage of more than 1.2 volts is required at the probe tip to forward-bias transistors T1 and T3 whose base-emitter junctions appear in series in the circuit. Any voltage less than this at the probe tip will not for-
ward-bias T3, resulting in the red LED (LED1) glowing. Assemble the circuit on a generalpurpose PCB and insert in a small plastic tube, say, a glue stick whose inner mechanism has been removed. For the probe tip, use the front portion of a gelpen refill. The proposed arrangement for the compact logic probe is shown
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in Fig. 2. Before checking the logic of any circuit, connect the black clip to the ground of the circuit and the red clip to the positive terminal of the circuit. The main feature of the circuit is that when the supply voltage is less than about 4.0V, the red LED (LED2) glows to indicate non-standard TTL voltage supply.
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SMART CELLPHONE HOLDER
T.K. HAREENDRAN
T
his smart cellphone holder makes sure that you don’t forget to carry your mobile phone. Fitted in the car, it keeps searching for the mobile phone within the holder using infrared (IR) rays and alerts you through a flashing LED when it doesn’t find one. You can attach the circuit to your existing cellphone holder or, with a little skill, construct one as per your requirement. The circuit, wired around IC LM555 (IC1), derives power from the 12V DC automobile battery. Diode D1 is an accidental wrong-polarity input guard. Resistor R7 limits the inrush current to IC1. When power is applied to the circuit, the low-frequency astable multivibrator built around IC1 is activated and LED2 at its output pin 3 flashes briefly. When ignition switch S2 is flipped to ‘on’ position, the +12V DC from the car’s battery disables the astable multivibrator via diode D2 and LED2 turns off. When the ignition is turned off and the mobile phone is in its holder, LED2 again starts blinking. In case the cellphone holder is empty, IR rays
Fig. 1: Circuit of smart cellphone holder
from IR LED1 fall on phototransistor T1 and it conducts to pull the base of LED driver transistor T2 Fig. 2: Pin configurations of BC547 and 2N5777 t o w a r d s ground to disable the visual indicator (LED2). If you’ve forgotten to carry your cellphone, LED2 fitted in the cellphone
94 • JULY 2005 • ELECTRONICS FOR YOU
IVEDI S.C. DW
holder will stop flashing to indicate that the mobile phone is not in the cell holder of the case. Resistor R1 limits the current flowing through IR LED1 and resistor R6 limits the operating current and hence luminance of LED2. Variable resistor VR1 determines the detection sensitivity of phototransistor T1. The blinking rate of LED2 can be changed by changing the value of capacitor C1 (or R3-R4 resistor combination). P i n configurations of BC547 and Fig. 3: Proposed cellphone holder phototransistor 2 N 5 7 7 7 , and the proposed cellphone holder are shown in Figs 2 and 3, respectively. z
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CIRCUIT IDEAS
MOSFET-BASED PREAMPLIFIER FOR FM RADIO DXing N.S. HARISANKAR,VU3NSH
F
M transmissions can be received within a range of 40 km. If you are in fringe areas, you may get a very weak signal. FM DXing refers to hearing distant stations (1500 km or more) on the FM band (88-108 MHz). The term ‘DX’ is
borrowed from amateur radio operators. It means ‘distance unknown’; ‘D’ stands for ‘distance’ and ‘X’ stands for ‘unknown.’ For an FM receiver lacking gain, or having a poor signal-to-noise ratio, using an external preamplifier improves the signal level. The dual-gate MOSFET preamplifier cir-
Fig. 1: Circuit of MOSFET-based preamplifier for FM DXing
Fig. 2: Different antennae used for FM DXing
cuit shown in Fig. 1 gives an excellent gain of about 18 dB. It costs less and is simple to design. Field-effect transistors (FETs) are superior to bipolar transistors in many applications as these have a
EO I TH SAN
much higher gain—approaching that of a vacuum tube. These are classified into junction FETs and MOSFETs. On comparing the FETs with a vacuum tube, the gate implies the grid, the source implies the cathode, and the drain implies the plate. In a transistor, the base implies the grid, the emitter implies the source, and the collector implies the drain. In dual-gate FETs, gate 1 is the signal gate and gate 2 is the control gate. The gates are effectively in series, making it easy to control the dynamic range of the device by varying the bias on gate 2. The MOSFET is more flexible because it can be controlled by a positive or negative voltage at gate 2. The resistance between the gate and rest of the device is extremely high because these are separated by a thin dielectric layer. Thus the MOSFET has an extremely high input impedance. Dual-gate MOSFETs (DG MOSFETs) are very popular among radio amateurs. These are being used in IF amplifiers, mixers, and preamplifiers in HF-VHF transceivers. The isolation between the gates (G1 and G2) is relatively high in mixer applications. This reduces oscillator pulling and radiation. The oscillator pulling is troublesome particularly in shortwave communications. It is a characteristic in many unsophisticated frequency-changer stages, where the incoming signal, if large, pulls the oscillator frequency slightly off the frequency set by the tuning knob and towards a frequency favourable to the (large) incoming signal. A DG MOSFET can also be used for automatic gain control in RF amplifiers. DG MOSFET BF966S is an n-channel depletion-type MOSFET that is used for general-purpose FM and VHF applications. In this configuration, it is used for FM radio band. The quadratic input characteristic of the FET input stage gives better results than the exponential characteristic of a bipolar transistor. Gate 1 is meant for input and gate 2 is for gain control. The input from the antenna is fed to gate G1 via C1 and L1. Trimmer VC1 is used to tune and select the input frequencies. Capacitor C4 (100 kpF) at the gain control electrode (gate 2) NOVEMBER 2003
ELECTRONICS FOR YOU
CIRCUIT IDEAS decouples any variation in G2 voltCoil & Capacitor Details for DXing age at radio frequencies to main10m band 6m band tain constant gain. Set preset VR (28 MHz) (50 MHz) (47k) to adjust the gain or connect amateur radio amateur radio a fixed resistor for fixed gain. The output of the circuit is obtained via L1 core 17T, 28 SWG 12T, 26 SWG capacitor C5 and fed to the FM reAmindoncore OnT-37-10,Tap ceiver amplifier. (T-50-6), Tap at 5T from GND For indoor use, connect a ¼at 6T from GND wavelength whip antenna, ½-waveL2 core 17T, 28 SWG 12T, 26 SWG length 1.5m wire antenna, or any Amindoncore On T-37-10, other indoor antenna set-up with T-50-6, without tap this circuit. You may use a 9V Without tap battery without the transformer and VC1 & VC2 60 pF 22 pF diode 1N4007, or any 6V-12V power supply to power the circuit (refer Fig. 1). The RF output can be taken on page 72 of Electronics Projects Vol. 8.) directly through capacitor C5. For an imMount the DG MOSFET BF966S at the proved input and output impedance, solder side of the PCB to keep parasitic change C1 from 1 kpF to 22 pF and C5 capacitance as small as possible. Use an from 1 kpF to 100 kpF. epoxy PCB. After soldering, clean the PCB For outdoor use at top mast, like a TV with isopropyl alcohol. Use a suitable booster, connect the C5 output to the enclosure for the circuit. All component power supply unit (PSU) line. Use RG58U/ leads must be small. Avoid shambled wirRG11 or RG174 cable for feeding the power ing to prevent poor gain or self oscillasupply to the receiver amplifier. The PSU tions. Connecting a single-element cubical for the circuit is the same as that of a TV quad antenna to the circuit results in booster. For TV boosters, two types of ‘Open Sesam’ for DXing. mountings are employed: The fixed tuned You can use a folded dipole or any booster is mounted on the mast of the other antenna. However, an excellent perantenna. The tunable booster consisting formance is obtained with a cubical quad of the PSU is placed near the TV set for antenna (refer Fig. 2) and Sangean ATSgain control of various TV channels. (For 803 world-band receiver. details, refer ‘High-Gain 4-Stage TV Booster’ In an amplifier, FET is immune to
ELECTRONICS FOR YOU
NOVEMBER 2003
of Various Frequency Bands 3m band (98 MHz) FM radio
2m band (144 MHz) FM radio
7T, 5mm dia., 20SWG Aircore, 1cm Length, Tap 3T from GND 8T, close winding, 5mm dia. Air-core, No tap
5T, 20SWG, ½-inch ID, ½-inch L Tap 2 from GND
22 pF
15 pF
4T, 20 SWG, No tap
strong signal overloading. It produces less cross-modulation than a conventional transistor having negative temperature coefficient, doesn’t succumb to thermal runaway at high frequencies, and decreases noise. In VHF and UHF, the MOSFET produces less noise and is comparable with JFETs. DG FETs reduce the feedback capacitance as well as the noise power coupled to the gate from the channel, giving stable unneutralised power gain for wide-band applications. This circuit can be used for other frequency bands by changing the input and the output LC networks. The table here gives details of the network components for DXing of stations at various frequency bands.
CIRCUIT IDEAS
FIRE ALARM USING THERMISTOR
I VED DWI S.C.
PRINCE PHILLIPS
I
n this fire alarm circuit, a thermistor works as the heat sensor. When temperature increases, its resistance decreases, and vice versa. At normal temperature, the resistance of the thermistor (TH1) is approximately 10 kilo-ohms, which reduces to a few ohms as the temperature increases beyond 100°C. The circuit uses readily available components and can be easily constructed on any generalpurpose PCB. Timer IC NE555 (IC1) is wired as an astable multivibrator oscillating in audio frequency band. Switching transistors T1 and T2 drive multivibrator NE555 (IC1). The output of IC1 is connected to npn transistor T3, which drives the loudspeaker (LS1) to generate sound. The frequency of IC1 depends on the values of resistors R5 and R6 and capacitor C2. When thermistor TH1 becomes hot, it provides a low-resistance path to extend positive voltage to the base of transistor T1 via diode D1 and resistor R2. Capacitor C1 charges up to the positive voltage
ELECTRONICS FOR YOU
NOVEMBER 2004
and increases the ‘on’ time of alarm. The higher the value of capacitor C1, the higher the forward voltage applied to the base of transistor T1 (BC548). Since the collector of transistor T1 is connected to the base of transistor T2, transistor T2 provides positive voltage to reset pin 4 of IC1 (NE555). Resistor R4 is used such that IC1 remains inactive in the absence of positive voltage. Diode D1 stops
discharging of capacitor C1 when the thermistor connected to the positive supply cools down and provides a high-resistance (10-kilo-ohm) path. It also stops the conduction of T1. To prevent the thermistor from melting, wrap it up in mica tape. The circuit works off a 6V-12V regulated power supply. LED1 is used to indicate that power to the circuit is switched on.
CIRCUIT IDEAS
ANTI-THEFT ALARM FOR BIKES
I VED DWI S.C.
PRAVEEN KUMAR M.P.
f anybody tries to steal your bike, this circuit turns on the horn of the bike to alert you of the impending theft. Usually, a handle lock is used on the handle bar for the safety of bikes, with the front mudguard in a slanted position.
I
module TSOP 1738 (IRX1), which is normally used in TV receivers. The receiver module senses the IR modulated frequency transmitted by the IR LED. When no IR rays are incident on the sensor, its output is high. But the output of the IR sensor goes low when it senses the modulated IR signal. The output of the
of the mobike’s horn, while the positive terminal of the horn is connected to the positive terminal of the battery via resistor R1. The energised relay drives the horn, which continues sounding until you press reset switch S2 momentarily. At night, lock your bike using the handle lock and switch on the circuit us-
When the handle lock is freed, the front mudguard can be aligned with the body of the bike. This circuit consists of transmitter and receiver sections. The transmitter (IR LED1) is fitted on the back end of the front mudguard and the receiver sensor (IRX1) is fitted on the central portion of the crash guard of the bike such that IR rays from the transmitter directly fall on the sensor when the front mudguard comes in line with the body of the bike. The transmitter section is built around timer 555 (IC2), which is wired as an astable multivibrator with a frequency of around 38 kHz. The output of IC2 is further amplified by transistor T1 and given to an infrared light-emitting diode (IR LED1), which continuously transmits the IR frequency. The receiver section uses IR receiver
receiver module is given to a negativevoltage comparator built around IC LM311 (IC3). The input voltage at pin 2 of IC3 is fixed by using the voltage-divider network comprising resistors R7 and R8. When IR rays are not incident on the IR receiver module, the voltage at pin 3 of IC3 is greater than the voltage at pin 2. As a result, the output of comparator IC3 is low. But when the receiver senses IR rays from IR LED1, the voltage at pin 3 of IC3 is lower than the voltage at pin 2. As a result, the output of the comparator goes high. The output of the comparator is given to a latch made up of JK flip-flop (IC4). The low-to-high going pulse from the comparator makes the output of IC4 high until it is reset. The output of IC4 is latched and used to energise relay RL1 via transistor T2. The relay is connected to the negative terminal
ing switch S1. Since the IR transmitter (IR LED1) and the receiver (IRX1) will not be in line of sight, IR rays from IR LED1 will not be incident on the sensor. When anyone tries to move the bike away, the IR transmitter and the IR receiver will come in line of sight and the IR rays from the IR transmitter will be incident on the receiver. This will make the output of the comparator (IC3) high. The pulse from the comparator will make the output of latch IC4 high and transistor T2 will conduct to sound the horn via relay RL1. Note. The circuit excluding the transmitter and the receiver can be housed in a small metal box and kept inside the tool box of the bike. Before you start your bike, make sure that the circuit is switched off using switch S1.
OCTOBER 2004
ELECTRONICS FOR YOU
CIRCUIT IDEAS
DIGITAL STOP WATCH C.H. VITHALANI
I VED DWI S.C.
ere’s a digital stop watch built around timer IC LM555 and 4-digit counter IC with multiplexed 7-segment output drivers (MM74C926). IC MM74C926 consists of a 4-digit counter, an internal output latch, npn output sourcing drivers for commoncathode, 7-segment display and an
push-on-switch S3. When S2 is momentarily pressed, the count value becomes 0, transistor T1 conducts and it resets IC1. Counting starts when S2 is in ‘off’ condition. A low signal on the latch-enable input pin 5 (LE) of IC2 latches the number in the counter into the internal output latches. When switch S2 is pressed, pin 5 goes low and hence the count value gets stored
Thus, when switch S3 is pressed, reset pin 13 of IC2 is connected to ground via transistor T1 and the oscillator does not generate clock pulses. This is done to achieve synchronisation between IC1 and IC2. First, reset the circuit so that the display shows ‘0000.’ Now open switch S2 for the stop watch to start counting the time. If you want to stop the clock, close
internal multiplexing circuitry with four multiplexing outputs. The multiplexing circuit has its own free running oscillator, and requires no external clock. The counter advances on negative edge of the clock. The clock is generated by timer IC LM555 (IC1) and applied to pin 12 of IC2. A high signal on reset pin 13 of IC2 resets the counter to zero. Reset pin 13 is connected to +5V through reset
in the latch. Display-select pin 6 (DS) decides whether the number on the counter or the number stored in the latch is to be displayed. If pin 6 is low the number in the output latch is displayed, and if pin 6 is high the number in the counter is displayed. When switch S2 is pressed, the base of pnp transistor T2 is connected to ground and it starts conducting. The emitter of T2 is connected to DS pin of IC2.
switch S2. Rotary switch S1 is used to select the different time periods at the output of the astable multivibrator (IC1). The circuit works off a 5V power supply. It can be easily assembled on a general-purpose PCB. Enclose the circuit in a metal box with provisions for four 7-segment displays, rotary switch S1, start/stop switch S2 and reset switch S3 in the front panel of the box.
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FULLY AUTOMATIC EMERGENCY LIGHT
DR C.H. VITHALANI
T
his simple automatic emergency light has the following advantages over conventional emergency lights: 1. The charging circuit stops automatically when the battery is fully charged. So you can leave the emergency light connected to AC mains overnight without any fear. 2. Emergency light automatically turns on when mains fails. So you don’t need a torch to locate it. 3. When mains power is available, emergency light automatically turns off. The circuit can be divided into inverter and charger sections. The inverter section is built around timer NE555, while the charger section is
built around 3-terminal adjustable regulator LM317. In the inverter section, NE555 is wired as an astable multivibrator that produces a 15kHz squarewave. Output pin 3 of IC 555 is connected to the Darlington pair formed by transistors SL100 (T1) and 2N3055 (T2) via resistor R4. The Darlington pair drives ferrite transformer X1 to light up the tubelight. For fabricating inverter transformer X1, use two EE ferrite cores (of 25×13×8mm size each) along with plastic former. Wind 10 turns of 22 SWG on primary and 500 turns of 34 SWG wire on secondary using some insulation between the primary and secondary. To connect the tubelight to ferrite transformer X1, first short both ter-
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minals of each side of the tubelight and then connect to the secondary of X1. (You can also use a Darlington pair of transistors BC547 and 2N6292 for a 6W tubelight with the same transformer.) When mains power is available, reset pin 4 of IC 555 is grounded via transistor T4. Thus, IC1 (NE555) does not produce squarewave and emergency light turns off in the presence of mains supply. When mains fails, transistor T4 does not conduct and reset pin 4 gets positive supply though resistor R3. IC1 (NE555) starts producing square wave and tubelight turns on via ferrite transformer X1. In the charger section, input AC mains is stepped down by transformer X2 to deliver 9V-09V AC at 500 mA. Diodes D1 and D2 rectify the output of the transformer. Capacitors C3 and C4 act as filters to eliminate ripples. The unregulated DC voltage is fed to IC LM317 (IC2). By adjusting preset VR1, the output voltage can be adjusted to deliver the charging voltage. When the battery gets charged above 6.8V, zener diode ZD1 conducts and regulator IC2 stops delivering the charging voltage. Assemble the circuit on a general-purpose PCB and enclose in a cabinet with enough space for the battery and switches. Connect a 230V AC power plug to feed charging voltage to the battery and make a 20W tube outlet in the cabinet to switch on the tubelight. WWW.EFYMAG.COM
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Automated Alarm Circuits Pallabi Sarkar and Anirban Sengupta
T
wo alarm circuits are presented here. One produces bird-chirping sound and the other British police siren tone. Fig. 1 shows the circuit of the birdchirping-sound alarm unit along with the circuit of the control unit. Fig. 2 shows the circuit of only the British police siren tone generator, which has to be integrated with the control circuit portion of Fig. 1 at points A and B to complete the circuit diagram of automated alarm. The control unit is built around ICs CD4047 and CD4027 (as shown on the left side of the dotted line in Fig. 1). As mentioned earlier, it is common to both the alarm circuits. IC CD4047 (IC1) is wired in positive-edge-triggering monostable multivibrator mode to set and reset IC CD4027 (IC2). The output pulse width of IC1 depends on the values of capacitor C2 and resistor R3 connected to its pins 1, 2 and 3.
Normally, when the door is closed, reed switch S1 is closed, transistor T1 conducts and the monostable multivibrator (IC1) remains in standby mode with ‘low’ output at pin 10.
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When the door is opened, reed switch S1 gets disconnected, T1 stops conducting and low-to-high pulse at pin
Fig. 2: Alarm circuit that generates police siren tone
Fig. 1: Alarm circuit that generates bird-chirping sound 9 6 • F e b r ua ry 2 0 0 9 • e l e c t ro n i c s f o r yo u
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8 of IC1 triggers the monostable and a short-duration positive pulse of about 10 seconds is available as Q output at pin 10. At the same time, complementary output Q goes low at pin 11. The output from IC1 is used to set and reset IC2. IC2 is a low-power, dual J-K master/slave flip-flop having independent J, K, set, reset and clock inputs. The flip-flops change states on the positive-going transition of the clock pulses. IC2 is wired such that its Q output turns ‘high’ when reset pin 4 receives a high pulse. When set pin 7 receives a high pulse, Q output goes low and Q output goes high. This lights up LED2 and drives transistor
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T2 (BC548), which enables the alarm circuit. The output at point A is used to enable the alarm tone generator circuit (on the right side of the dotted line) consisting of two 555 timer ICs marked as IC3 and IC4. The R-C network determines the frequency of the sound produced. The triangular waveform of the astable multivibrator is taken out from the junction of pins 2 and 6 of IC3. This waveform is fed as the control voltage at pin 5 of IC4 through resistor R18. The output received from pin 3 of IC4 is fed to the base of transistor T3 to drive an 8-ohm loudspeaker (LS1), which generates the bird-chirp-
ing sound. For the chirping-sound alarm generator, assemble the circuit shown in Fig. 1 on a separate general-purpose PCB and enclose in a small box. And if you want an alarm circuit with British police siren tone, assemble the circuit shown in Fig. 2 on another generalpurpose PCB and connect it to points A and B of the control unit shown in Fig. 1 after removing the circuit on the right side of the dotted line. Use a 9V, 500mA standard adaptor to power the circuit. This circuit may be used as a security alarm in banks, households and motorcars.
e l e c t ro n i c s f o r yo u • F e b r ua ry 2 0 0 9 • 9 7
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FLYING SAUCER IVEDI S.C. DW
ASHOK K. DOCTOR
T
his unidentified flying object (UFO) is nothing but an electronic toy depicting the fantacy. It comprises three separate sections, viz, rim flasher, dome flasher and sound generator. The rim flasher is a simple sequential circuit built around timer IC 555 (IC1) and decade counter IC CD4017 (IC2) as shown in Fig. 1. IC1 is wired as an astable multivibrator whose output is fed to clock pin 14 of decade counter IC2. All the eight outputs of IC2 are connected with two LEDs each. These 16 LEDs (LED1 through LED16) are arranged round the rim of a flying-saucer-like toy. The colour of LEDs used may be yellow, pink orange or even white to give a good colour effect. The dome flasher circuit is built around a 14-stage ripple-carry binary counter and oscillator IC CD4060 (IC3) as shown in Fig. 2. Three outputs are used here. Three groups of LEDs with six LEDs in each are arranged such that each group flashes at a different rate. Preset VR1 (47-kilo-ohm) is used to vary the flash cycle. These 18 LEDs (LED17 through LED34) are arranged around the grove (disk) of a general-purpose PCB or veroboard, which is covered by a transparent dome. Use differentcoloured LEDs for each group to create the required light effect. Red, blue, yellow or green LEDs will create a nice effect. If a transparent dome is not possible, drill holes around the top to fix the LEDs. The sound generator is built around two 555 timers, two transistors and some discrete components as shown in Fig. 3. Timer IC5 is config-
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Fig. 1: Rim flasher
Fig. 2: Dome flasher
Fig. 3: Sound generator
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Fig. 5: Assemble unit of unidentify bird Fig. 4: Fittings of LEDs on rim
ured as an astable multivibrator. The charge-discharge cycle of capacitor C8 (47µF) generates a sawtooth waveform which rises rapidly but falls slowly. This waveform is fed to the base of transistor T2 (BC327), which is an emitter follower. Its output is used to control frequency modulation. It is fed to
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pin 5 of timer IC4. The rectangular-wave output at pin 3 of timer IC5 is fed to transistor BC548 (T1) to operate timer IC4, which is also an asymmetrical multivibrator. If a 75ohm-impedance speaker is available, there is no need to use resistor R16 (68 ohms). For assembling the circuit, use two deep, plastic bowls of about 20 cm di-
ameter each. Make sure that bowls have rims to facilitate fixing of LEDs with small screws. For fixing the LEDs, refer to Fig. 4. Assemble the rim flasher, dome flasher and sound generator circuits on separate general-purpose PCBs and mount these on the deep bowls along with batteries and speaker. PCB1, PCB2 and PCB3 are for rim flasher, dome flasher and sound generator, respectively. The assembled flying saucer is shown in Fig. 5. When you switch on the circuit, rim LEDs and dome LEDs flash, and at the same time, a sound is generated. This gives the simulated effect of an unidentified flying object.
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Portable Lamp Flasher
T.K. Hareendran
H
ere is a portable, high-power incandescent electric lamp flasher. It is basically a dual flasher (alternating blinker) that can handle two separate 230V AC loads (bulbs L1 and L2). The circuit is fully transistorised and battery-powered. The free-running oscillator circuit is realised using two low-power, low-noise transistors
T1 and T2. One of the two transistors is always conducting, while the other is blocking. Due to regular charging and discharging of capacitors C1 and C2, the two transistors alternate between conduction and non-conduction states. The collector of transistor T1 is connected to the base of driver transistor T4 through current-limiting resistor R5. Similarly, the collector of transistor T2 is connected to the base of driver
Fig. 1: Circuit for portable lamp flasher
Fig. 2: Pin configurations of MOC3021, BT136 and BC550/547
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transistor T3 through limiting resistor R6. These transistors are used to trigger Triac1 and Triac2 (each BT136) through optotriacs IC1 and IC2, respectively, and switch on the power supply to external loads L1 and L2. IC1 and IC2 operate alternatively at a low frequency determined by the values of
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capacitors C1 and C2. The oscillator circuit built around transistors T1 and T2 generates low frequencies. When transistor T3 conducts, IC1 is enabled to fire Triac1 and bulb L1 glows. Similarly, when transistor T4 conducts, IC2 is enabled to fire Triac2 and bulb L2 glows. Connect the power supply line (L) of mains to bulbs L1 and L2, and neutral (N) to T1 terminals of Triac1 and Triac2. You can also connect neutral (N) line of the external 230V mains supply to both loads (bulbs L1 and L2) as a common line and then route supply line (L) to respective loads (bulbs L1 and L2). The circuit works off only 3 volts. Since current consumption is fairly low, two AA-type cells are sufficient to power the circuit. Assemble the circuit on a general-purpose PCB and enclose in a suitable plastic cabinet with integrated AA-size pen-light cell holder. Drill holes for mounting the ‘on’/‘off’ switch and power switching terminals. Also connect two bulb holders for bulbs L1 and L2. Refer Fig. 2 for pin configurations. EFY note. While assembling, testing or repairing, take care to avoid the lethal electric shock.
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Car Anti-theft Guard
T.K. Hareendran
H
ere is an easy-to-build car anti-theft guard. The circuit, shown in Fig. 1, is simple and easy to understand. When key-operated switch S2 of the car is turned on, 12V DC supply from the car battery is extended to the entire circuit through polarity-guard diode D5. Blinking LED1 flashes to indicate that the guard circuit is enabled. It works off 12V power supply along with current-limiting resistor R4 in series. When the car door is closed, door switch S1 is in ‘on’ position and 12V power supply is available across resis-
Fig. 1: Circuit of car anti-theft guard
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tor R1, which prevents transistor T1 from conducting. In this position, antitheft guard circuit is in sleep mode. W h e n someone opens the car door, switch S1 becomes ‘off’ as shown in Fig. 2. As a result, transistor T1 conducts to fire r e l a y - d r i v e r Fig. 2: Wiring diagram for door switch (S1) SCR1 (BT169) after a short delay introduced by capacitor C1. Electromagnetic relay RL1 energises and its N/O contact connects the power supply to piezobuzzer PZ1, which starts sounding to indicate that someone is trying to steal your car. To reset the circuit, turn off switch S2 using car key. This will cutoff the power supply to the circuit and stop the buzzer sound. Assemble the circuit on a general-purpose PCB and house in a small box. Connect switch S1 to the car door and keep piezobuzzer PZ1 at an appropriate place in the car.
e l e c t ro n i c s f o r yo u • M ay 2 0 0 8 • 7 5
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DICE WITH 7-SEGMENT DISPLAY
EFY LAB
A
digital dice circuit can be easily realised using an astable oscillator circuit followed by a counter, display driver and a display. Here we have used a timer NE555 as an astable oscillator with a frequency of about 100 Hz. Decade counter IC CD4026 or CD4033 (whichever available) can be used as countercum-display driver. When using
CD4026, pin 14 (cascading output) is to be left unused (open), but in case of CD4033, pin 14 serves as lamp test pin and the same is to be grounded. The circuit uses only a handful of components. Its power consumption is also quite low because of use of CMOS ICs, and hence it is well suited for battery operation. In this circuit two tactile switches S1 and S2 have been provided. While switch S2 is used for initial resetting of the display to ‘0,’ depression of S1 simulates throwing of
Decoded Segment Outputs for Counts 0 through 9
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the dice by a player. When battery is connected to the circuit, the counter and display section around IC2 (CD4026/4033) is energised and the display would normally show ‘0’, as no clock input is available. Should the display show any other decimal digit, you may press re-set switch S2 so that display shows ‘0’. To simulate throwing of dice, the player has to press switch S1, briefly. This extends the supply to the
IVEDI S.C. DW
astable oscillator configured around IC1 as well as capacitor C1 (through resistor R1), which charges to the battery voltage. Thus even after switch S1 is released, the astable circuit around IC1 keeps producing the clock until capacitor C1 discharges sufficiently. Thus for duration of depression of switch S1 and discharge of capacitor C1 thereafter, clock pulses are produced by IC1 and applied to clock pin 1 of counter IC2, whose count advances at a frequency of 100 Hz until C1 discharges sufficiently to deactivate IC1. When the oscillations from IC1 stop, the last (random) count in counter IC2 can be viewed on the 7-segment display. This count would normally lie between 0 and 6, since at the leading edge of every 7th clock pulse, the counter is reset to zero. This is achieved as follows. Observe the behavior of ‘b’ segment output in the Table. On reset, at count 0 until count 4, the segment ‘b’ output is high. At count 5 it changes to low level and remains so during count 6. However, at start of count 7, the output goes from low to high state. A differentiated sharp high pulse through C-R combination of C4-R5 is applied to reset pin 15 of IC2 to reset the output to ‘0’ for a fraction of a pulse period (which is not visible on the 7-segment display). Thus, if the clock stops at seventh count, the dis-
ELECTRONICS FOR YOU • NOVEMBER 2007 • 97
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play will read zero. There is a probability of one chance in seven that display would show ‘0.’ In such a situation, the concerned player is given an-
other chance until the display is nonzero. Note. Although it is quite feasible to inhibit display of ‘0’ and advance
98 • NOVEMBER 2007 • ELECTRONICS FOR YOU
the counter by ‘1,’ the same makes the circuit somewhat complex and therefore such a modification has not been attempted.
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ideas
Night Alert
s.c.
dwivedi
D. Mohan Kumar
I
dea of this circuit came to me at midnight when my pet dog started barking continuously on sensing a moving shadow, perhaps that of an intruder. Dogs have a night adaptation capability to maximise the sensitivity of vision in low light. They are well adapted to see moving objects rather
Transistor T1 along with transistor T2 amplifies the sound signals and provides current pulses from the collector of T2. The input trigger pulse is applied to the collector of transistor T3 and coupled by capacitor C3 to the base of transistor T4 causing T4 to cut off. The
than stationary ones in darkness. This circuit turns a lamp ‘on’ for a short duration when the dog barks, giving an impression that the occupants have been alerted. The condenser microphone fitted in the dog’s cage senses barking sound and generates AC signals, which pass through DC blocking capacitor C1 to the base of transistor BC549 (T1).
collector voltage of transistor T4 forward biases transistor T3 via resistor R8. Transistor T1 conducts and capacitor C3 discharges to keep transistor T4 cut-off. Transistor T4 remains cut-off until capacitor C3 charges enough to enable it to conduct. When transistor T4 conducts, its collector voltage goes low to drive transistor T3 into cut-off state. Resistor R9 and
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capacitor C3 are timing components. When fully charged, capacitor C3 takes about two minutes to discharge. So when sound is produced in front of the condenser mic, triac1 (BT136) fires and the bulb (B1) glows for about two minutes. Assemble the circuit on a generalpurpose PCB and enclose in a plastic cabinet. Power to the circuit can be derived from a 12V, 500mA step-down transformer with rectifier and smoothing capacitor. Solder the triac ensuring sufficient spacing between the pins to avoid short circuit. Fix the unit in the dog’s cage, with the lamp inside or outside as desired. Connect the microphone to the circuit using a short length of shielded wire. Enclose the microphone in a tube to increase its sensitivity. Caution. Since the circuit uses 230V AC, many of its points are at AC mains voltage. It could give you lethal shock if you are not careful. So if you don’t know much about working with line voltages, do not attempt to construct this circuit. EFY will not be responsible for any kind of resulting loss or damage.
e l e c t ro n i c s f o r yo u • N o v e m b e r 2 0 0 8 • 8 5
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REMOTE-CONTROLLED POWER-OFF SWITCH
DEBARAJ KEOT
R
emote controllers for various audio/video systems are usually provided with a power ‘on’/‘off’ or standby-mode selector button. But turning the system off from the remote handset actually does not cut off the whole system from mains. Some circuitry inside the system continues to get power from mains even when the power is turned off using the remote handset. One needs to turn off the mechanical switch provided on the system’s front panel or wall outlet in order to turn off the entire system. Also, accessories like TV boosters, stabilisers and additional amplispeaker systems cannot be turned off from the remote handset. And it is very annoying to get out of bed to switch off mains after watching some programme on TV or listening to music. The circuit given here can disconnect the entire system along with the
accessories, including the circuit itself, from mains using the remote for the audio or video systems. The circuit consists of a timer IC NE555, a decade counter IC HCF4017, three BC548 transistors, an infrared (IR) sensor IC TSOP1738 and a few discrete components. Transformer X1, diodes D1 and D2, and capacitor C1 form power supply for the circuit. Zener diode ZD1 provides regulated voltage to IR sensor TSOP1738 (IC3). Timer IC NE555 (IC1) is configured as an astable multivibrator that produces a clock pulse every two seconds. The clock pulse is fed to decade counter IC HCF4017 (IC2), whose Q7 output is inverted by transistor T1 and applied to the base of transistor T2 to drive the relay. The output of sensor IC3 is used to drive transistor T3 and activate the relay via transistor T2. The outputs of transistors T1 and T3 are ORed and the resultant is applied to transistor T2. Thus if any one or both the inputs connected to the
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base of transistor T2 are high, relay RL1 energises. The relay de-energises if both the inputs to transistor T2 go low. Initially, to switch on mains supply for the audio/video system and the circuit itself, pushbutton switch S1 is pressed momentarily. Normally, the output of IR receiver module IC3 is high when it is not being activated by a remote, and the relay energises to close the N/O contact and place a short across switch S1. This circuit and the load continue to get power through the N/O contact of relay RL1 even when pushbutton S1 is released. At the same time, the output of IC2 starts scrolling around its output pins, i.e., pins 5 and 6 go high and low alternately for the clock pulses received. When Q6 output goes high the ‘warning’ LED (yellow) glows, and when Q7 output goes high the ‘off’ LED (red) glows. Yellow LED (LED1) indicates that it’s time to switch off the audio or
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video system. The entire system can be turned off by pressing any key on the handset once when the red LED (LED2) is glowing. The reason is that red LED2 glows when the Q7 output of IC2 is high. Due to this, the output of transistor T1 is low. Now if any key on
the remote handset is pressed, the sensor output goes low for a while. Since both the inputs connected to the base of transistor T2 become low at this time, the transistor stops conducting and the relay de-energises. As a result, the N/O contact of the relay opens to switch off the circuit and
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the load connected across the transformer’s primary winding. Any normal operation (increasing/ decreasing volume, changing channels, etc) can be performed when either both the LEDs are ‘off’ or the remote is not oriented towards IR receiver module IC3. z
ELECTRONICS FOR YOU • OCTOBER 2005 • 105
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LAPTOP PROTECTOR
D. MOHAN KUMAR
P
rotect your valuable laptop against theft using this miniature alarm generator. Fixed inside the laptop case, it will sound a loud alarm when someone tries to take the laptop. This highly sensitive circuit uses a homemade tilt switch to activate the alarm through tilting of the laptop case. The circuit uses readily available components and can be assembled on a small piece of Vero board or a general-purpose PCB. It is powered by a 12V miniature battery used in remote control devices. IC TLO71 (IC1) is used as a voltage comparator with a potential divider comprising R2 and R3 providing half supply voltage at the non-inverting input (pin 3) of IC1. The inverting input receives a higher volt-
age through a water-activated tilt switch only when the probes in the tilt switch make contact with water. When the tilt switch is kept in the horizontal position, the inverting input of IC1 gets a higher voltage than its noninverting input and the output remains low. IC CD4538 (IC2) is used as a monostable with timing elements R5 and C1. With the shown values, the output of IC2 remains low for a period of three minutes. CD4538 is a precision monostable multivibrator free from false triggering and is more reliable than the popular timer IC 555. Its output becomes high when power is switched on and it becomes low when the trigger input (pin 5) gets a low-tohigh transition pulse. The unit is fixed inside the laptop case in horizontal position. In this position, water inside the tilt switch ef-
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fectively shorts the contacts, so the output of IC1 remains low. The alarm generator remains silent in the standby mode as trigger pin 5 of IC2 is low. When someone tries to take the laptop case, the unit takes the vertical position and the tilt switch breaks the electrical contact between the probes. Immediately the output of IC1 becomes high and monostable IC2 is triggered. The low output from IC2 triggers the pnp transistor (T1) and the buzzer starts beeping. Assemble the circuit as compactly as possible so as to make the unit matchbox size. Make the tilt switch using a small (2.5cm long and 1cm wide) plastic bottle with two stainless pins as contacts. Fill two-third of the bottle with water such that the contacts never make electrical path when the tilt switch is in vertical position. Make the bottle leakproof with adhesive or wax. Fix the tilt switch inside the enclosure of the circuit in horizontal position. Fit the unit inside the laptop case in horizontal position using adhesive. Use a miniature buzzer and a micro switch (S1) to make the gadget compact. Keep the laptop case in horizontal position and switch on the unit. Your laptop is now protected.
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Short-Circuit Protection in DC Low-Voltage Systems
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Pratik Panchal
M
any a times you need to power an adjoining accessory circuit from the power supply used in the main module cir-
cuit. Here is a circuit to derive the additional power supply from the main circuit. The main circuit is protected from any damage due to short-circuit in the additional power supply circuit by cutting off the derived supply voltage. The derived supply voltage restores automatically when shorting is removed. An LED is used to indicate whether short-circuit exists or not. Author’s prototype of short-circuit protection module is shown in Fig. 1. In the main power supply circuit, 230V AC is Fig. 1: Prototype of short-circuit protection in DC low-voltage systems stepped down by transformer X1 (230V AC primary to 0-9V, 300mA secondary), rectified by a fullwave rectifier comprising diodes D1 through D4, filtered by capacitor C1 and regulated by IC 7805 to give regulated 5V (O/P1). Transistors SK100 and BC547 are used to derive the secondary output of around 5V (O/P2) from the main 5V Fig. 2: Circuit diagram of short-circuit protection supply (O/P1).
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Working of the circuit is simple. When the 5V DC output from regulator IC 7805 is available, transistor BC547 conducts through resistors R1 and R3 and LED1. As a result, transistor SK100 conducts and short-circuit protected 5V DC output appears across O/P2 terminals. The green LED (LED2) glows to indicate the same, while the red LED (LED1) remains off due to the presence of the same voltage at both of its ends. When O/P2 terminals short, BC547 cuts off due to grounding of its base. As a result, SK100 is also cut-off. Thus during short-circuit, the green LED (LED2) turns off and the red LED (LED1) glows. Capacitors C2 and C3 across the main 5V output (O/P1) absorb the voltage fluctuations occurring due to short-circuit in O/P2, ensuring disturbance-free O/P1. The design of the circuit is based on the relationship given below: RB = (HFE X Vs)/(1.3 X IL) where, RB = Base resistances of transistors of SK100 and BC547 HFE = 200 for SK100 and 350 for BC547 Switching Voltage Vs = 5V 1.3 = Safety factor IL = Collector-emitter current of transistors Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. Connect O/P1 and O/P2 terminals on the front panel of the cabinet. Also connect the mains power cord to feed 230V AC to the transformer. Connect LED1 and LED2 for visual indication.
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DESKTOP POWER SUPPLY
T.K. HAREENDRAN
U
seful for electronics hobbyists, this linear workbench power supply converts a high input voltage (12V) from the SMPS of a PC into low output voltage (1.25 to 9 volts). An adjustable three-pin voltage regulator chip LM317T (IC1) is used here to provide the required voltages. The LM317T regulator, in TO-220 pack, can handle current of up to 1 amp in practice. Fig. 1 shows the circuit of the desk-
top power supply. Regulator IC LM317T is arranged in its standard application. Diode D1 guards against polarity reversal and capacitor C1 is an additional buffer. The green LED (LED1) inFig. 2: Pin dicates the status of the configuration of power input. Diode D2 LM317 prevents the output voltage from rising above the input voltage when a capacitive or induc-
Fig. 1: Circuit of desktop power supply
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tive load is connected at the output. Similarly, capacitor C3 suppresses any residual ripple. Connect a Fig. 3: Suggested power standard digi- supply box tal voltmeter in parallel with the output leads to accurately set the desired voltage with the help of variable resistor VR1. You can also use your digital multimeter if the digital voltmeter is not available. Switch on S1 and set the required voltage through preset VR1 and read it on the digital voltmeter. Now the power supply is ready for use. The circuit can be wired on a common PCB. Refer Fig. 2 for pin configuration of LM317 before soldering it on the PCB. After fabrication, enclose the circuit in a metallic cover as shown in Fig. 3. Then open the cabinet of your PC and connect the input line of the gadget to a free (hanging) four-pin drive power connector of the SMPS carefully.
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ELECTRIC WINDOW/FENCE CHARGER
T.K. HAREENDRAN
H
ere is the circuit of a simple electric window charger. With a couple of minor circuit variations, it can be used as an electric fence charger too. A standard 12V, 7Ah sealed maintenance-free (SMF) UPS battery is required for powering the entire unit. Any component layout and mounting plan can be used. However, try to keep the output terminals of transformer X1 away from the circuit board. Timer NE555 (IC1) is wired as a free-running oscillator with narrow negative pulse at the output pin 3. The pulse frequency is determined by resis-
tors R2 and R3, preset VR1 and capacitor C3. The amplitude of the output pulse can be varied to some extent by adjusting variable resistor VR1. You can vary the frequency from 100 Hz to 150 Hz. X1 is a small, iron-core, step-down transformer (230V AC primary to 12V, 1A secondary) that must be reverse connected, i.e., the secondary winding terminals of the transformer should be connected between the emitter and ground and the output taken across the primary winding. Switch S1 is used for power ‘on’/‘off’ and LED1 works as a power-‘on’ indicator. LED2 is used to indicate the pulse activity. The output pulse from pin 3 of IC1
1 1 0 • S e p t e m b e r 2 0 0 8 • e l e c t ro n i c s f o r yo u
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drives pnp transistor T1 into conduction for the duration of the time period. The collector of T1 is connected to the base of driver transistor T2 through resistor R5. When transistor T1 conducts, T2 also conducts. When T2 conducts, a high-current pulse flows through the secondary winding of transformer X1 to generate a very high-voltage pulse at the primary winding. This dangerously high voltage can be used to charge the window rails/fences. Ordinary silicon diode D1 (1N4001) protects T2 against high-voltage peaks generated by X1 inductance during the switching time. You can replace X1 with another transformer rating, and, if necessary, replace T2 with another highercapacity transistor. The circuit can be used to charge a 1km fence with some minor modifications in the output section. Caution. Take all the relevant electrical safety precautions when assembling, testing and using this highvoltage generator.
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Electric Guitar Preamplifier
T.K. HAREENDRAN
H
ere is the circuit of a guitar preamplifier that would accept any standard guitar
pickup
jack Headstock
Fig. 1: A typical example of mounting the guitar pickup
up attached to a guitar headstock is shown in Fig. 1. The pickup device has a transducer on one end and a jack on the other end. The jack can be plugged into a preamplifier circuit and then to a power amplifier system. The pickup device captures mechanical vibrations, usually from stringed instruments such as guitar or violin, and converts them into an electrical signal, which can then be amplified by an audio amplifier. It is most often mounted on the body of the instrument, but can also be attached to the bridge, neck, pickguard or headstock. The first part of this preamplifier circuit shown in Fig. 2 is a single-tran-
Fig. 2: Guitar preamplifier circuit
pickup. It is also versatile in that it has two signal outputs. A typical example of using a pick-
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sistor common-emitter amplifier with degenerative feedback in the emitter and a boot-strapped bias divider to
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secure optimal input impedance. With the component values shown here, the input impedance is above 50 kilo-ohms and the peak output voltage is about 2V RMS. Master-level-control potmeter VR1 should be adjusted for minimal distortion. The input from guitar pickup is fed to this preamplifier at J1 terminal. The signal is buffered and processed by the op-amp circuit wired around IC TL071 (IC1). Set the gain using preset VR2. The circuit has a master and a slave control. RCA socket J2 is the master signal output socket and socket J3 is the slave. It is much better to take the signal from J2 as the input to the power amplifier system or sound mixer. Output signals from J3 can be used to drive a standard headphone amplifier. Using potmeter VR3, set the slave output signal level at J3. House the circuit in a metallic case. VR1 and VR3 should preferably be the types with metal enclosures. To prevent hum, ground the case and the enclosures. A wellregulated 9V DC power supply is crucial for this circuit. However, a standard 9V alkaline manganese battery can also be used to power the circuit. Switch S1 is a poweron/off switch.
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CIRCUIT IDEAS
HIT SWITCH T.A. BABU
O I THE SAN
his versatile hit switch is the electronic equivalent of a conventional switch. It can be used to control the switching of a variety of electronic devices. The circuit of the hit switch uses a piezoelectric diaphragm (piezobuzzer) as the hit sensor. A piezoelectric material develops electric polarisation when strained
amplified by transistor BC547 (T1). The combination of transistor T1 and the bridge rectifier comprising diodes D1 through D4 acts as a voltage-control switch. The inverter gates of IC CD4069 (IC1) together with associated components form a bistable switch. IC CD4069 is a CMOS hex inverter. Out of the six available inverter gates, only three are used here. IC1 operates at any voltage between 3V and 15V and offers a
by an applied stress. The hit sensor makes use of this property. When you hit or knock the piezo element (hit plate) with your fingertip, a small voltage developed by the piezo element is
high immunity against noise. The recommended operating temperature range for this IC is –55°C to 125°C. This device is intended for all general-purpose inverter applications.
T
ELECTRONICS FOR YOU
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Initially, the input of gate N1 is low, while the input of gate N2 is high. Triggering the voltage-control switch by hitting the sensor pulls the input of gate N1 to high level and causes the bistable to toggle. The capacitor gets charged via resistor R1 and the circuit changes its state. This latch continues until the bistable switch gets the next triggering input. Every time the hit plate receives a hit, the voltage-control switch triggers the bistable circuit. That means every subsequent hit at the sensor will toggle the state of the switch. The red LED (LED1) connected at the output of gate N3 indicates ‘on’/‘off’ position of the switch. Relay RL1 is activated by the hit switch to control the connected load. The circuit works off 12V DC. It can be constructed on any general-purpose PCB. For the desired results, proper connections and installation of the hit sensor are necessary. Remove the cover of the piezobuzzer and connect its two leads to the circuit. Mount the plate such that it receives the hit properly. The piezoelectric material on the plate can easily get damaged, so hit the switch gently.
CIRCUIT IDEAS
ELECTRONIC WATCHDOG TAPAN KUMAR MAHARANA
H
ere’s an electronic watchdog for your house that sounds to inform you that somebody is at the gate.
I VED DWI S.C.
The circuit comprises a transmitter unit and a receiver unit, which are mounted face to face on the opposite
Fig. 4: Mounting arrangement for transmitter and receiver units
Fig. 1: 38kHz IR transmitter circuit
Fig. 3: Pin configurations of TSOP1738 and UM66
Fig. 2: Receiver circuit
transmitter is derived from the receiver circuit by connecting its points A and B to the respective points of the receiver circuit. The receiver is powered by regulated 6V DC. For the purpose, you can use a 6V battery. The transmitter and receiver units are aligned such that the IR beam falls directly on the IR sensor. As long as IR beam falls on the sensor, its output remains low, transistor T1 does not conduct
and trigger pin 2 of IC2 remains high. When anyone interrupts the IR beam falling on the sensor, its output goes high to drive transistor T1 into conduction and pin 2 of IC2 goes low momentarily. As a result, IC2 gets triggered and its pin 3 goes high to supply 3.3V to melody generator IC3 at its pin 2, which produces a sweet melody through the speaker fitted inside the house. Output pin 3 of IC2 remains high for around 30 seconds.
pillars of the gate such that the IR beam gets interrupted when someone is standing at the gate or passing through it. The transmitter circuit (see Fig. 1) is built around timer NE555 (IC1), which is wired as an astable multivibrator producing a frequency of about 38 kHz. The infrared (IR) beam is transmitted through IR LED1. The receiver circuit is shown in Fig. 2. It comprises IR sensor TSOP1738 (IR RX1), npn transistor BC548 (T1), timer NE555 (IC2) and some resistors and capacitors. IC2 is wired as a monostable multivibrator with a time period of around 30 seconds. The melody generator section is built around melody generator IC UM66 (IC3), transistor T2 and loudspeaker LS1. Fig. 3 shows pin configurations of IR sensor TSOP1738 and melody generator IC UM66. The power supply for the Fig. 4 shows mounting arrangement for both the transmitter and receiver units on the gate pillars. To achieve a high directivity of the IR beam towards the sensor, use a reflector behind the IR LED. After both the units have been built, connect 6V power supply to the receiver circuit. You should hear a continuous melody from the speaker. Now connect 6V power to the transmitter also and orient IR LED1 towards IR receiver. The NOVEMBER 2004
ELECTRONICS FOR YOU
CIRCUIT IDEAS melody should stop after about 30 seconds. Now the transmitter and the receiver units are ready for use. When somebody enters through the
ELECTRONICS FOR YOU
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door, the IR beam is interrupted and the alarm sounds for 30 seconds. The alarm keeps sounding as long as one stands between the transmitter and receiver units.
Using preset VR1, you can set the volume of the loudspeaker. This circuit can also be used as a doorbell or burglar alarm.
CIRCUIT IDEAS
SOLIDSTATE REMOTE CONTROL SWITCH SEEMANT SINGH
from the transmitter, the output of the IR receiver module is high (approx. 5V). When the transmitter is pointed at the receiver and switch S1 is momentarily pressed, the transmitted IR rays are sensed by the receiver module and its output pulses low to trigger the monostable (IC2). The output of IC2 goes high for about five seconds. Thus, even if you press the remote
H
ere is a solidstate remote control switch which uses readily available electronic components. The control circuit comprises the transmitter and receiver sections. The range of the transmitter is around seven metres. The transmitter circuit (shown in Fig. 1) is built around a timer IC (555) wired as an astable multivibrator. It works off a 9V battery. When remote control switch S1 is pressed, the astable multivibrator built around IC1 starts oscillating at a frequency of about 38 kHz. The signal frequency at output pin 3 of IC1 is transmit-
EO I TH SAN
Fig. 1: Transmitter circuit
Fig. 2: Receiver circuit
ted through two infrared diodes (IR LED1 and IR LED2). A green LED (LED1) connected to pin 3 glows whenever S1 is pressed, indicating the presence of a signal for transmission at the output of the multivibrator. The output frequency F at pin 3 of IC1 depends on the timing components, viz, resistors R1 and R2 and capacitor C2. It is given by the following relationship: F = 1.443/(R1+2R2)C2 This frequency is fed to npn transistors T1 and T2 (each BC547) through resistor R4 (470-ohm) to drive the IR LEDs. Resistor R5 limits the current flowing through the IR LEDs. The receiver circuit (shown in Fig. 2)
consists of regulator IC 7806 (IC4), IR receiver module (TSOP1738), timer 555 (IC2) and decade counter CD4017 (IC3). Timer 555 (IC2) is wired as a monostable multivibrator. The 9V DC power supply for the receiver circuit is regulated by regulator IC 7806. The presence of power in the circuit is indicated by glowing of the red LED (LED2). The IR receiver module (TSOP1738), which gets 5.1V power supply through zener diode ZD1, receives the transmitted signal of about 38 kHz. The signal is amplified by transistor BC558 (T3) and given to triggering pin 2 of IC2 through coupling capacitor C6. Initially, when no signal is received
switch more than one time by mistake, there won’t be any change in the output of the receiver within this period and hence no undesired switching of the appliance. The signal reception is indicated by glowing of the green LED (LED3). The output of IC2 is given to the clock input (pin 14) of IC3. Here, IC3 is wired as a bistable circuit. For every clock input, pins 2 and 3 of IC3 alternately go high. Initially, when the power to the receiver circuit is switched on, pin 3 of IC3 is high and therefore the yellow LED (LED4) connected to it glows. The glowing of LED4 indicates that the appliance is in ‘off’ condition. When a clock pulse is received at pin 14 SEPTEMBER 2004
ELECTRONICS FOR YOU
CIRCUIT IDEAS of IC3, pin 3 goes low to turn off LED4, while pin 2 becomes high. The high output at pin 2 triggers the gate of triac BT136, which, in
ELECTRONICS FOR YOU
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turn, controls the appliance. Precautions. Don’t touch the leads of the triac as it is connected across the 230V
AC mains. Also, make sure that the neutral point of mains is connected to the ground line of the circuit and not vice versa.
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IDEAS
INFRARED FIRECRACKER IGNITER
PRADEEP VASUDEVA
irecrackers are normally ignited by using a matchstick or a candle. You have to run away quickly after igniting the fuse of the firecracker. This method of igniting
F
with the remote control. In the figure shown here, normally the output of IC1 is low and green LED2 is ‘on’ and the red LED3 ‘off.’ This indicates that the circuit is ready for use. When any key on the remote control is pressed, output pin
firecracker is unsafe, because the danger of the firecracker bursting before you reach a safe distance is always there. The device described here uses remote control, usually used with TV receivers or CD players, to burst the firecracker. Thus the firecracker can be ignited from a safe distance using the circuit described below in conjunction
3 of IRX1 (IR receiver module TSOP1738) goes low. This output is connected to pin 2 of IC1 via LED1 and resistor R4 to trigger the monostable operation of IC1. The output of IC1 remains high for a period equal to 1.1×R2×C2. With the values of the components given in the circuit diagram here, the period works out to 3.5 seconds approximately. This
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activates relay RL1 and red LED3 glows and green LED2 turns off. ‘On’ state of red LED3 indicates that the firecracker is about to burst. R7 is a small part of the element of an electric heater (220V, 1000W), which is kept away from the electronic circuit and connected to the relay contacts through a thick electric cable. The resistance value of short length of the heater element (R7) is 3 to 3.5 ohms. A current of around 4 amperes flows through it when connected to a 12V battery. Flow of 4A current through R7 for 3.5 seconds makes it red hot, which ignites the firecracker. The circuit is powered by a 12V, 7AH battery. IC2 provides about 9V for the operation of the circuit. The circuit should be housed in a metallic cabinet to prevent it from being damaged by bursting of the firecracker. The IR receiver and the two LEDs should be fixed on the front panel of the cabinet. Wiring and relay used in the circuit should be chosen such that they are able to carry more than 5 amperes of current.
ELECTRONICS FOR YOU • AUGUST 2007 • 97
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VERSATILE WATER-LEVEL CONTROLLER
A. SHAFEEK AHAMED
T
his simple, economical and versatile circuit switches on the motor pump when water in the overhead tank falls below the lowest level and turns it ‘off’ when the tank is full. Moreover, if the pump is running dry due to low voltage, it sounds an alarm to alert you to switch off the controller circuit (and hence the mo-
IVEDI S.C. DW
trodes are suspended into the tank such that they don’t touch each other. Points B, L and U of the water-level controller circuit are connected to the respective points of the sensor electrodes assembly. When water in the tank is below the lowest level L1, all the electrodes are electrically separated and hence points L and U (pins 6 and 2 of IC2, respectively) are pulled up to the sup-
When water rises to the overflow level L2 and touches electrode E3, point U (pin 2 of IC2) is connected to already sunken ground electrode E2, thereby triggering it. IC2 resets to give a high output at pin 3. This is inverted by transistor T1 to cut off transistor T2 and de-energise relay RL1. The motor pump now stops to prevent water overflow. As water is consumed, the water
ply voltage through resistors R2 and R3, respectively. Therefore, to reset IC2 the output of IC2 at pin 3 goes low. As a result, transistor T1 stops conducting to drive transistor T2 and relay RL1 energises. The motor pump now starts running to fill the tank with water. Freewheeling diode D5 prevents chattering of the relay due to the back emf produced by the relay coil. When the water level rises to bridge the electrodes, because of the conductivity of water, pin 6 (E1) is pulled down to ground (E2). This does not alter the output state of IC2, which maintains its previous state, and the motor keeps running.
level comes down leaving electrode E3 isolated from ground electrode E2. Now point U (pin 2 of IC2) is pulled up to the supply voltage. This does not change the output state of IC2 and the motor remains switched off. When water level again falls below electrode E2, IC2 resets to cut off transistor T1. Transistor T2 conducts to energise relay RL1 and the motor is powered to run. This is how the process continues. LED1 glows whenever the relay energises, indicating that the motor pump is running. As the values of resistors R2 and R3 are very high, corrosion of electrodes is very little. Capacitors C2 through C7
Fig. 1: Circuit of water-level controller
tor pump) to avoid coil burn and power wastage. The water-level controller circuit (see Fig. 1) is built around IC 555 (IC2) to monitor the water level in the overhead tank and ‘on’/‘off’ status of the motor through the inverter and driver circuits. The transistor switch circuitry monitors the flow of water and raises an alarm if the pump runs dry. Power supply is obtained through step-down transformer X1, diodes D1 through D4, capacitor C1, series currentlimiting resistor R1, regulator IC1, and noise-filtering capacitors C2 and C3. The set-up for the water-level sensing electrodes is shown in Fig. 2. ElecWWW.EFYMAG.COM
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filter out unwanted noise. Switches S2 and S3 can be used to manually switch on and off the motor pump, respectively, when water is in between the upper and lower levels. Switch S1 is used to disable the unit during dry pump run or while flushing the tank. For the sensor electrodes, use a moulded-type AC chord (used for tape recorders) with its pair of wires sleeved at the end and connected together to form the electrode. Other electrodes can be made similarly. These three AC chords are suspended inside the tank from a longitudinally cut PVC pipe (used for electrical wiring). The arrangement for the dry pump sensor is shown in Fig. 3. A mouldedtype AC chord with its pair of wires sleeved at the end can be attached firmly to the delivery pipe such that water falls onto the plug leads. The sleeved ends are connected to points A and B of the water-level controller circuit. The circuit for dry-run alarm comprises transistors T3 and T4, piezobuzzer PZ1, resistors R6 and R7, and capacitor C7. When points A and B of the dry-running sensor (see Fig. 3) are bridged by water being delivered by the pipe, transistor T3 conducts to drive transistor T4 into cutoff state and therefore the DC buzzer remains silent. When the pump runs dry, points A and B are electrically apart causing transistor T3 to cut off because of pullup resistor R6. Transistor T4 conducts due to the emitter drop of transistor T3, which activates the DC buzzer to
Fig. 2: Water-level electrodes set-up for overhead tank
sound an alarm indicating dry running of the pump. The alarm circuit is enabled only when transistor T2 conducts, i.e., only when the motor pump runs. Diode D6 isolates the relay driver circuitry to prevent transistor T3 from extending ground to the relay through transistor T3 and water being delivered. As soon as the pump is switched on, the alarm sounds until water reaches the delivery port. House the controller circuit (including the power supply) in a cabinet. Use a four-core shielded cable for wiring the tank electrodes to the controller unit fixed near the motor switch. To test the circuit, proceed as follows: 1. Switch on power to the circuit. 2. LED1 glows and relay RL1 energises to produce an alarm from piezobuzzer PZ1, indicating that none of the circuit points A, B, U and L is shorted through water (i.e., water in the tank is below the lowest limit). The
96 • DECEMBER 2005 • ELECTRONICS FOR YOU
energised relay indicates ‘on’ status of the motor. 3. Immerse points A and B in Fig. 3: Dry pump water. The buzzer sensor set-up stops sounding to indicate that water is flowing out of the pipe to short points A and B. This confirms no dry run. 4. Immerse points B and L in water, as would be the case when the water level rises. Momentarily touch point U to water. LED1 goes off and the relay de-energises to turn the pump ‘off.’ This would be the case when water touches the overflow limit. 5. Remove points A and B from water assuming that the flowing water that was shorting points A and B has stopped. Now, although water is not flowing, the buzzer does not sound as the relay is already de-energised. 6. Remove points U and B from water, assuming that water has fallen below the lowest limit because of consumption. Two seconds later, LED1 glows and the relay energises. Precautions. 1. Make sure that water being delivered from the water pipe doesn’t touch any of the suspended water-level sensors. 2. Mount the alarm sensor firmly onto the water pipe such that electrodes A and B are shorted by water flowing out of the pipe. 3. Use a properly shielded cable to carry signals from the tank to the water-level controller unit. z
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SAFETY GUARD
A. RAMESH BABU
P
rotect your home appliances from voltage spikes with this simple time delay circuit. At the heart of the circuit is IC CD4060, which consists of two inverter gates for clock generation and
UMAR SUNIL K
a 14-bit binary ripple counter. Here the clock oscillations are governed by resistor R1 and capacitor C1. In this circuit, only two outputs of the IC (Q5 and Q14) have been used. Q5 is connected to an LED (LED1) and Q14 is used to trigger the gate of the SCR through D4 as well as reset the
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counter. The anode of the SCR is connected to +9V and the cathode is connected to the relay coil. The other pin of the relay coil is connected to the negative supply, while its contacts are used for switching on the appliances. Whenever power to the appliances is switched on or resumes after mains failure, the oscillator starts oscillating and LED1 blinks. This continues for three minutes. After that, Q14 output of IC CD4060 goes high to trigger the gate of the SCR through D4. At this moment, the voltage is available at the cathode of the SCR, which energises the relay coil to activate the appliance and LED2 glows. Switch S1 is used for quick start without waiting for delay. z
ELECTRONICS FOR YOU • JUNE 2005 • 91
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Electronic Dice Using AT89C2051
Debdoot Sheet
T
his simple circuit demonstrates the capability of an AT89C2051 microcontroller chip to function as a random number generator based on the flying counter principle. The pro- Fig. 1: Suggested gram in the chip con- LED arrangement for electronic dice stantly updates the display
Fig. 2: Circuit for electronic dice using AT89C2051
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counter variable, which, on being interrupted by an external trigger, latches the counter value and displays a random number through its output ports. This method is similar to the one used in PCs or calculators for generating random numbers at any instance. The application of this dice is similar to the one used in a game of dice. The random numbers generated are not displayed numerically, but represented by the number of glowing LEDs. The LEDs are the dot representation on the face of a dice. Suggested LED arrangement for the electronic dice display is shown in Fig. 1. The use of IC AT89C2051 (IC1) module in the design is quite simple. It operates off 35.5V DC supply
o
sani the
and uses an 18MHz crystal to generate the clock (refer Fig. 2). Switch S1 connected at pin 1 is used as a reset switch. Interrupt occurs at pin 6 of IC1 on logic 0. Switch S2 connected to pin 6 (INT0) of IC1 is used to trigger an external interrupt to make pin 6 low. It is used as input to generate the random number. The random number is indicated by glowing of the LEDs (LED1 through LED7) connected to port pins P1.2-P1.7 and P3.7. TL0 and TH0 act as free-wheeling counters in auto-increment mode and constantly count up from the initial value. When the interrupt occurs, the value from the counter is latched and glowing LEDs indicate the random number generated by the microcontroller chip. Assembly language is used for programming the chip. The Assembly code listing is self-explanatory. EFY note. The source code is included in this month’s EFY-CD and
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Smart Hearing Aid
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ormally, hearing aid circuits consume battery power continuously once they are switched on. The circuit given here saves battery power by switching on the sound amplifier section only
N
which is used as a comparator. The reference voltage (Vref) at the noninverting terminal (pin 3) of IC1(A) is set using preset VR1. The preset is also used to control the sensitivity of the sound signals received by the circuit. The output from pin 1 of IC1(A) is fed to the trigger input (pin 2) of timer NE555, which is configured in
signal received from the mic is fed to the non-inverting pin of the second op-amp of IC1(B) which is wired in unity follower configuration. The unity follower mode resolves the problem of impedance mismatch which would have occured if the output of the mic
when sound is detected. The sensitivity of the detection section and the ‘on’ time duration of the sound amplifier circuit can be set by the user. Also the circuit uses only a single condenser mic for sound detection and amplification. As is clear from the above, this hearing aid consists of a condenser microphone, earphone, and sound detection and amplification sections. The sound detection section employs a quad op-amp IC LM324 (IC1(A)) and a timer NE555 (IC2). The sound signal received at the mic is pre-amplified by transistor BC549 (T1). The voltage at its collector is fed to the inverting terminal (pin 2) of op-amp IC1(A),
monostable mode. When sufficient sound signal strength is detected at the base of transistor T1, the pulsating voltage at its collector exceeds the reference voltage at pin 3. As a result, output pin 1 of IC1(A) goes low. The low output from IC1(A) triggers the NE555 timer and its output goes high for a preset duration. R4 and C2 are the timing components for setting the time duration. The high output of the timer is directly used as the power source for the sound amplifier section. The sound amplifier section is built around transistors T2 through T5. The last amplifier stage T5 (pnp transistor BC558) drives the earphone. The sound
is fed directly to amplifier stage. The output from pin 7 of IC1(B) is fed to the base of transistor T2. The weak signal received at transistor stage T2 is further amplified by transistors T3, T4 and T5. An earphone to listen to the sound is connected between the collector of T5 and ground. It is recommended to use a mono earphone with volume control attached. With 9V DC supply, when sound is detected through the mic, the amplifier section is automatically triggered and the current consumption of the circuit is about 96 mA. When the amplifier circuit is ‘off,’ the circuit draws a current of about 6 mA only, thus saving considerable amount of battery power.
Devrishi Khanna and Rohit Modi
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Four-Stage FM Transmitter Pradeep G.
edi
s.c. dwiv
his FM transmitter circuit uses four radio frequency stages: a VHF oscillator built around transistor BF494 (T1), a preamplifier
T
the pre-driver stage. You can also use transistor 2N5109 in place of 2N2219. The preamplifier is a tuned class-A RF amplifier and the driver is a class-C amplifier. Signals are finally fed to the class-C RF power amplifier, which de-
frequency generated. You can also use a 12V battery to power the circuit. Assemble the circuit on a generalpurpose PCB. Install the antenna prop-
built around transistor BF200 (T2), a driver built around transistor 2N2219 (T3) and a power amplifier built around transistor 2N3866 (T4). A condenser microphone is connected at the input of the oscillator. Working of the circuit is simple. When you speak near the microphone, frequency-modulated signals are obtained at the collector of oscillator transistor T1. The FM signals are amplified by the VHF preamplifier and
livers RF power to a 50-ohm horizontal dipole or ground plane antenna. Use a heat-sink with transistor 2N3866 for heat dissipation. Carefully adjust trimmer VC1 connected across L1 to generate frequency within 88108 MHz. Also adjust trimmers VC2 through VC7 to get maximum output at maximum range. Regulator IC 78C09 provides stable 9V supply to the oscillator, so variation in the supply voltage will not affect the
erly for maximum range. Coils L1 through L5 are made with 20 SWG copper-enamelled wire wound over air-cores having 8mm diameter. They have 4, 6, 6, 5 and 7 turns of wire, respectively. EFY note. This transmitter is meant only for educational purposes. use of this transmitter with outdoor antenna is illegal in most parts of the world. The author and EFY will not be responsible for any misuse of this transmitter.
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ELECTRONIC BICYCLE LOCK
T.K. Hareendran
T
he electronic bicycle lock described here is a worthwhile alternative for bicycle own-
J2 are two standard RCA sockets. A home-made security loop can be used to link these two input points. Around 50cm long, standard 14/36 flexible wire with one RCA plug per end is
edi
s.c. dwiv
Fig. 3: Lock fitted on the bicycle
Fig. 1: Circuit of electronic bicycle lock
ers who want to make their bicycles ‘intelligent’ at reasonable cost. One of the benefits of building it yourself is that the circuit can Fig. 2: Lock box be used for virtually any make of bicycles. In the circuit, input jacks J1 and
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enough for the security loop. Fig. 1 shows the circuit of the electronic bicycle lock. It is powered by a compact 9V battery (6F22). Key lock switch S1 and smoothing capacitor C2 are used for connecting the power supply. A connected loop cannot activate IC1 and therefore the speaker does not sound. When the loop is broken, zener diode ZD1 (3.1V) receives operating power supply through resistor R2 to enable
tone generator UM3561 (IC1). IC1 remains enabled until power to the circuit is turned off using switch S1 or the loop is re-plugged through J1 and J2. Assemble the circuit on a generalpurpose PCB and house in a small tinplate enclosure. Fit the system key lock switch (S1) on the front side of the enclosure as shown in Fig. 2. Place RCA sockets (J1 and J2) at appropriate positions. Now, mount the finished unit in place of your existing lock (as shown in Fig. 3) by using suitable clamps and screws.
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SENSITIVE VIBRATION DETECTOR
T.K. HAREENDRAN
T
his vibration detector is realised using readily available, lowcost components. One of its many applications is in a rolling shutter guard for offices and shops. The detector will sense vibration caused by activities like drilling and switch on the connected load (bulb, piezobuzzer, etc) to alert you. The circuit works off a 6V battery or 6V regulated power supply and uses a piezoceramic element as the vibration detector. The same is easily available from electronics/telephone component vendors or you can take it out from an active buzzer. Initially, when the power is switched on, decade counter IC1 is reset by power-on-reset components C2 and R1. As a result, Q0 output (pin 3)
IVEDI S.C. DW
of IC1 goes high and the entire circuit is in idle state. LED1 indicates the power status. In the event of vibrations, IC2 is clocked by the pulses from the piezoceramic element connected to its clock pin 14. Q1 Fig. 2: Pin configuration of through Q9 outputs SCR1 BT169 and of IC2 are fed to reback view of the piezo element lay-driver switching transistor T1 through diodes D1 through D9 connected in OR mode. Immediately after clocking, any of the outputs Q1 through Q9 would go high and npn transistor T1 would conduct. As a result, SCR1 is fired through
Fig. 1: Circuit of the sensitive vibration detector
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Fig. 3: Arrangement for rolling shutter guard for shops, offices and banks
its gate. This, in turn, energises relay RL1. The relay contacts can be used to switch any alarm device to indicate vibration detection. The circuit can be reset by momentarily pressing switch S1. Zener diodes ZD1 and ZD2 at the clock input of IC1 are used for protection against high voltage input. In the case of repeated false triggering of IC1, add a 100nF capacitor in parallel to the piezoceramic element. The pin configuration of SCR BT169 and the back view of the piezo element are shown in Fig. 2. Fig. 3 shows suggested location of the vibration detector for rolling shutters of banks, shops, etc. z
ELECTRONICS FOR YOU • NOVEMBER 2005 • 103
CMYK
CIRCUIT
IDEAS
FUEL RESERVE INDICATOR FOR VEHICLES
D. MOHAN KUMAR
H
ere is a simple circuit for monitoring the fuel level in vehicles. It gives an audiovisual indication when the fuel level drops alarmingly below the reserve level, helping you to avoid running out of petrol on the way. Nowadays vehicles come with a dash-mounted fuel gauge meter that indicates the fuel levels on an analogue display. The ‘reserve’ level is indicated by a red marking in some vehicles, but the needle movement through the red marking may be confusing and not precise. This circuit monitors the fuel tank below the reserve level and warns through LED indicators and audible beeps when the danger level is approaching. The fuel sensor system consists of a tank-mounted float sensor and a current meter (fuel meter), which are connected in series. The float-driven sensor attached to an internal rheostat offers high resistance when the tank is empty. When the tank is full, the resistance decreases, allowing more current to pass through the meter to give a higher reading. The fuel monitoring circuit works
by sensing the voltage variation developed across the meter and activates the beeper when the fuel tank is almost empty. Its point A is connected to the input terminal of the fuel meter and point B is connected to the body of the vehicle. The circuit consists of an op-amp IC CA3140 (IC1), two 555 timer ICs (IC2 and IC3) and decade counter CD4017 (IC4). Op-amp IC CA3140 is wired as a voltage comparator. Its inverting input (pin 2) receives a reference voltage controlled through VR1. The noninverting input (pin 3) receives a variable voltage tapped from the input terminal of the fuel meter through resistor R1. When the voltage at pin 3 is higher than at pin 2, the output of IC1 goes high and the green LED (LED1) glows. This condition is maintained until the voltage at pin 3 drops below that at pin 2. When this happens, the output of IC1 swings from high to low, sending a low pulse to the trigger pin of the monostable (usually held high by R3) via C1. The monostable triggers and its output goes high for a predetermined time based on the values of R5 and C2. With the given values, the
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EO SANI TH
‘on’ time will be around four minutes. The output of IC2 is used to power the astable circuit consisting of timer 555 (IC3) via diode D2. Oscillations of IC3 are controlled by R6, R7, VR2 and C4. With the given values, the ‘on’ and ‘off’ time periods are 27 and 18 seconds, respectively. The pulses from IC3 are given to the clock input (pin 14) of decade counter CD4017 (IC4) and its outputs go high one by one. When the circuit is switched on, LED1 and LED2 glow if your vehicle has sufficient petrol in the tank. When the fuel goes below the reserve level, the output of IC1 goes low, LED1 turns off and a negative triggering pulse is received at pin 2 of IC2. The output of IC2 goes high for around four minutes and during this time period, clock pin 14 of IC4 receives the clock pulse (low to high) from the output of IC3. For the first clock pulse, Q0 output of IC4 goes high and the green LED (LED2) glows for around 50 seconds. On receiving the second clock pulse, Q1 goes high to light up the yellow LED (LED3) and sound the buzzer for around 45 seconds. This audio-visual signal warns you that the vehicle is running out of fuel. On receiving the
ELECTRONICS FOR YOU • JULY 2005 • 97
CMYK
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IDEAS
third clock pulse, LED3 and the buzzer go off. There is a gap of around twoand-a-half minutes before Q5 output goes high. By the time Q5 goes high and the red LED (LED4) glows, four minutes elapse and the power supply to IC3 is cut off. The output state at Q5 will not change unless a low-to-high clock input is received at its pin 14. Thus LED4 will glow continuously along with the beep. The continuous glowing of the red LED (LED4) and the beep from the buzzer indicate that the vehicle will run out of fuel very shortly.
Q6 output of IC4 is connected to its reset pin 15 via diode D3. This means that after ‘on’ state of Q5, the count will always start from Q0. Capacitor C5 provides power-on reset to IC4 when switch S1 is closed. The output of IC1 is also connected to reset pin of IC4 via diode D1 (1N4148). So when your vehicle is refueled above the reserve level, LED2 glows to indicate that the tank has sufficient fuel. IC5 provides regulated 12V DC for proper functioning of the circuit even when the battery is charged to more than 12V.
98 • JULY 2005 • ELECTRONICS FOR YOU
The circuit can be assembled on a perforated board. Adjust VR1 until the voltage at pin 2 of IC1 drops to 1.5V. When point A is connected to the fuel meter (fuel gauge) terminal that goes to the fuel sensor, green LEDs (LED1 and LED2) glow to indicate the normal fuel level. VR2 can be varied to set the ‘on’ time period of IC3 at around 20 seconds. Enclose the circuit in a small case and mount on the dashboard using adhesive tape. The circuit works only in vehicles with negative grounding of the body. z
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CMYK
CIRCUIT
IDEAS
DIGITAL AUDIO/VIDEO INPUT SELECTOR
IVEDI S.C. DW
T.K. HAREENDRAN
eed to connect more than one audio-video (AV) source to your colour television? Don’t worry, here’s an AV input expander for your TV. It is inexpensive and easy to construct. The working of the circuit is simple
N
S1 once. To select the second AV signal, press switch S1 twice. In the same way, you can select the other two signals. Momentarily pressing of switch S1 once results in clocking of the decade counter and relay driver transistor T1
and straightforward. Whenever 12V DC is applied to the circuit, power-on LED1 glows. Now reset the decade counter by momentarily pressing switch S2 to make Q0 output of IC1 high. LED2 glows to indicate that the circuit is ready to work. Switch S1 is used for selecting a particular audio-video (AV) signal. To select the first AV signal, press switch
conducts to energise relay RL1. Now normally opened (N/O) contacts of two-changeover relay RL1 connect the television set’s inputs to the first AV signal (marked as Video-In 1 and Audio-in 1). LED3 glows to indicate this. When you press switch S1 twice, the Q2 output of IC1 goes high. Consequently, 2C/O relay RL2 (not shown in the circuit) energises and television
PCB FOR 8085 MICROPROCESSOR KIT (EFY NOVEMBER 99) WITH ALL ITS ICs
inputs are connected to the second AV signal (not shown in the figure). LED4 (not shown in figure) glows to indicate this. Similarly, pressing switch S1 thrice makes the Q3 output of IC1 high. Consequently, 2C/O relay RL3 (not shown in the figure) energises and the television inputs are connected to the third AV signal source. LED5 (not shown in the figure) glows to indicate this. Again, pressing switch S1 four times makes the Q4 output of IC1 high. Consequently, 2C/O relay RL4 energises and the TV inputs are connected to the fourth AV signal source (marked as Video-in 4 and Audio-in 4). LED6 glows to indicate this. Further pressing of switch S1 resets the decade counter and LED2 glows again. Thereafter, the cycle repeats. The circuit is wired for four-input selection, therefore the Q5 output of IC1 is connected to reset pin 15 of IC1. Enclose the assembled PCB along with the relays in a cabinet with the input/output sockets and indicators mounted on the body of the cabinet. z
Available at:
Kits‘n’Spares 303, Dohil Chambers, 46, Nehru Place, New Delhi 110019; Phone: 26430523, 26449577; E-mail:
[email protected]
108 • MARCH 2005 • ELECTRONICS FOR YOU
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CMYK
CIRCUIT IDEAS
TOUCH DIMMER K. KRISHNA MURTY
B
y simply touching this touch dimmer you can increase the light intensity of incandescent lamps in three steps. The touch dimmer is built around 8-pin CMOS IC TT8486A/TT6061A specifically manufactured for touch dim-
I VED DWI S.C.
second touch, the bulb gives medium light. At the third touch, the bulb is driven fully. Another touch puts off the light. Since the IC is highly sensitive, use a long wire to connect the IC to the touch sensor. The circuit uses minimum external components. For touch plate, you can use a simple copper plate of 1cm×1cm or
touch signal is connected to the counter/ decoder via a resistor and clock input CK is connected to the counter/decoder via a frequency generator. Line frequency signal is taken through R4 at pin 2 of IC TT6061A. At zero crossing, the triac (BT136) triggers to drive a 200W bulb. The 6.8V power supply is taken directly from mains through resistors R1 and R3, diode D3, capacitor C4, and zener diode and fed to power-input pin 3 of the IC. Capacitors C1, C2, and C3 connected between touch input pin 4 and touch plate Pin Assignments of IC TT6061A Pin No. 1 2 3 4 5 6 7 8
mer applications. Initially, when mains switch is ‘on,’ the bulb is ‘off’. Now, if you touch the touch plate, the bulb glows dimly. On
even the end of the lead wire. Touch plate is coupled to the touch detector through 820pF, 2kV capacitors C1, C2, and C3 connected in series. Internally IC TT6061A’s
Pin name CK FI VDD TI CI NC VSS AT
Function description System clock input 50Hz line frequency Power input pin for VDD Touch input Sensor control input Not connected Power input pin for VSS Angle-trigger output
remove the shock potential from the touch plate, so do not replace these capacitors with a single capacitor or with a capacitor of a lower voltage rating. Mains potential exists in the circuit. Needless to say, it is dangerous to touch the circuit when mains is ‘on.’ Note. The IC had been procured by the author from SM Semiconductors, Santacruz (W), Mumbai.
DECEMBER 2003
ELECTRONICS FOR YOU
CIRCUIT
IDEAS
MOBILE BUG
IVEDI S.C. DW
D. MOHAN KUMAR
his handy, pocket-size mobile transmission detector can sense the presence of an activated mobile phone from a distance of oneand-a-half metres. So it can be used to prevent use of mobile phones in examination halls, confidential rooms, etc. It is also useful for detecting the use of mobile phone for spying and unauthorised video transmission.
T
quired for a mobile bug. Here the circuit uses a 0.22µF disk capacitor (C3) to capture the RF signals from the mobile phone. The lead length of the capacitor is fixed as 18 mm with a spacing of 8 mm between the leads to get the desired frequency. The disk capacitor along with the leads acts as a small gigahertz loop antenna to collect the RF signals from the mobile phone.
The circuit can detect both the incoming and outgoing calls, SMS and video transmission even if the mobile phone is kept in the silent mode. The moment the bug detects RF transmission signal from an activated mobile phone, it starts sounding a beep alarm and the LED blinks. The alarm continues until the signal transmission ceases. An ordinary RF detector using tuned LC circuits is not suitable for detecting signals in the GHz frequency band used in mobile phones. The transmission frequency of mobile phones ranges from 0.9 to 3 GHz with a wavelength of 3.3 to 10 cm. So a circuit detecting gigahertz signals is re-
Op-amp IC CA3130 (IC1) is used in the circuit as a current-to-voltage converter with capacitor C3 connected between its inverting and non-inverting inputs. It is a CMOS version using gate-protected p-channel MOSFET transistors in the input to provide very high input impedance, very low input current and very high speed of performance. The output CMOS transistor is capable of swinging the output voltage to within 10 mV of either supply voltage terminal. Capacitor C3 in conjunction with the lead inductance acts as a transmission line that intercepts the signals from the mobile phone. This capacitor
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creates a field, stores energy and transfers the stored energy in the form of minute current to the inputs of IC1. This will upset the balanced input of IC1 and convert the current into the corresponding output voltage. Capacitor C4 along with high-value resistor R1 keeps the non-inverting input stable for easy swing of the output to high state. Resistor R2 provides the discharge path for capacitor C4. Feedback resistor R3 makes the inverting input high when the output becomes high. Capacitor C5 (47pF) is connected across ‘strobe’ (pin 8) and ‘null’ inputs (pin 1) of IC1 for phase compensation and gain control to optimise the frequency response. When the mobile phone signal is detected by C3, the output of IC1 becomes high and low alternately according to the frequency of the signal as indicated by LED1. This triggers monostable timer IC2 through capacitor C7. Capacitor C6 maintains the base bias of transistor T1 for fast switching action. The low-value timing components R6 and C9 produce very short time delay to avoid audio nuisance. Assemble the circuit on a generalpurpose PCB as compact as possible and enclose in a small box like junk mobile case. As mentioned earlier, capacitor C3 should have a lead length of 18 mm with lead spacing of 8 mm. Carefully solder the capacitor in standing position with equal spacing of the leads. The response can be optimised by trimming the lead length of C3 for the desired frequency. You may use a short telescopic type antenna. Use the miniature 12V battery of a remote control and a small buzzer to make the gadget pocket-size. The unit will give the warning indication if someone uses mobile phone within a radius of 1.5 metres.
ELECTRONICS FOR YOU • JANUARY 2008 • 135
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IDEAS
HEARING AID
T.K. HAREENDRAN
T
his low-cost, general-purpose electronic hearing aid works off 3V DC (2x1.5V battery). In this circuit, transistor T1 and associated components form the audio signal preamplifier for the acoustic signals picked up by the condenser microphone and converted into corresponding electrical signals. Resistor R5 and capacitor C3 decouple the power supply of the preamplifier stage. Re-
IVEDI S.C. DW
sistor R1 biases the internal circuit of the low-voltage condenser microphone for proper working. The audio output from the preamplifier stage is fed to the input of the medium-power amplifier circuit via capacitor C2 and volume control VR1. The medium-power amplifier section is wired around popular audio amplifier IC TDA2822M (not TDA2822). This IC, specially designed for portable low-power applications, is readily available in 8-pin mini
DIP package. Here the IC is wired in bridge configuration to drive the 32-ohm general-purpose monophonic earphone. Red LED (LED1) indicates the power status. Resistor R8 limits the operating current of LED1. The audio Fig. 2: Enclosure output of this circuit is 10 to 15 mW and the quiescent current drain is below 1 mA. The circuit can be easily assembled on a
Fig. 3: Pin descriptions of cond. mic, C1740 and BC547
veroboard. For easy assembling and maintenance, use an 8-pin DIP IC socket for TDA2822M. Proposed enclosure (with earphone socket) for the assembled unit is shown in Fig. 2. Note. The complete kit is available at Kits‘n’Spares.
Fig. 1: Hearing aid circuit
96 • AUGUST 2006 • ELECTRONICS FOR YOU
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CMYK
CIRCUIT
IDEAS
ANTI-COLLISION REAR LIGHT
ASHOK K. DOCTOR
D
uring poor visibility, i.e., when there is fog, or at dawn or dusk, or when your vehicle gets stalled on a lonely stretch of a highway, this flashing light will provide safety and attract the attention of people to help you out. It uses highbrightness yellow LEDs. The circuit uses a dual binary counter CD4520, quadruple 2-input
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NAND schmitt trigger CD4093, 8-stage shift-and-store bus register CD4094 and some descrete components. An oscillator is built around gate A, whose frequency can be varied through preset VR1 when required. The output of the oscillator is fed to IC1 and IC3. When the circuit is switched on, the oscillator starts oscillating, the counter starts counting through IC1 and the data is shifted on positive-going clock through IC3. As a result, the four groups
IVEDI S.C. DW
of LEDs flash one by one. All the LEDs will then glow for some time and switch off for some time, and the cycle will repeat. Input pins 12 and 13 of the unused gate D must be tied to ground and pin 11 left open. Preset VR1 should be of cermet type and used to change the flashing rate of each group of LEDs. The circuit works off regulated 12V. Assemble it on a general-purpose PCB and house suitably.
ELECTRONICS FOR YOU • JUNE 2007 • 83
CCI IRRC UCIU T II TD E IADS E A S
CELL-PHONE-CONTROLLED AUDIO/VIDEO MUTE SWITCH
I VED DWI . C . S
T.K. HAREENDRAN
T
his cell-phone-controlled audio/ video mute switch is highly useful in automobiles. The circuit automatically disconnects power supply to the audio/video system whenever the mobile handset is lifted off the holder for making
Fig. 2: The circuit of the cell phone-controlled audio/video mute switch
Fig. 1: Proposed cell-phone holder
or receiving a call. You can use any readily available cell-phone holder with some minor alterations or fabricate it yourself as shown in Fig. 1. The circuit is wired around IC LM555 (IC1), the CMOS version of timer NE555, as shown in Fig. 2. IC1 is used as a medium-current line driver with either an inverting or non-inverting output. It can sink (or source) current of up to 50 mA only, so take care while handling it. The audio/
ELECTRONICS FOR YOU
JUNE 2004
video system is connected to the circuit via normally opened (N/O) contacts of the relay. When the cell phone is in its holder, LDR1 does not receive any light from white LED1 and its resistance is high. As a result, the voltage at pin 2 of IC1 remains high to provide a low output at pin 3. The low output of IC1 activates relay RL1 and the audio/video system gets power supply via its N/O contacts. LED3 glows to indicate that the audio/video system is ‘on.’ When the handset is taken off the holder, light rays from LED1 fall on
LDR1 and its resistance decreases. As a result, the voltage at pin 2 of IC1 decreases to provide a high output at its pin 3. The high output of IC1 deactivates relay RL1 and the audio/video system does not get power supply. LED2 glows to indicate that the audio/video system is ‘off.’ Preset VR1 is used to control the sensitivity of the circuit. Zener diode ZD1 is used for protecting white LED1 from the higher voltage. The circuit works off a 12V car battery. Switch S1 can be used to manually switch on/off the audio/video system.
C I R CCUI RICTU IITDIEDAE ASS
CHILD’S LAMP
SAN
I THE
O
D. MOHAN KUMAR
H
ere is a mini emergency lamp that you can use as a tabletop lamp in your child’s study room. It is battery-operated and gives sufficient light for the child to move out of the room when power fails. The white LED in the circuit automatically turns on when light in the room goes off following a power cut. The LED gives a flashing light instead of glowing continuously to reduce power consumption. The circuit comprises a light sensor and an LED flasher designed around CMOS IC CD4093 (IC1). The light sensor switch comprises a light-dependent resistor (LDR) and npn transistors T1 and T2. When ambient light is present, the low resistance of LDR1 drives transistor T1 into conduction. This keeps transistor T2 cut-off due to low base bias. The flasher circuit does not get power as long as ambient light falls on LDR1. When the resistance of LDR1 becomes high in darkness, transistor T1 stops conducting and transistor T2 starts conducting to turn on the LED lamp. IC1 is designed as a simple oscillator using its gate 1 (comprising input pins 1
and 2 and output pin 3). The oscillator’s external components comprise resistor R2 and capacitor C1. Diode D1 and resistor R4 help in rapid charging of capacitor C1. When capacitor C1 charge to around 50% of Vcc, output of gate 1 of IC1 goes low to discharge capacitor C1. The output from pin 3 of IC1 again goes high to charge capacitor C1 again. This cycle repeats and sets up an oscillation, which is given to
gate 2 (comprising input pins 5 and 6 and output pin 4) of IC1. Gate 2 serves as a buffer to drive the white LED (LED1). For the given values of resistor R2 and capacitor C1, the flashing rate of LED1 is one per second (1 Hz). It can be increased by decreasing the value of capacitor C1. Pin 14 of IC1 is Vcc and all the unused input pins are tied to the positive rail (pin 14) to prevent floating. The circuit can be constructed on a small veroboard. Use a reflective holder for LED1, which should be directed downwards at an angle of 45 degrees to prevent direct viewing of LED1 which gives a high-intensity light that is harmful for eyes. Preset VR1 can be adjusted to control the sensitivity of LDR1. You can enclose the circuit in a plastic doll with LED1 as its headlamp to make it an attractive gadget for your child. Mount LDR1 such that ambient light falls on it directly.
MAY 2004
ELECTRONICS FOR YOU
CIRCUIT
IDEAS
SPEED CONTROL OF DC MOTOR USING PULSE-WIDTH MODULATION
age value is 2.5V, and if the duty cycle is 75%, the average voltage is 3.75V and so on. The maximum duty cycle ulse-width modulation (PWM) can be 100%, which is equivalent to a or duty-cycle variation methods DC waveform. Thus by varying the are commonly used in speed pulse-width, we can vary the average control of DC motors. The duty cycle voltage across a DC motor and hence is defined as the percentage of digital its speed. ‘high’ to digital ‘low’ plus digital ‘high’ The circuit of a simple speed conpulse-width during a PWM period. troller for a mini DC motor, such as that used in tape recorders and toys, is shown in Fig. 2. Here N1 inverting Schmitt trigger is configured as an astable multivibrator with constant period Fig. 1: 5V pulses with 0% through 50% duty cycle but variable duty cycle. Although the total in-circuit resistance of VR1 during a complete cycle is 100 kilo-ohms, the part used during positive and negative periods of each cycle can be varied by changing the position of its wiper contact to obtain variable pulse-width. Schmitt Fig. 2: DC motor speed control using PWM method gate N2 simply acts as a buffer/driver to drive transistor T1 during positive inFig. 1 shows the 5V pulses with 0% cursions at its base. Thus the average through 50% duty cycle. amplitude of DC drive pulses or the The average DC voltage value for speed of motor M is proportional to 0% duty cycle is zero; with 25% duty the setting of the wiper position of VR1 cycle the average value is 1.25V (25% potmeter. Capacitor C2 serves as a of 5V). With 50% duty cycle the averEFY LAB
P
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UMAR SUNIL K
storage capacitor to provide stable voltage to the circuit. Thus, by varying VR1 the duty cycle can be changed from 0% to 100% and the speed of the motor from ‘stopped’ condition to ‘full speed’ Fig. 3: Pin in an even and continuconfiguration of BC337A ous way. The diodes effectively provide different timing resistor values during charging and discharging of timing capacitor C1. The pulse or rest period is approximately given by the following equation: Pulse or Rest period ≈ 0.4 x C1 (Farad) x VR1 (ohm) seconds. Here, use the in-circuit value of VR1 during pulse or rest period as applicable. The frequency will remain constant and is given by the equation: Frequency ≈ 2.466/(VR1.C1) ≈ 250 Hz (for VR1=100 kilo-ohms and C1=0.1 µF) The recommended value of in-circuit resistance should be greater than 50 kilo-ohms but less than 2 megaohms, while the capacitor value should be greater than 100 pF but less than 1 µF.
ELECTRONICS FOR YOU • AUGUST 2006 • 97
CMYK
CIRCUIT
IDEAS
LIGHT FENCE
IVEDI S.C. DW
D. MOHAN KUMAR
he basic problem with most of standard light sensors is that they require precise alignment of light beam to mute the circuit dur-
T
ent day light or fluorescent electric light. The beep generated from the circuit will be loud enough to detect the entry of a person in the room or the protected area being guarded.
ing standby mode. The circuit described here is so sensitive that it will detect a moving person at a distance of few metres in daylight or under electric lighting without cumbersome alignment of light beam. It requires virtually no set up, and may be simply placed within the line-of-sight of almost any light source including ambi-
The circuit uses a voltage comparator and a monostable timer to give the warning alarm on detecting a moving person. IC µA741 (IC1) is used as a voltage comparator with two potential dividers in its inverting and noninverting inputs. Resistors R1 and R2 provide half-supply voltage of 4.5 volts to its inverting input (pin 2).
84 • JUNE 2007 • ELECTRONICS FOR YOU
LDR1 and preset VR1 form another potential divider to provide a variable voltage input to the non-inverting input (pin 3). If VR1 is properly adjusted for the required light level, the output of IC1 will be high, which drives pnp transistor T1 out of conduction. This is due to the high potential at the base of T1. The emitter voltage of T1 will be high in this condition, which inhibits IC2 from oscillation and LED1 from lighting. IC2 is wired as a monostable timer. R6 and C2 provide a preset time delay. As a person crosses the protected area, his shadow will be sensed by LDR1 due to change in the light intensity level and the voltage at the non-inverting input of IC1 will drop momentarily. The output of IC1 suddenly becomes low, allowing T1 to conduct. This triggers the monostable (IC2) and the alarm sounds. Assemble the circuit on a common PCB and house in a plastic case. Keep LDR1 inside a black tube to increase its sensitivity. Adjust preset VR1 until LED1 turns off at the particular light level. Keep LDR1 facing the entrance of the room or the area to be protected. Sensitivity of the circuit depends on the proper adjustment of VR1. If VR1 is correctly adjusted, the circuit can detect a moving person from a distance of about three metres.
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CIRCUIT IDEAS
DTMF RECEIVER IC MT8870 TESTER RONE POUMAI
T
oday, most telephone equipment use a DTMF receiver IC. One common DTMF receiver IC is the Motorola MT8870 that is widely used in electronic communications circuits. The MT8870 is an 18-pin IC. It is used in telephones and a variety of other applications. When a proper output is not obtained in projects using this IC, engineers or technicians need to test this IC separately. A quick testing of this IC could save a lot of time in research labs and manufacturing industries of communication instruments. Here’s a small and handy tester circuit for the DTMF IC. It can be assembled on a multipurpose PCB with an 18-pin IC base. One can also test the IC on a simple breadboard. For optimum working of telephone equipment, the DTMF receiver must be designed to recognise a valid tone pair greater than 40 ms in duration and to accept successive digit tone-pairs that are greater than 40 ms apart. However, for other applications like remote controls and
radio communications, the tone duration may differ due to noise considerations. Therefore, by adding an extra resistor and steering diode the tone duration can be set to different values. The circuit is configured in balancedline mode. To reject common-mode noise signals, a balanced differential amplifier input is used. The circuit also provides an excellent bridging interface across a properly terminated telephone line. Transient protection may be achieved by splitting the input resistors and inserting zener diodes (ZD1 and ZD2) to achieve voltage clamping. This allows the transient energy to be dissipated in the resistors and diodes, and limits the maximum voltage that may appear at the inputs. Whenever you press any key on your local telephone keypad, the delayed steering (Std) output of the IC goes high on receiving the tone-pair, causing LED5 (connected to pin 15 of IC via resistor R15) to glow. It will be high for a duration depending on the values of capacitor and resistors at pins 16 and 17.
I VED DWI S.C.
The Status of LEDs on Pressing Keys on the Telephone Keypad Key No.
LED4 (MSB)
LED3
LED2
LED1 (LSB)
1 2 3 4 5 6 7 8 9 0 A B C D
Off Off Off Off Off Off Off On On On On On On Off
Off Off Off On On On On Off Off Off On On On Off
Off On On Off Off On On Off Off On Off On On Off
On Off On Off On Off On Off On Off On Off On Off
Note. 1. LED5 blinks momentarily whenever any key is pressed. 2. On = 1, while Off = 0
The optional circuit shown within dotted line is used for guard time adjustment. The LEDs connected via resistors R11 to R14 at pins 11 through 14, respectively, indicate the output of the IC. The tone-pair DTMF (dual-tone multi-frequency) generated by pressing the telephone button is converted into binary values internally in the IC. The binary values are indicated by glowing of LEDs at the output pins of the IC. LED1 represents the lowest significant bit (LSB) and LED4 represents the most significant bit (MSB). So, when you dial a number, say, 5, LED1 and LED3 will glow, which is equal to 0101. Similarly, for every JUNE 2003
ELECTRONICS FOR YOU
CIRCUIT IDEAS other number dialled on your telephone, the corresponding LEDs will glow. Thus, a non-defective IC should indicate proper binary values corresponding to the decimal number pressed on your telephone keypad. To test the DTMF IC 8870/KT3170, proceed as follows: 1. Connect local telephone and the circuit in parallel to the same telephone line. 2. Switch on S1. (Switch on auxiliary
ELECTRONICS FOR YOU
JUNE 2003
switch S2 only if keys A, B, C, and D are to be used.) 4. Now push key ‘*’ to generate DTMF tone. 5. Push any decimal key from the telephone keypad. 6. Observe the equivalent binary as shown in the table. 7. If the binary number implied by glowing of LED1 to LED4 is equivalent to the pressed key number (decimal/A, B, C,
or D), the DTMF IC 8870 is correct. Keys A, B, C, and D on the telephone keypad are used for special signalling and are not available on standard pushbutton telephone keypads. Pin 5 of the IC is pulled down to ground through resistor R8. Switch on auxiliary switch S2. Now the high logic at pin 5 enables the detection of tones representing characters A, B, C, and D. This circuit costs around Rs 80.
CIRCUIT IDEAS
LOW-COST HEARING AID
EDI DWIV S.C.
PRADEEP G.
C
ommercially available hearing aids are quite costly. Here is an inexpensive hearing aid circuit that uses just four transistors and a few passive components. On moving power switch S to ‘on’ position, the condenser microphone detects the sound signal, which is amplified by transistors T1 and T2. Now the amplified signal passes through coupling capacitor C3 to the base of transistor T3. The signal is further amplified by pnp transistor T4 to drive a lowimpedance earphone. Capacitors C4 and C5 are the power supply decoupling capacitors. The circuit can be easily assembled on a small, general-purpose PCB or a Vero board. It operates off a 3V DC sup-
ply. For this, you may use two small 1.5V cells. Keep switch S to ‘off’ state when the circuit is not in use. To increase the
sensitivity of the condenser microphone, house it inside a small tube. This circuit costs around Rs 65.
AUGUST 2003
ELECTRONICS FOR YOU
CIRCUIT IDEAS
SAN
I THE
O
S.C. DWIVEDI
T
his LED-based message display is built around readily availble, lowcost components. It is easy to fabricate and makes use of 3mm red LEDs. A total of 172 LEDs have been arranged to display the message “HAPPY NEW YEAR 2004.” The arrangement of LED1 through LED11 is used to display ‘H’ as shown in Fig. 1. The anodes of LED1 through LED11 are connected to point A and the cathodes of these LEDs are connected to point B. Similarly, letter ‘A’ is built using LED12 through LED21. All the anodes of LED12 through LED21 are connected to point A, while the cathodes of these LEDs are connected to resistor R8 (not shown in the circuit diagram). Other letters/words can also be easily arranged to make the required sentence. The power supply for the message display circuit (Fig. 2) comprises a 0-9V, 2A step-down transformer (X1), bridge rectifier comprising diodes D1 through D4, and a filter capacitor (C1). IC 7806 (IC1)
Fig. 2: Circuit diagram of LED-based message display
LED-BASED MESSAGE DISPLAY
Fig. 1: LED arrangement for word ‘H’ JANUARY 2004
ELECTRONICS FOR YOU
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CIRCUIT IDEAS provides regulated 6V DC to the display circuit comprising timer 555 (IC2) and decade counter CD4017 (IC3). The astable multivibrator built around IC2 produces 1Hz clock at its output pin 3. This output is connected to clock pin (pin 14) of the decade counter. The decade counter can count up to 10. The output of IC3 advances by one count every second (depending on the time period of astable multivibrator IC2). When Q1 output of IC3 goes high, transistor T1 conducts and the current flows through LED1 through LED48 via resistors R7 through R11. Now the word ‘HAPPY’ built around LED1 through LED48 is displayed on the LED arrangement board. Next, when Q2 output of IC3 goes
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high, transistor T2 conducts and the current flows through LED49 through LED87 via resistors R12 through R14. Now the word ‘NEW’ is displayed on the LED arrangement board. Again, when Q3 output goes high, transistor T3 conducts and the current flows through LED88 through LED128 via resistors R15 through R18. Now the word ‘YEAR’ is displayed on the LED arrangement board. Similarly, when Q4 output goes high, transistor T4 conducts and the current flows through LED129 through LED172 via resistors R19 through R22. Now digits ‘2004’ are displayed on the LED arrangement board. During the entire period when Q5,
Q6, Q7, or Q8 output go high, transistor T5 conducts and the current flows through all the LEDs via diodes D9 through D12 and resistors R7 through R22. Now the complete message “HAPPY NEW YEAR 2004” is displayed on the LED arrangement for four seconds. Thus, the display board displays ‘HAPPY,’ ‘NEW,’ YEAR’ and ‘2004’ one after another for one second each. After that, the message “HAPPY NEW YEAR 2004” is displayed for 4 seconds (because Q5 through Q8 are connected to resistor R6 via diodes D5 through D8). At the next clock input output Q9 goes high, and IC3 is reset and the display is turned off for one second. Thereafter the cycle repeats.
CIRCUIT IDEAS
REMOTE-OPERATED MUSICAL BELL
EDI DWIV S.C.
PRADEEP G.
T
his infrared lightcontrolled 12-tone musical bell can be operated using any TV remote control. It can be operated from up to 10 metres, provided the remote control is directed towards the sensor. The circuit uses the popular 3-lead IR sensor TK1836 to trigger musical bell built around IC UM3481(IC1). (You can also use IC UM3482, UM3483, or UM3484 in place of IC UM3481.) The sensor responds only to 36 kHz. Most TV remote controls transmit this frequency. When any button on the TV remote control is pressed, the sensor’s output
ELECTRONICS FOR YOU
OCTOBER 2003
pulses low. Transistor T1 conducts to apply a triggering pulse to IC1 at its pin 4. After playing one musical tone, the circuit automatically resets. If you again press any of the remote’s buttons, another music is
heard. This way, twelve different musical tones can be generated. The circuit works off a 5V power supply. Regulator IC 7805, powered from a 912V DC source, provides regulated 5V.
CIRCUIT IDEAS
INFRARED REMOTE CONTROL TIMER
T
his infrared remote control timer can be used to turn an appliance on/off for a period of 0.11 second to 110.0 seconds. The circuit comprises two sections, namely, the transmitter section and the receiver section. Fig. 1 shows the IR transmitter section. The astable multivibrator NE555 (IC1) is used to generate a 10kHz modulated IR signal. The output of IC1 is connected to the base of pnp transistor T1 via resistor R2. Two infrared LEDs (IR1 and IR2) are connected in series between the collector (via resistor R3) and ground. When switch S1 is pressed, the IR LEDs transmit the modulated IR signal of 10-11 kHz. This frequency can be changed with the help of VR1 potmeter. In the receiver section shown in Fig. 2, two photodiodes (IR3 and IR4) receive the IR signal transmitted by the IR transmitter. Transistors T2 and T3 amplify the weak signal. The amplified signal is filtered by capacitors C6 and C7. The amplified and filtered signal is now fed to the inverting input pin 2 of op-amp IC2 (IC 741). The output of IC2 is further connected to trigger pin 2 of timer NE555 (IC3) that is used as a monostable multivibrator
Fig. 1: IR transmitter section
whose frequency may be varied with the help of potmeter VR3. When switch S1 of the transmitter is pressed, the modulated IR rays are generated, which are received by photodiodes in the receiver section and amplified by the amplifier circuit. The output of op-amp goes low to trigger the monostable. Then high output at pin 3 of IC3 activates the twochangeover relay RL via transistor T3 (BC548) for a preset time. The on/off time can be set in the timer with the help of VR3 and C10. Switch S2 is used to reset the monostable. If you want to turn the appliance on for a preset time, connect the appliance via relay RL(a). On the other hand, if you want to turn the appliance off for a preset time, connect the appliance via relay RL(b). The timer can be reset by pressing reset switch S2. The circuit works up to 3 metres without using any focusing lens. However, you can increase the operating range by using focusing lens. This circuit costs around Rs 100.
Fig. 2: IR receiver section
DIPANJAN BHATTACHARJEE
I VED DWI S.C.
MAY 2003
ELECTRONICS FOR YOU
CIRCUIT IDEAS
PULSE GENERATOR A. JEYABAL
T
his circuit is very useful while checking/operating counters, stepping relays, etc. It avoids the procedure of setting a switch for the required number of pulses. By pressing appropriate switches S1 to S9, one can get 1 to 9 negative-going clock pulses, respectively. Schmitt trigger NAND gate N1 of IC2, resistor R1, and capacitor C1 are wired to produce clock pulses. These pulses are taken out through NAND gate N3 that is controlled by decade counter CD4017 (IC1). Initially no switch from S1 to S9 is
I VED DWI . C . S
depressed and the LED is glowing. As pins 5 and 6 of NAND gate N2 are pulled up by resistor R3, its output pin 4 goes low. This disables NAND gate N3 to take its output pin 10 to high state, and no pulse is available. IC1 is a decade counter whose Q outputs normally remain low. When clock pulses are applied, its Q outputs go high successively, i.e. Q0 shifts to Q1, Q1 shifts to Q2, Q3 shifts to Q4, and so on. If any one of switches S1 through S9, say, S5 (for five pulses), is momentarily depressed, pins 5 and 6 of NAND gate N2 go low, making its output pin 4 high, which
fully charges capacitor C2 via diode D. At the same time, this high output of N2 enables NAND gate N3 and clock pulses come out through pin 10. These are the required number of pulses used to check our device. The clock pulses are fed to clock-enable pin 13 of IC1, which starts counting. As soon as output pin 1 (Q5) of IC1 turns high, input pins 5 and 6 of NAND gate N2 will also become high via switch S5 because high-frequency clock allowed five pulses during momentary pressing. This high input of N2 provides low output at pin 4 to disable NAND gate N3 and finally no pulse will be available to advance counter IC1. Before the next usage, counter IC1 must be in the standby state, i.e. Q0 output must be in the high state. To do this, a time-delay pulse generator wired around NAND gate N4, resister R4, diode D, capacitor C2, and differentiator circuit comprising C3 and R5 is used. When output pin 4 of NAND gate N2 is low, it discharges capacitor C2 slowly through resistor R4. When the voltage across capacitor C2 goes below the lower trip point, output pin 11 of NAND gate N4 turns high and a high-going sharp pulse is produced at the junction of capacitor C3 and resistor R5. This sharp pulse resets counter IC1 and its Q0 output (pin 3) goes high. This is represented by the glowing of LED. Ensure the red LED is glowing before proceeding to get the next pulse. Press any of the switches momentarily and the LED will glow. If the switch is kept pressed, the counter counts continuously and you cannot get the exact number of pulses. This circuit costs around Rs 70.
JUNE 2003
ELECTRONICS FOR YOU
CIRCUIT IDEAS
OVER-/UNDER-VOLTAGE PROTECTION OF ELECTRICAL APPLIANCES C.H. VITHALANI
MAR IL KU SUN
his circuit protects refrigerators as well as other appliances from overand under-voltage. Operational amplifier IC LM324 (IC2) is used here as a comparator. IC LM324 consists of four operational amplifiers, of which only two op-
T
amplifier is zero and transistor T1 remains off. The relay, which is connected to the collector of transistor T1, also remains deenergised. As the AC supply to the electrical appliances is given through the normally closed (N/C) terminal of the relay, the supply is not disconnected during normal operation.
are protected against over-voltage. Now let’s consider the under-voltage condition. When the line voltage is below 180V, the voltage at the inverting terminal (pin 6) of operational amplifier N2 is less than the voltage at the non-inverting terminal (6V). Thus the output of operational amplifier N2 goes high and it
erational amplifiers (N1 and N2) are used in the circuit. The unregulated power supply is connected to the series combination of resistors R1 and R2 and potmeter VR1. The same supply is also connected to a 6.8V zener diode (ZD1) through resistor R3. Preset VR1 is adjusted such that for the normal supply of 180V to 240V, the voltage at the non-inverting terminal (pin 3) of operational amplifier N1 is less than 6.8V. Hence the output of the operational
When the AC voltage increases beyond 240V, the voltage at the non-inverting terminal (pin 3) of operational amplifier N1 increases. The voltage at the inverting terminal is still 6.8V because of the zener diode. Thus now if the voltage at pin 3 of the operational amplifier is higher than 6.8V, the output of the operational amplifier goes high to drive transistor T1 and hence energise relay RL. Consequently, the AC supply is disconnected and electrical appliances turn off. Thus the appliances
energises the relay through transistor T1. The AC supply is disconnected and electrical appliances turn off. Thus the appliances are protected against under-voltage. IC1 is wired for a regulated 12V supply. Thus the relay energises in two conditions: first, if the voltage at pin 3 of IC2 is above 6.8V, and second, if the voltage at pin 6 of IC2 is below 6V. Over-voltage and under-voltage levels can be adjusted using presets VR1 and VR2, respectively. This circuit costs around Rs 110.
AUGUST 2003
ELECTRONICS FOR YOU
CIRCUIT IDEAS
PC-BASED DC MOTOR SPEED CONTROLLER R. KARTHICK
SAN
I THE
O
his circuit allows you to control the speed of a DC motor (in eight levels) from your PC’s parallel port. The PC uses a software program to control the speed of the motor. The motor is connected to the PC through an interface circuit. The interface
The resistor network comprising presets VR1 through VR8, resistors R1 and R2 and capacitor C1 are the timing components of timer IC 555 (IC4), which is configured in astable mode. The output of IC4 is a square wave, which is fed to the base of transistor T1 via current-limiting resistor R3. Transistor T1 is used to drive the motor.
The software (speedM.c) is written in ‘C’ language and compiled using Turbo C compiler. Initially, when the motor is ‘off,’ the program prompts you to press ‘Enter’ key to start the motor. Once you press the key, the motor starts running at full speed. After a few seconds, the program asks you to press any key from the keyboard to go
circuit consists of 1-of-8 decoder IC 74LS138 (IC1), hex inverter ICs 74LS04 (IC2 and IC3), resistor networks, timer IC 555 (IC4) and motor driver transistor SL100 (T1). The decoder IC accepts binary weighted inputs A0, A1 and A2 at pins 1, 2 and 3, respectively. With active-low enable input pins 4 and 5 of the decoder grounded, it provides eight mutually exclusive active-low outputs (Q0 through Q7). These outputs are inverted by hex inverters IC2 and IC3.
The pulse-width modulation (PWM) method is used for efficient control of the motor. The output of the PC is decoded to select a particular preset (VR1 through VR8). The value of the selected preset, along with resistors R1 and R2 and capacitor C1, changes the output pulse width at pin 3 of IC4. Thus the motor speed can be increased/decreased by choosing a particular resistance. For high-power motors, the transistor can be replaced by an IGBT or a power MOSFET.
to the next screen for controlling the speed of the motor. This screen has options for increasing and decreasing the motor speed and also for exiting from the program. For increasing the speed enter choice 1 and press ‘Enter’ key, and for decreasing the speed enter choice 2 and press ‘Enter’ key. This action changes the speed by one step at-a-time and the message “Speed decreased” or “Speed increased” is displayed on the screen. To go to the main menu, again press ‘Enter’ key.
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SPEEDM.C //R.KARTHICK,III ECE,K.L.N.C.E.,MADURAI //
[email protected] #include
#include int a[7],i,c; void start(void); void main(void) { int P=0x0378,j,c=7,c1,x,y;
clrscr(); outportb(P,0); textbackground(9); textcolor(3); for(x=0;x<=80;x++) for(j=0;j<=25;j++) { gotoxy(x,j); cprintf(" ");
} for(i=0;i<8;i++) a[i]=i; gotoxy(23,11); printf("Press Enter to start the motor"); getch(); gotoxy(28,13); printf("WAIT STARTING MOTOR"); start(); JUNE 2004
ELECTRONICS FOR YOU
CIRCUIT IDEAS gotoxy(25,15); printf("Motor started sucessfully"); gotoxy(22,17); printf("Press any key for speed control"); getch(); while(1) { clrscr(); gotoxy(25,3); for(j=0;j<79;j++) { gotoxy(j+1,2); printf("*"); } gotoxy(23,3); printf("DC MOTOR SPEED CONTROL USING PC"); for(j=0;j<79;j++) { gotoxy(j+1,4); printf("*"); } printf("\n"); printf("\t\t\t1.INCREASE SPEED\n\t\t\t2.DECREASE SPEED\n\t\t\t3.EXIT") ; for(j=0;j<79;j++) { gotoxy(j+1,8); printf("*"); } for(j=0;j<79;j++) { gotoxy(j+1,10); printf("*"); }
ELECTRONICS FOR YOU
JUNE 2004
gotoxy(1,9); printf("Enter your choice:"); scanf("%d",&c1); switch(c1) { case 1:if(c==7) { clrscr(); gotoxy(23,13); printf("MOTOR IS RUNNING IN FULL SPEED"); getch(); } if(c<7) { clrscr(); c++; outport(P,a[c]); gotoxy(33,13); printf("SPEED INCREASED"); getch(); } break; case 2: if(c==0) { clrscr(); gotoxy(23,13); printf("MOTOR IS RUNNING IN LOW SPEED"); getch(); } if(c>0) { clrscr(); c--;
outport(P,a[c]); gotoxy(33,13); printf("SPEED DECREASED"); getch(); } break; case 3 : for(j=c;j>=0;j--) { outportb(0X0378,j); delay(100); } outportb(P,0); clrscr(); gotoxy(17,13); textcolor(2); cprintf("KARTHICK.R\nECE\nK.L.N.COLLEGE OF ENGG\nMADURAI."); getch(); exit(1); } } } void start() { outportb(0x0378,0); for(i=0;i<8;i++) { outportb(0X0378,i); delay(1000); } }
CIRCUIT IDEAS
ELECTRONIC MOTOR STARTER T.A. BABU
MAR IL KU SUN
his motor starter protects singlephase motors against voltage fluctuations and overloading. Its salient feature is a soft on/off electronic switch for easy operation. The transformer steps down the AC voltage from 230V to 15V. Diodes D1 and
preset VR1 such that T1 conducts when voltages goes beyond upper limit (say, 260V). When T1 conducts, it switches off T2. Transistor T2 works as the under-voltage protector. The under-voltage setting is done with the help of preset VR2 such that T2 stops conducting when voltage is below lower limit (say, 180V). Zener diodes ZD1 and ZD2 provide base bias to transistors T1
While making over-/under-voltage setting, disconnect C2 temporarily. Capacitor C2 prevents relay chattering due to rapid voltage fluctuations. Regulator IC 7809 gives the 9V regulated supply to soft switch as well as the relay after filtering by capacitor C4. A suitable miniature circuit breaker is used for automatic over-current protection. Green
D2 rectify the AC voltage to DC. The unregulated power supply is given to the protection circuit. In the protection circuit, transistor T1 is used to protect the motor from over-voltage. The over-voltage setting is done using
and T2, respectively. Transistors T3 and T4 are connected back to back to form an SCR configuration, which behaves as an ‘on’/ ‘off’ control. Switch S1 is used to turn on the pump, while switch S2 is used to turn off the pump.
LED (LED1) indicates that the motor is ‘on’ and red LED (LED2) indicates that the power is ‘on’. The motor is connected to the normally-open contact of the relay. When the relay energises, the motor turns on.
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OCTOBER 2003
ELECTRONICS FOR YOU
CIRCUIT IDEAS
WATCHMAN WATCHER JAYAN A.R.
I VED DWI S.C.
ere is a circuit that can be used in offices, stores, warehouses, etc during night to check whether the watchman of your establishment is on duty. For operation, it uses an existing telephone (e.g. in office or store) closest to the watchman’s post. The watchman is given an audio alert signal by just ringing the office/store telephone once (minimum) from your residence or any other place,
an instruction to register his presence by simply pointing his torch-light beam towards a wall-mounted LDR sensor unit (without lifting the handset off-cradle of the ringing telephone). This is to be done within the time period during which the alert lamp glows. If he fails to do it within the permissible time, the circuit registers his absence by incrementing a count. If he does, the count remains unaltered. Up to nine separate alert rings are considered here. The count displayed is the
the circuit are connected across optocoupler MCT2E (IC1) through a resistor-capacitor (R1-C1) combination. The diode in the optocoupler conducts only during ring pulses. The collector of the optocoupler transistor is normally off and a 5V signal is available here. This signal is connected to the trigger input of IC 555 (IC2) configured in monostable mode. The time constant of IC2 is set to nearly one minute (1.1RxC). Its output pin 3 is low during normal mode of operation and the relay is
preferably using your mobile phone. The ring is detected by the given circuit and the watchman is also given a visual alert signal by a glowing lamp. The lamp remains ‘on’ for a duration of nearly 60 seconds soon after the ringtone. The watchman is given
number of times the watchman failed to register his presence. The mobile phone records the called number and call time, and it can be used with the displayed count to get the timing details. The telephone lines (TIP and RING) in
de-energised. When the phone rings, the internal transistor of the optocoupler conducts to cause a high-to-low transition at trigger pin 2 of monostable IC2. Timer IC2 gets triggered on this trailing edge to energise
H
ELECTRONICS FOR YOU
MAY 2004
CIRCUIT IDEAS the torch is used as a remote for triggering MRpin 14 PL pin 11 UPpin5 DNpin4 Mode monostable IC6 and this triggering is enabled only H X X X Reset when alert lamp L1 is L L X X Preset ‘on.’ L H H H No change Monostable IC6 has L H H Count up a time constant of nearly L H H Count down one minute (1.1RxC). It Note: X = Don’t care is used to form a down relay RL1. This relay is used to switch on clock signal for 4-bit up-/down-counter alert lamp L1. The circuit doesn’t respond 74LS192 (IC7). Counter IC7 has two sepato additional trigger inputs for the set du- rate clocks for up and down counts (refer ration of the monostable. The caller may to the table). For correct counting, it needs cut the phone call after hearing ringback one clock line to be high during high-tolow transition of the other clock line. Othtone from the called phone. The sensor circuit formed using LDR1 erwise, it counts erratically. To operate counter IC7, the voltage activates another monostable 555 (IC6). LDR1 has a resistance of 2.2 kilo-ohms in levels and timings of the two clock inputs daylight, which drops below 50 ohms when (up and down) are to be properly adjusted. torchlight beam falls on it. (An LDR of Both trigger inputs, i.e. up and down nearly 2cm diameter has been used in this clocks, are asynchronous. The output of monostable IC2 is filcircuit.) Comparator LM358 (IC5) compares the level set at pin 3 (nearly 1V, set using a tered using capacitor C4 to remove unwanted transitions and inverted using 10k pot) with the level at pin 2. When no light is falling on LDR1, its Schmitt trigger inverter 74LS14 (IC3). This voltage is above 1V and IC5 has a low forms a signal with correct rising and output at its pin 1. When light is falling on falling edges. The inverted signal from LDR1, its voltage drops below 1V and IC5 pin 6 of gate N3 is used as the up clock. Counter 74LS192 (IC7) is reset to zero output at its pin 1 becomes high. This lowto-high transition is NANDed with the out- state by making its reset pin 14 high through put of monostable IC2 (via inverters gates reset switch S1. The 7-segment, commonN1 and N2) to form the trigger signal for anode display DIS1 is driven through IC monostable IC6. So the trigger input is 74LS47 (IC8). When the phone rings, count normally high, which falls when torchlight ‘1’ is displayed after nearly one minute. beam is focused on LDR1. It returns to high This happens if the watchman fails to focus state when torchlight is switched off. So the torchlight beam on LDR1. Mode-Select Table of 74LS192
If LDR1 receives light from the torch of the watchman within the allowed time period, the down clock remains high until the up clock is high. The counter counts up and then down, so, in effect, the count remains unchanged. All components, except LDR1, are kept in a sealed cabinet with locking arrangement. Only LDR1 is wall-mounted and visible outside. This is done to avoid manual resetting of the counter. The circuit is to be powered by a battery to avoid resetting of the count during power failure. The working procedure can be summarised as follows: 1. Initially, when the power supply is switched on, power-on-reset components C8 and R13 reset counter IC7 and the display shows ‘0.’ 2. Now dial the telephone number (where parallel system is installed) from outside or from your mobile. For the first ring, relay RL1 energises and alert lamp L1 glows. 3.When alert lamp L1 is off, the counter is incremented by ‘1.’ 4. If the watchman focuses the torchlight beam on LDR1 within the glowing time of alert lamp L1, the counter first counts up and then counts down and finally the display shows 0. This indicates that the watchman is present. 5. If the watchman focuses the torchlight beam on LDR1 after alert lamp L1 goes off, up-counting takes place and the display shows ‘1.’ This indicates that the watchman is absent.
MAY 2004
ELECTRONICS FOR YOU
CIRCUIT IDEAS
MULTIBAND CW TRANSMITTER REJIMON G. VU2RGQ
A
radio frequency oscillator is at the heart of all radio transmitters and receivers. It generates high-
Fig. 1: Circuit of multiband CW transmitter
frequency oscillations, which are known as carrier waves. Here’s a continuouswave (CW) transmitter for transmitting Morse code signals in the shortwave band (see Fig. 1). It is basically a variable frequency oscillator (VFO) whose frequency can be varied from 5.2 MHz to 15 MHz. The signal can be received in the shortwave band by any radio re-
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ceiver. The circuit works off a 9V battery. Connect the Morse key (S1) across capacitor C5 as shown in the figure. Attach a telescopic antenna (capable of transmitting over a short distance) at the output terminal. The coil and gang capacitor C2 form the tank circuit. The coil (L) has a total of 60 t u r n s . Winding details are given in Fig. 2. Tappings on the coil allow selection of the required band. The frequency can be varied using C2 (main tuning). On reducing turns of the coil (using selector switch S2), the oscillator’s frequency increases because frequency is inversely proportional to inductance. Capacitor C1 couples the signal from the tank circuit to the base of transistor T1 (2N2222). Transistor T1 provides
SAN
I THE
O
the required positive feedback for oscillation and transistor T2 (BC547) functions as the emitter follower. The output is taken from the emitter of T2.
Fig. 2: Details of the inductor
For stable oscillations, use a polystyrene capacitor as C1. All other capacitors may be ceramic disk type. Enclose the circuit in a metal box for better shielding.
CIRCUIT
IDEAS
SIMPLE SHORT-WAVE TRANSMITTER
IVEDI S.C. DW
PRINCE PHILLIPS
T
his low-cost short-wave transmitter is tunable from 10 to 15 MHz with the help of ½J gang condenser VC1, which determines the carrier frequency of the transmitter in conjunction with inductor L1. The frequency trimming can be done with VC2. The carrier is amplified by transistor T4 and coupled to RF amplifier transistor T1 (BD677) through transformer X1*. The transmitter does not use any modulator transformer. The audio output from condenser MIC is preamplified by transistor T3 (BC548). The audio output from T3 is further amplified by transistor T2 (BD139), which modulates the RF amplifier built around transistor T1 by varying the current through it in accordance with the audio signal’s amplitude. RFC1 is used to block the carrier RF signal from transistor T2 and the power supply. The modulated RF is coupled to the antenna via capacitor C9. For antenna, one can use a 0.5m long telescopic aerial. Details of RF choke, inductor L1 and coupling
Fig. 1: Simple shortwave transmitter
Fig. 2: Pin configurations of BD139, BD677 and BC548
transformer are given in the figure. EFY Lab. During testing, in place of coupling
WWW.EFYMAG.COM
transformer X1, we used a readymade short-wave antenna coil with tuning slug (Jawahar make), which worked satisfactorily. We tested the transmitter reception up to 75 metres and found good signal strength.
ELECTRONICS FOR YOU • APRIL 2006 • 95
CMYK
CIRCUIT
IDEAS
SOLAR LIGHTING SYSTEM
ASHISH AHUJA
IVEDI S.C. DW
he world cannot continue to rely for long on fossil fuels for its energy requirements. Fossil fuel reserves are limited. In addition, when burnt, these add to global warming, air pollution and acid rain. So solar photovoltaic systems are ideal for providing independent electrical power and lighting in isolated rural areas that are far away from the
ing occurs moments after the voltage across it falls below 12V. Capacitor C1 also filters the rectified output if the battery is charged through AC power. The higher the value of the capacitor, the more the delay in switching. The switching time is to be properly adjusted because the charging would practically stop in the early evening while we want the light to be ‘on’ during late evening. During daytime, relay RL1
ing and the battery is in the charging mode. At night, there will be no generation of electricity. The relay will not energise and charging will not take place. The solar energy stored in the battery can then be used to light up the lamp. A 3W lamp glows continuously for around 6 hours if the battery is fully charged. Instead of a 3W lamp, you can also use a parallel array of serially connected white LEDs and lim-
power grid. These systems are nonpolluting, don’t deplete the natural resources and are cheap in the long run. The aim of this circuit is to demonstrate how we can utilise solar light to electrify the remote areas, i.e., how we can store the solar energy and then use it for small-scale lighting applications. Solar cells generate direct current, so make sure that DPDT switch S1 is towards the solar panel side. The DC voltage from the solar panel is used to charge the battery and control the relay. Capacitor C1 connected in parallel with a 12V relay coil remains charged in daytime until the relay is activated. Capacitor C1 is used to increase the response time of the relay, so switch-
energises, provided DPDT switch S1 is towards the solar panel side. Due to energisation of relay RL1, the positive terminal of the battery is connected to the output of regulator IC 7808 (a 3terminal, 1A, 8V regulator) via diode D1 and normally-open (N/O) contacts of relay RL1. Here we have used a 6V, 4.5Ah maintenance-free, lead-acid rechargeable battery. It requires a constant voltage of approx. 7.3 volts for its proper charging. Even though the output of the solar panel keeps varying with the light intensity, IC 7808 (IC1) is used to give a constant output of 8V. Diode D1 causes a drop of 0.7V, so we get approx. 7.3V to charge the battery. LED1 indicates that the circuit is work-
iting resistors to provide sufficient light for even longer duration. In case the battery is connected in reverse polarity while charging, IC 7808 will get damaged. The circuit indicates this damage by lighting up LED2, which is connected in reverse with resistor R2. However, the circuit provides only the indication of reverse polarity and no measure to protect the IC. A diode can be connected in reverse to the common terminal of the IC but this would reduce the voltage available to the battery for charging by another 0.7 volt. There is also a provision for estimating the approximate voltage in the battery. This has been done by connecting ten 1N4007 diodes (D2 through D11)
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in forward bias with the battery. The output is taken by LED3 across diodes D2, D3, D4 and D5, which is equal to 2.8V when the battery is fully charged. LED3 lights up at 2.5 volts or above. Here it glows with the voltage drop across the four diodes, which indicates that the battery is charged. If the bat-
tery voltage falls due to prolonged operation, LED3 no longer glows as the drop across D2, D3, D4 and D5 is not enough to light it up. This indicates that the battery has gone weak. Microswitch S1 has been provided to do this test whenever you want. If the weather is cloudy for some
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consecutive days, the battery will not charge. So a transformer and full-wave rectifier have been added to charge the battery by using DPDT switch S1. This is particularly helpful in those areas where power supply is irregular; the battery can be charged whenever mains power is available.
ELECTRONICS FOR YOU • APRIL 2006 • 99
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CIRCUIT IDEAS
FLASHING-CUM-RUNNING LIGHT A. SIVASUBRAMANIAN
his circuit generates flashing lights in running pattern. In conventional running lights, the LEDs glow one
quencies, which are given to decade counter IC2. The decade counter is designed to count Q0, Q1 and Q2 outputs, while its fourth output (Q3) is used to reset it. The Q0, Q1 and Q2 outputs of IC2
by one. In this circuit, the LEDs flash a number of times one by one. The circuit comprises two astable multivibrators (IC1 and IC3) and a decade counter (IC2). Astable multivibrator IC1 produces approximately 0.72Hz clock fre-
are fed to npn transistors T1, T2 and T3, respectively. The collectors of transistors T1, T2 and T3 are connected to the emitter of transistor T4, while their emitters are connected to LED1, LED2 and LED3 via 150-ohm resistors R6, R7 and R8, re-
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ELECTRONICS FOR YOU
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spectively. The LEDs are activated one by one by the decade counter outputs. Astable multivibrator IC3 produces approximately 8.4Hz clock, which is given to transistor T4 via resistor R9 to switch on the supply to transistors T1 through T3 for each positive half cycle of IC3 output. Now for each output period of IC2, a particular LED blinks at the rate of 8.4 Hz. The blinking then shifts to the next LED when the output of IC2 advances by one count (after about 1.3 seconds). Similarly, the blinking effect shifts to the next LED after another 1.3 seconds and the cycle repeats thereafter. Flashing frequencies can be changed by changing the values of R10 and R11 and capacitor C4. The circuit can be easily assembled on any general-purpose PCB. It works off a 12V regulated power supply. You can also add more LEDs in series with LED1, LED2 and LED3, respectively.
CIRCUIT IDEAS
QUALITY FM TRANSMITTER TAPAN KUMAR MAHARANA
T
his FM transmitter for your stereo or any other amplifier provides a good signal strength up to a distance of 500 metres with a power output of about 200 mW. It works off a 9V battery. The audio-frequency modulation stage is built around transistor BF494 (T1), which is wired as a VHF oscillator and modulates the audio signal present at the base. Using preset VR1, you can adjust the audio signal level. The VHF frequency is decided by coil L1 and variable capacitor VC1. Reduce the value of VR2 to have a greater power output. The next stage is built around transistor BC548 (T2), which serves as a Class-A power amplifier. This stage is inductively coupled to the audio-frequency modulation stage. The antenna matching network consists of variable capacitor VC2 and capacitor C9. Adjust VC2 for the maximum transmission of power or signal strength at the receiver.
SUN
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For frequency stability, use a regulated DC power supply and house the transmitter inside a metallic cabinet. For higher antenna gain, use a telescopic antenna in place of the simple wire. Coils L1 and L2
L1: 5 turns of 24 SWG wire closely wound over a 5mm dia. air core L2: 2 turns of 24 SWG wire closely wound over the 5mm dia. air core L3: 7 turns of 24 SWG wire closely
are to be wound over the same air core such that windings for coil L2 start from the end point for coil L1. Coil winding details are given below:
wound over a 4mm dia. air core L4: 5 turns of 28 SWG wire on an intermediate-frequency transmitter (IFT) ferrite core
AUGUST 2004
ELECTRONICS FOR YOU
CIRCUIT
IDEAS
ULTRASONIC PROXIMITY DETECTOR
PRADEEP G.
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e the humans can hear sound of up to 20kHz frequency only. This proximity detector works at a frequency of 40 kHz. It uses two specially made ultrasonic transducers: One transducer emits 40kHz sound, while the other receives 40kHz sound and converts it into electrical variation of the same frequency. Fig. 1 shows the block diagram of the ultrasonic proximity detector and Fig. 2 shows its circuit. Mount the transducers (transmitter as well as receiver) about 5 cm apart on a piece of
general-purpose PCB as shown in Fig. 3 and connect to identical points (‘a’ through ‘d’) of the detector circuit (Fig. 2) via external wires. The 40kHz oscillator is built around transistors T1 and T2. If there is a solid object in front of the ultrasonic transmitter module (TX1), some signals will be reflected back and sensed by the receiver transducer (RX1). The 40kHz ultrasonic signals are converted into 40kHz electric signals by the receiver and then amplified by transistors T3 and T4. The amplified signals are still in the inaudible range, i.e., these can’t be heard. So a frequency-divider stage us-
Fig. 1: Block diagram of ultrasonic proximity detector
IVEDI S.C. DW
ing CMOS decade counter IC4017 (IC1) is used at the output of the amplifier. IC1 divides the input frequency by ’10,’ so the 40kHz signal becomes 4 kHz, which is within the audible range. The 4kHz signals are fed to op-amp IC 741 (IC2), which is wired as an earphone amplifier. This circuit can be used as an electronic guard for the blind. Keep it (along with 9V battery) in their pocket with earphone plugged to their ear. The transducer modules should be directed towards the walking path. If any object comes up in front or n e a r b y , Fig. 3: Transducers mounted they will on the PCB
Fig. 2: Circuit of ultrasonic proximity detector
88 • DECEMBER 2006 • ELECTRONICS FOR YOU
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hear 4kHz sound through the earphone and can change their path accordingly. One thing to be noted here is that while using this device, avoid the company of your pets. The reason is that
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pets can hear ultrasonic sound, which will irritate them and they will bark unnecessarily. EFY note. A similar device is used in some cars, such as Skoda’s Laura model, to help the drivers in backing
up and avoid banging against some invisible objects. However, instead of earphones the sound in this case is heard through a speaker and there is also an LCD screen to visually assist the driver.
ELECTRONICS FOR YOU • DECEMBER 2006 • 89
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UMAR SUNIL K
SECRET BELL
D. MOHAN KUMAR
M
any people move through the corridors and steps in multistoried buildings. As most of them are strangers for the inhabitants of the flats, it becomes necessary to verify the identity of the visitor before opening the door as he can be a burglar. This circuit helps you identify the members of your family. It is basically
a switchless musical bell that activates with a single puff of breath. The condenser mic fitted inside the existing door-bell switch box will trigger the bell on detecting air-pressure changes following the breath. As only the members of your family know the secret of the bell and hence puff out before the hole for the switch box, the door can be opened without fear. The front end of the circuit is a condenser mic amplifier with fixed sensi-
Fig. 1: Secret bell circuit
94 • DECEMBER 2006 • ELECTRONICS FOR YOU
tivity. Transistor T1 amplifies the signal received from the condenser mic through capacitor C1. When transistor T1 conducts, a short negative pulse triggers the monostable wired around IC1. The monostable time is decided by resistor R7 and capacitor C5. Reset pin 4 of IC1 is made stable by R6 and C3. Resistor R5 acts as a pull-up Fig. 2: Pin configuration resistor for trigof UM66 and BC548/549 ger pin 2 of IC1 to keep the trigger pin high in the standby mode. The high output from IC1 is used to power IC UM66 (IC2). IC2 generates a soft melody on receiving 3 volts at pin 2. Transistor T2 amplifies the music notes. A zener diode maintains the power for IC2 at a safer level of 3 volts. Assemble the circuit on any general-purpose PCB and enclose in a suitable cabinet. The condenser mic should be connected to the circuit using a single-core shielded wire to reduce noise interference. Drill a 1mm hole in the cover of the existing bell switch box and fix the mic inside the box with adhesive. The front side of the mic should face the hole.
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SHADOW ALARM
D. MOHAN KUMAR
EO SANI TH
his opto-sensitive circuit sounds an alarm whenever a shadow falls on it. So it can be used at night by shopkeepers to protect the valuables in their showrooms. A dim lighting in the room is necessary to detect the moving shadow. Unlike opto-interruption alarms based on light-dependent resistors (LDRs), it does not require an aligned light beam to illuminate the photo-sensor.
The non-inverting input of IC1 gets a controlled voltage from potential divider R2 and VR1. In the presence of ambient light, the phototransistor conducts and the inverting input (pin 2) of IC1 gets a lower voltage than its non-inverting input (pin 3). This makes the output of IC1 high, which is indicated by the glowing of LED1. When a shadow falls on the photosensor, the output of IC1 goes low. This low pulse triggers the monostable
and zener diode ZD1 provide 3.1V DC to IC UM3561. The circuit is easy to assemble as it requires only a few low-cost components. Enclose it in a cabinet with the photo-sensor inside. Drill a 5mm hole on the front panel of the cabinet to let ambient light fall on the photosensor. Adjust potmeter VR1 (47k) until LED2 stops glowing and the buzzer stops beeping while LED1 glows. This is the position of VR1 to be main-
The circuit is powered by a 9V PP3 battery and uses the most sensitive photo-sensor L14F1 to detect shadows. It is portable and can be used at any place that is to be monitored. Op-amp µA741 (IC1) is used as a voltage comparator. Its inverting input is biased by the voltage obtained from the junction of 100k resistor R1 and the collector of phototransistor T1.
(IC2) designed for a delay of 51 seconds using R6 and C3. The output of IC2 is used to light up LED2 and activate the alarm. Slide switch S2 is used to select either the buzzer or siren. When it is towards left the buzzer beeps, and when it is towards right IC UM3561 (IC3) activates to give a loud alarm simulating a police siren. Resistor R8
tained for that particular intensity of light. LED1 will continue to glow even when a shadow is detected. The circuit is now ready to use. To test it, move a paper in front of the unit. If LED2 glows along with the beep of the buzzer, it means that the photo-sensor has detected a shadow. z
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100 • JANUARY 2006 • ELECTRONICS FOR YOU
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CMYK
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IR BURGLAR DETERRENT
T.K. HAREENDRAN
IVEDI S.C. DW
readily-available electronic components. LED2 is used for indicating power-‘on.’ The transmitter section is built around timer 555 (IC1), while the re-
siren-driver transistor T2. This condition is indicated by the glowing of LED1. The time-out period can be increased or decreased by changing the value of capacitor C6.
ceiver-cum-alarm section consists of IR sensor TSOP1738, dual precision monostable multivibrator CD4538 and three-siren sound generator IC UM3561. Pin configurations of UM3561 and TSOP1738 are shown in Fig. 2. The astable multivibrator (IC1) oscillates at a frequency of around 38 kHz, which is transmitted by the infrared LED (IR LED1). Resistor R2 Fig. 2: Pin configurations of UM3561 and TSOP1738 limits the current across the IR LED. when the rays falling on its sensor are The transmitted IR signal directly interrupted. falls on IR sensor TSOP1738. WhenThe circuit of IR burglar deterrent ever the IR signal is interrupted, its (shown in Fig. 1) comprises transmitoutput pin 3 goes low and IC2 is trigter and receiver-cum-alarm sections. It gered at pin 5 through transistor T2. works off 6V DC, 500mA uninterAs a result, its output at pin 7 goes rupted supply and uses low-cost low (for a preset time) to forward bias
Now siren-sound generator IC3 is activated and its output signal is amplified by transistor T4 to produce a sound resembling that of police siren. Resistor R14 limits the loudspeaker current. The output tone of siren-sound generator IC3 can be set by connecting its pin 6 to either Vcc or GND. When you connect pin 6 to Vcc IC3 produces the sound of fire-alarm siren, but when you connect it to GND it produces the sound of ambulance siren. Assemble the transmitter and receiver-cum-alarm circuits on two separate general-purpose PCBs and house in suitable cabinets. Mount the units on the opposite sides of the entrance gate such that IR rays from IR LED1 fall directly on the IR receiver module (TSOP1738). z
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hwart any burglary attempt using this infrared proximity detector that triggers an alarm
Fig. 1: Circuit for IR burglar deterrent
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ELECTRONICS FOR YOU • JANUARY 2006 • 101
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Traffic Baton
Ashok K. Doctor
I
n small towns, there are no traffic lights and the police regulates the traffic with hand signals. Since
their hand signals may not be visible at night, it is necessary to have some illuminated direction indicator. Here we present two circuits for the same. One uses 6V bulbs and the
Fig. 1: Circuit of LED flasher
Fig. 2: Circuit of bulb flasher
9 4 • j a n ua ry 2 0 0 9 • e l e c t ro n i c s f o r yo u
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other uses bright LEDs. Both the circuits operate off a 6V, 4.5Ah rechargeable battery, which is clipped to the policeman’s waistband. Fig. 1 shows the circuit of the LED flasher. It is wired as an astable multivibrator. The ‘on’ time of the LED cluster is about 108 milliseconds and ‘off’ time is around 105 milliseconds. The frequency is around 5 Hz. A diode is used in series with the base of BD140 to increase the forward voltage in order to ensure that when BD139 conducts, BD140 is cut-off. Select the LED which consumes low current (20 mA or so) but flashes bright. Fig. 2 shows the circuit of the bulb flasher. Timer NE555 is wired as an astable multivibrator. The ‘on’ period of flashing bulb is around 344 milliseconds and ‘off’ period is around 329 milliseconds. The frequency is around 1.5 Hz. Bulb-driver transistors 2N3053/ BD139 and 2N2905/BD140 are used to light up the lamp. Two diodes are used in series with the base of 2N2905 to increase the forward voltage in order to ensure that when BD139 is conducting, BD140 is cut-off. Slide switch S2 is used to change the colour status of the w w w. e f y m ag . co m
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Fig. 3: Traffic baton for LED flasher
flashing bulb. Assemble the LED flasher and bulb flasher circuits on separate general-purpose PCBs. Enclose the LED flasher in a transparent acrylic pipe as shown in Fig. 3. The bulb flasher can be enclosed in another transparent acrylic pipe as shown in Fig. 4. Slide switches and red and green acrylic sheets are used for appropriate colour emissions. Now your traffic baton is ready to use.
Fig. 4: Traffic baton for bulb flasher
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Laser-guided Door Opener
T.K. Hareendran
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his automatic door opener can be made using readily available components. The electromagnetic relay at the output of this gadget can be used to control the DC/AC door-opener motor/solenoid of an electromechanical door opener assembly, with slight intervention in its electrical wiring. A laser diode (LED1) is used here as the light transmitter. Alternatively, you can use any available laser pointer. The combination of resistor R1 and diode D1 protects the laser diode from over-current flow. By varying muliturn trimpot VR1, you can adjust the sensitivity. (Note that ambient light reflections may slightly degrade the performance of this unit.) Initially, when the laser beam is falling on photo-transistor T1, it con-
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ducts to reverse-bias transistor T3 and the input to the first gate (N1) of IC1 (CD4001) is low. The high output at pin 3 of gate N1 forward biases the LED-driver transistor (T4) and the green standby LED (LED2) lights up continuously. The rest of the circuit remains in standby state. When someone interrupts the laser beam, photo-transistor T1 stops conducting and transistor T3 becomes forward-biased. This makes the output of gate N1 go low. Thus LED-driver transistor T4 becomes reverse-biased and LED2 stops glowing. At the same time, the low output of gate N1 makes the output of N2 high. Instantly, this high level at pin 4 of gate N2 triggers the monostable multivibrator built around the remaining two gates of IC1 (N3 and N4). Values of resistor R8 and capacitor C1 determine the time period of the monostable.
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The second monostable built around IC2 (CD4538) is enabled by the high-going pulse at its input pin 12 through the output of gate N4 of the first monostable when the laser beam is interrupted. As a result, relay RL1 energises and the door-opener motor starts operating. LED3 glows to indicate that the door-opener motor is getting the supply. At the same time, piezobuzzer PZ1 sounds an alert. Transistor T5, whose base is connected to Q output (pin 10) of IC2, is used for driving the relay. Transistor T6, whose base is connected to Q output of IC2, is used for driving the intermittent piezobuzzer. ‘On’ time of relay RL1 can be adjusted by varying trimpot VR2. Resistor R9, variable resistor VR2 and capacitor C3 decide the time period of the second monostable and through it
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on time of RL1. The circuit works off 12V DC power supply. Assemble it on a general-purpose PCB. After construction, mount the laser diode and the pho-
totransistor on opposite sides of the doorframe and align them such that the light beam from the laser diode falls on the phototransistor directly. The motor connected to the pole of
9 8 • j a n ua ry 2 0 0 9 • e l e c t ro n i c s f o r yo u
relay contacts is the one used in electromechanical door-opener assembly. If you want to use a DC motor, replace mains AC connection with a DC power supply.
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CCI IRRC UCIU T II TD E IADS E A S
MOBILE CELLPHONE CHARGER D. MOHAN KUMAR
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harging of the cellphone battery is a big problem while travelling as power supply source is not generally accessible. If you keep your cellphone switched on continuously, its battery will go flat within five to six hours, making the cellphone useless. A fully charged battery becomes necessary especially when your distance from the nearest relay station increases. Here’s a simple charger that replenishes the cellphone battery within two to three hours. Basically, the charger is a current-lim-
ited voltage source. Generally, cellphone battery packs require 3.6-6V DC and 180200mA current for charging. These usually contain three NiCd cells, each having 1.2V rating. Current of 100mA is sufficient for charging the cellphone battery at a slow rate. A 12V battery containing eight pen
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cells gives sufficient LED Status for Different Charging Conditions current (1.8A) to charge the battery con- Load across the output Output frequency (at pin 3) LED1 nected across the out765 kHz On put terminals. The cir- No battery connected Charging battery 4.5 Hz Blinks cuit also monitors the 0 Off voltage level of the bat- Fully charged battery tery. It automatically cuts off the charging process when its out- to take output pin 3 high. When the battery put terminal voltage increases above the is fully charged, the output terminal voltage increases the voltage at pin 2 of IC1 above predetermined voltage level. Timer IC NE555 is used to charge and the trigger point threshold. This switches off monitor the voltage level in the battery. the flip-flop and the output goes low to Control voltage pin 5 of IC1 is provided terminate the charging process. Threshold with a reference voltage of 5.6V by zener pin 6 of IC1 is referenced at 2/3Vcc set by VR1. Transistor T1 is used to enhance the charging current. Value of R3 is critical in providing the required current for charging. With the given value of 39-ohm the charging current is around 180 mA. The circuit can be constructed on a small general-purpose PCB. For calibration of cut-off voltage level, use a variable DC power source. Connect the output terminals of the circuit to the variable power supply set at 7V. Adjust VR1 in the middle position and slowly adjust VR2 until LED1 goes off, indicating low output. LED1 should turn on when the voltage of the variable power supply reduces below 5V. Enclose the circuit in a small plastic case and use suitable connector for connecting diode ZD1. Threshold pin 6 is supplied to the cellphone battery. Note. At EFY lab, the circuit was tested with a voltage set by VR1 and trigger pin with a Motorola make cellphone battery 2 is supplied with a voltage set by VR2. When the discharged cellphone battery rated at 3.6V, 320 mAH. In place of 5.6V is connected to the circuit, the voltage given zener, a 3.3V zener diode was used. The to trigger pin 2 of IC1 is below 1/3Vcc and charging current measured was about 200 hence the flip-flop in the IC is switched on mA.The status of LED1 is shown in the table.
CIRCUIT IDEAS
SMART FOOT SWITCH JAYAN A.R.
I VED DWI S.C.
uch jobs as jewel cutting and polishing require the workers to switch on/ off two electrical appliances one after another repeatedly for two different ser-
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two flip-flops are cross-connected, i.e. CLK1 (pin 1) is conneted to CLR2 (pin 8) and CLR1 (pin 3) is connected to CLK2 ( pin 6). Clock input pins 1 and 6 are pulled up high through resistors R1 and R4 (each 4.7k), respectively.
also connected to clear input CLR2 of flip-flop IC1(B) so as to clear it asynchronously. Switch debounces don’t affect the circuit as the same J1 state is being transferred to Q1 output on succeeding trailing edges. At the same time, device 2 is
vices on the same workpiece. This is cumbersome as they need to fully concentrate on delicate handwork on precious jewels. Switching in such situations cannot be done by hand, and doing it by foot using ordinary switches is too tedious. This is mainly because of the difficulty in sensing and controlling the switch position by foot. Ordinary pushbutton switches make or break a contact momentarily, and they cannot hold the keypress status. You need a bistable multivibrator with two independent trigger inputs to solve this problem. Here’s a smart foot switch based on dual negative-edge triggered master slave JK flip-flop IC 74LS76 (IC1). J1 and J2 inputs are conneted to 5V through resistors R2 and R5 (each 10k), respectively. K1 and K2 inputs are grounded. Preset pins 2 and 7 are shorted and connected to 5V via resistor R7 (10k). Push-to-on switch S3 connected to the preset inputs is also grounded. Clock and clear inputs of the
Push-to-on switches S1 and S2 are connected between clock and ground of the flip-flops. Switch S1 activates device 1, while switch S2 activates device 2. Switch S3 activates both device 1 and device 2 simultaneously. Device status is indicated by LED1 and LED2. Glowing of LED1 and LED2 indicates that device 1 and device 2, respectively, are in on condition. The LEDs are connected from +5V to Q1 (pin 14) and Q2 (pin 10) of IC1 through resistors R3 and R6, respectively. Initially when the power supply is switched on, Q1 and Q2 outputs of the JK flip-flops are at low level (logic 0). When switch S1 is pressed for the first time, the high level (logic 1) present at J1 input is transferred to Q1 output on the trailing edge of clock (CLK1). The high level (logic 1) at Q1 activates relay RL1 through pin 16 of IC ULN2003 (IC2), turning on device 1 via its normally-opened (N/O) contacts. Clock CLK1 of flip-flop IC1(A) is
switched off. When switch S2 is pressed, flip-flop IC1(A) gets cleared via CLR1 and the high state of J2 input of flip-flop IC1(B) is transferred to its Q2 output on the trailing edge of clock (CLK2). This high level (logic1) activates relay RL2 through pin 15 of IC2, turning on device 2 via its N/O contacts. At the same time, device 1 is switched off. Now if you want to turn on both the devices simultaniously, press switch S3 momentarily. Switch S3 provides ground to preset inputs PRE1 and PRE2 of flipflops IC1(A) and IC1(B), making their Q1 and Q2 outputs high, which energises both the relays turning on the two devices. LEDs glow to indicate that both the devices are ‘on.’ Place all the three switches (S1 through S3) where you can easily press them by foot when required. The LEDs can also be mounted at a convenient location to know whether the devices are turned on.
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CIRCUIT IDEAS
ELECTRONIC SECURITY SYSTEM K. BHARATHAN
T
his reliable and easy-to-operate electronic security system can be used in banks, factories, commercial establishments, houses, etc. The system comprises a monitoring system and several sensing zones. Each sensing zone is provided with a closed-loop switch known as sense switch. Sense switches are fixed on the doors of premises under security and connected to the monitoring system. As long as the doors are closed, sense switches are also closed. The monitoring system can be installed at a convenient central place for easy operation. Fig. 1 shows the monitoring circuit only for zone 1 along with the common alarm circuit. For other zones, the monitoring circuit is identical, with only the prefixes of components changing as per zone number. Encircled points A, B, and C of each zone monitoring circuit need to be joined to the corresponding points of the alarm circuit (upper half of Fig. 1). When zone 1 sensing switch S11, zone on/off slide switch S12, and system on/off switch S1 are all on, pnp transistor T12 reverse biases to go in cut-off condition, with its collector at around 0 volt. When the door fitted with sensor switch S11 is opened, transistor T12 gets forward biased and it conducts. Its collector voltage goes high, which forward biases transistor T10 via resistor R10 to turn it on. (Capacitor C10 serves as a filter capacitor.) As a result, the collector voltage of transistor T10 falls to forward bias transistor T11, which conducts and its collector voltage is sustained at a high level. Under this latched condition, sensor switch S11 and the state of transistor T12 have no effect. In this state, red LED11 of the zone remains lit. Simultaneously, the high-level voltage from the collector of transistor T11 via diode D10 is applied to VDD pin 5 of siren sound generator IC1 (UM3561) whose pin 2 is grounded. Resistor R3 connected across pins 7 and 8 of IC1 determines the frequency of the in-built oscillator. As a result, IC1 starts generating the audio signal output at pin 3. The output voltage from IC1 is further amplified by Darlington pair of transistors T1 and T2. The amplified
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output of t h e Darlington pair drives the loudspeaker whose output volume can be controlled by potentiometer VR1. Capacitor C1 serves as a filter capacitor. Y o u can alter the alarm sound as desired by changing the connections of IC1 as shown in the table. T h e circuit continues to sound the alarm until zone door Fig. 1: Monitoring circuit along with the alarm circuit
Fig. 2: Physical layout of sensors and monitoring/alarm system
is closed (to close switch S11) and the reset switch is pressed momentarily (which causes transistor T10 to cut off, returning the circuit to its initial state).
The system operates off a 3V DC battery or recharging battery with charging circuit or battery eliminator. If desired, more operating zones can be added. MARCH 2003
ELECTRONICS FOR YOU
CIRCUIT IDEAS Alarm sound
Circuit connections IC pin 1 connected to IC pin 6 connected to
Police siren Ambulance siren Fire engine Sound Machinegun sound
NC NC NC VSS
Note. NC indicates no connection
Initially keep the monitoring system switch S1 off. Keep all the zone doors fixed with sensing switches S11, S21, S31, S41, etc closed. This keeps the sensing switches
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NC VDD VSS NC
for respective zones in closed position. Also keep zone slide switches S12, S22, S32, S42, etc in ‘on’ position. This puts the system in operation, guarding all the zone
doors. Now, if the door of a particular zone is opened, the monitoring system sounds an audible alarm and the LED correspond-
ing to the zone glows to indicate that the door of the zone is open. The alarm and the LED indication will continue even after that particular door with the sensing switch is immediately closed, or even if that switch is removed/damaged or connecting wire is cut open. Any particular zone in the monitoring system can be put to operation or out of operation by switching on or switching off the corresponding slide switch in the monitoring system. The circuit for monitoring four zones costs around Rs 400.
CIRCUIT IDEAS
MULTI-SWITCH DOORBELL WITH INDICATORS T.K. HAREENDRAN
H
ere’s the circuit of a multi-switch input musical doorbell (shown in Fig.1). The circuit is built around the popular and less expensive quad D-latch CD4042B (IC1). When switch S6 is pushed to ‘on’ condition, the circuit gets +9V and the four data inputs (D1 through D4) of ICI are in low state because these are tied to ground via resistors R1 through R4. Polarity input (POL) pin 6 of IC1 is
Fig. 1: Multi-switch doorbell with indicators
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also pulled down by resistor R5. Clock input (pin 5) of the quad D-latch is wired in normally low mode and hence all the four outputs (Q0 through Q3) have the same states as their corresponding data inputs. As a result, LED1 through LED4 are in off condition. There are four switches fitted at four different doors/gates outside the home and a monitoring panel (as shown in Fig. 2) in the common room of the home. If any switch is pressed by a visitor (for example,
Fig. 2: Suggested panel layout of musical doorbell
switch S1 at door 1), pins 2 and 4 of IC1 go high. Simultaneously, pin 3 to IC1 (Q0 output) goes low and LED1 starts glowing to indicate that switch S1 is pressed by someone. Next, output pin 13 of the dual 4-input NOR gate (IC2, here wired as a single 4-input OR gate) goes high to forward bias buzzerdriver transistor T1 via resistor R10. The final result is a soft and pleasing musical bell, which lasts until reset switch S5 is pressed by the owner. For this latching arrangement , output pin 13 of IC2 from the NOR gate is fed back to the clock input of IC1. The circuit costs around Rs 100.
CIRCUIT IDEAS
LEAD-ACID BATTERY CHARGER WITH VOLTAGE ANALYSER D. MOHAN KUMAR
N
owadays maintenance-free lead-acid batteries are common in vehicles, inverters, and UPS systems. If the battery is left in a poor state of charge, its useful life is shortened. It also reduces the capacity and rechargeability of the battery. For older types of batteries, a hygrometer can be used to check the specific gravity of the acid, which, in turn, indicates the charge condition of the battery. However, you cannot use a hygrometer for sealedtype maintenance-free batteries. The only way to know their charge level is by checking their terminal voltage. The circuit presented here can replenish the charge in a battery within 6-8 hours. It also has a voltage analysing circuit for quick checking of voltage before start of charging, since overcharging may damage the battery. The voltage analyser gives an audio-visual indication of the battery voltage level and also warns about the critical voltage level at which the battery requires immediate charging.
ELECTRONICS FOR YOU
MARCH 2003
Red <9.8V >9.8V 11.5V 12.0V 12.5V
Off On On On On
I THE
O
comprising resistors R1 through R5. Thus the voltage applied to any non-inverting input is the ratio of the resistance between that non-inverting terminal and ground to the total resistance (R1+R2+R3+R4+R5). The resistor chain provides a positive voltage of above 5V to the non-inverting inputs of all op-amps when battery voltage is 12.5V or more. A reference voltage of 5V is applied to the inverting inputs of op-amps via 5V zener diode ZD1. When the circuit is connected to the battery and pushswitch S2 is pressed (with S1 open), the battery voltage is sampled by the analyser circuit. If the supply voltage sample applied to the non-inverting input of an op-amp exceeds the reference voltage applied Status of LEDs Comments to the inverting inputs, the Green Yellow Orange output of the Off Off Off Buzzer off op-amp goes Off Off Off Danger level high and the On Off Off Low level LED connected On On Off Normal level at its output On On On High level lights up.
The charger circuit consists of a standard step-down 12V AC (2-amp) transformer and a bridge rectifier comprising diodes D1 through D4. Capacitor C1 smoothes the AC ripples to provide a clean DC for charging the battery. The battery voltage analyser circuit is built around the popular quad op-amp LM324 that has four separate op-amps (A through D) with differential inputs. Opamps have been used here as comparators. Switch S2 is a pushswitch, which is pressed momentarily to check the battery voltage level before charging the battery. The non-inverting terminals of op-amps A through D are connected to the positive supply rail via a potential divider chain Battery voltage
SAN
CIRCUIT IDEAS The different levels of battery voltages are indicated by LED1 through LED4. All the LEDs remain lit when the battery is fully charged (above 12.5V). The buzzer connected to the output of IC1 also sounds (when S2 is pressed with S1 kept open) as long as the voltage of battery is above
9.8V. If the voltage level goes below 9.8V, the buzzer goes off, which indicates that it’s time to replace the battery. The status of LEDs for different battery voltages is shown in the table. The circuit can be assembled on a general-purpose PCB or a veroboard. Use 4mm
wire and crocodile clips to connect the charger to the battery. A 2.5-amp fuse connected to the output of the charger protects the analyser circuit against accidental polarity reversal. The circuit costs around Rs 120 with all accessories.
MARCH 2003
ELECTRONICS FOR YOU
CIRCUIT IDEAS
TELEPHONE RECEIVER S.K. ROUSHON
EDI DWIV S.C.
his simple telephone receiver without a dialling section can be connected in parallel to a telephone line. It can be easily assembled on a small vero board or a PCB. A geometry box made in the shape of a telephone receiver will be an excellent cabinet for it. No external
bridge rectifier consisting of diodes D1 through D4 protects the circuit from any polarity change in the telephone line. PNP transistor MPS-A92 (T1) is the main interface transistor. The output of T1 is regulated by zener diode ZD and capacitor C2 to get 6.8V for powering the amplifier section. This power is also used to bias the transmitter section.
voice input for the amplifier comes directly from the positive end of the bridge rectifier. The amplifier section is built around high-performance, low-wattage power amplifier IC LM386. This circuit is designed as a high-gain amplifier. A small 8-ohm speaker is good enough for the output. After all soldering is done, adjust pre-
power supply is needed, which makes the circuit handy. The ringer section comprises R1, C1, and a buzzer. If your telephone has a loud ringer, this circuit can be avoided. A
The transmitter section comprises transistor BC548 (T2) together with a few discrete components and a condenser microphone. The transmit signal is fed to the base of interface transistor T1. The
sets VR1 and VR2 to their middle position and connect the circuit to the telephone line in parallel. Adjust VR1 and VR2 for optimum reception as well as transmission.
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ELECTRONICS FOR YOU
SEPTEMBER 2003
CIRCUIT IDEAS
VOLTAGE-BASED CONTROLLER FOR SWITCHES
I VED DWI S.C.
RAJ K. GORKHALI
H
ere’s a simple circuit for controlling four switches from a distance through just a pair of
wires. In the circuit, the inverting inputs (pin 2) of operational amplifiers IC1 through IC4 are set to refrence voltages of +12V, +9V, +6V, and +3V, respectively, through a chain of four 1k resistors (R1 through R4). The reference voltage (VREF) can be simply calculated by the following relationship:
VREF =
Total applied voltage×Resistance across reference voltage Total resistance
For example, reference voltage VREF3 is calculated as follows: VREF3 =
12V×(R3+R4) 12V×2k VREF2 = 4k R1+R2+R3+R4 VREF2 = 6V
The non-inverting inputs (pin 3) of the four op-amps ( IC1 through IC4) are tied together and connected to a pair of wires that provide +3V to +12V input voltage for controlling the switches. Four 12V, 200-ohm, singlechangeover relays are connected to four BC548 relay driver transistors (T1 through T4) via resistors R5 through R8, respectively. These relays energise depending on the voltage present at the controlling volt-
age input terminal; for example, relay RL4 energises when controlling voltage input of +3V is available at non-inverting pin 3 of IC4. Four electrical equipment can be
connected to the terminals of the relays through the 220V AC, 50Hz mains. This circuit, excluding relays, costs around Rs 60.
JULY 2003
ELECTRONICS FOR YOU
CIRCUIT IDEAS
WASHING MACHINE MOTOR CONTROLLER
EDI DWIV S.C.
trolled as shown in Fig. 2. When switch S1 is in position A, coil L1 of the motor receives the current directly, whereas coil ashing machines usually employ L2 receives the current with a phase shift a single-phase motor. In semidue to capacitor C. So the rotor rotates in automatic washing machines, a clockwise direction (see Fig. 2(a)). When purely mechanical switch controls the timswitch S1 is in position B, the reverse haping and direction of the motor. These pens and the rotor rotates in anti-clockswitches are costly and wear out easily. wise direction (see Fig. 2(b)). Thus switch Here’s a controller for single-phase S1 can change the rotation direction. motors of washing machines (Fig. 1) that The motor cannot be reversed instantly. It needs a brief pause between switching directions, or else it may get damaged. For this purpose, another spin direction control timer (IC2) is employed. It is realised with an IC 555. This timer gives an alternate ‘on’ and ‘off’ time duration of 10 seconds and 3 seconds, respectively. So after every l0 seconds of running (either in clockwise or anticlockwise direction), the motor stops for a brief duration of 3 seconds. The values of R3 and R4 are calculated accordingly. The master timer is realised with monostable IC 555 (IC1) and its ‘on’ time is decided by the resistance of 1-mega-ohm potmeter VR. A 47-kilo-ohm resistor is added in series so that Fig. 1: Circuit diagram of washing machine motor controller even when the VR knob is in zero resistance position, the net series resistance is not zero. The on-off cycle in the master timer Fig. 2: Direction of motor Fig. 3: Timing diagram for rotation of motor should SANTHOSH VASUDEVAN
W
efficiently replaces its mechanical equivalent. Basically, a single-phase motor requires a master timer, which decides the time for which the motor should keep rotating (washing time), and a spin direction controller, which stops the motor for 3 seconds after every 10 seconds and then resumes rotation in opposite direction. The direction of rotation can be con-
SEPTEMBER 2003
ELECTRONICS FOR YOU
CIRCUIT IDEAS go on only for the set time (here it is 18 minutes). Once the master timer goes off, the cycle should stop. To achieve this, the outputs of both the timers are connected to NAND gate N1 (IC3), which gives a low output only when both the timers are giving high outputs.The output pin 2 of N1 is connected to relay RL1 via pnp transistor T1, so the relay energises
ELECTRONICS FOR YOU
SEPTEMBER 2003
only when the output from NAND gate N1 is low. As the mains 220V line is taken through relay RL1, the motor turns off during the 3-second off period after the set time of 10 seconds is over. The graph is shown in Fig. 3. During ‘on’ time of spin direction timer IC2, the output of negative-edge triggerd JK flip-flop at pin 2 goes low to energise
relay RL2 and washing machine motor rotates in one direction. During the off time of IC2, the output of N1 goes high again to de-energise relay RL1, which cuts off the mains supply to RL2 and the motor stops rotating. Floating point trouble may occur at trigger pin 2 of IC1. Resistor R8 overcomes this problem by holding pin 2 high.
CMYK
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AUTO TURN-OFF BATTERY CHARGER
Y.M. ANANDAVARDHANA
IVEDI S.C. DW
his charger for series-connected 4-cell AA batteries automatically disconnects from mains to stop charging when the batteries are fully charged. It can be used to charge partially discharged cells as well. The circuit is simple and can be divided into AC-to-DC converter, relay
to energise electromagnetic relay RL1. Relay RL1 is connected to the collector of transistor T1. Transistor T1 is driven by pnp transistor T2, which, in turn, is driven by pnp transistor T3. Resistor R4 (10-ohm, 0.5W) is connected between the emitter and base of transistor T3. When a current of over 65 mA flows through the 12V line, it causes a
Pushing switch S1 latches relay RL1 and the battery cells start charging. As the voltage per cell increases beyond 1.3V, the voltage drop across resistor R4 starts decreasing. When it falls below 650 mV, transistor T3 cuts off to drive transistor T2 and, in turn, cuts off transistor T3. As a result, relay RL1 de-energises to cut off the charger and red LED1 turns off.
driver and charging sections. In the AC-to-DC converter section, transformer X1 steps down mains 230V AC to 9V AC at 750 mA, which is rectified by a full-wave rectifier comprising diodes D1 through D4 and filtered by capacitor C1. Regulator IC LM317 (IC1) provides the required 12V DC charging voltage. When you press switch S1 momentarily, the charger starts operating and the power-on LED1 glows to indicate that the charger is ‘on.’ The relay driver section uses pnp transistors T1, T2 and T3 (each BC558)
voltage drop of about 650 mV across resistor R4 to drive transistor T3 and cut off transistor T2. This, in turn, turns transistor T1 ‘on’ to energise relay RL1. Now even if the pushbutton is released, mains is still available to the primary of the transformer through its normally open (N/O) contacts. In the charging section, regulator IC1 is biased to give about 7.35V. Preset VR1 is used for adjusting the bias voltage. Diode D6 connected between the output of IC1 and battery limits the output voltage to about 6.7V, which is used for charging the battery.
You may determine the charging voltage depending on the NiCd cell specifications by the manufacturer. Here, we’ve set the charging voltage at 7.35V for four 1.5V cells. Nowadays, 700mAH cells are available in the market, which can be charged at 70 mA for 10 hours. The open-circuit voltage is about 1.3V. The shut-off voltage point is determined by charging the four cells fully (at 70 mA for 14 hours). After measuring the output voltage, add the diode drop (about 0.65V) and bias LM317 accordingly. z
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96 • FEBRUARY 2005 • ELECTRONICS FOR YOU
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CMYK
CIRCUIT
IDEAS
ATMEL AVR ISP DONGLE
EFY LAB
A
tmel’s AVR microcontroller chips are in-system programmable (ISP), i.e. these can be programmed directly in the target circuit. A special programmer software is used to download the program from the PC into the AVR’s flash memory. Atmel offers a software package called the Atmel AVR ISP that allows programming of the AVR microcontrollers in the circuit using a simple dongle. A dongle is nothing but an adaptor cable that connects the PC’s parallel port with the ISP pins of the AVR chip for programming. For programming, the four lines required from the AVR chip to the ISP adaptor (dongle) are: 1. MOSI (Master Out, Slave In): Data being transmitted to the AVR being programmed is sent on this pin 2. MISO (Master In, Slave Out): Data received from the AVR being pro-
grammed is sent on this pin 3. SCK (Shift Clock): Serial clock generated by the programmer from the PC. 4. RST (Reset): Reset (low pulse) generated by the program. The AVR is programmed while in reset state. Here’s a dongle circuit for in-system programming of Atmel’s AVR chip AT90S8515 using such software packages as Atmel ISP 2.65 and PonyProg2000. Though not exactly the same, a similar dongle circuit can be found at the Website ‘www.iready.org/ projects/uinternet/ispdongle.pdf.’ The PC’s parallel-port pins 4 and 5 drive buffer IC 74LS244 by enabling its pins 19 and 1, respectively. A low pulse on these pins will allow the passing of the serial clock and data during programming. MOSI, LED, SCK and RST outputs are buffered from the parallel port’s pins 7, 8, 6 and 9, respectively. The MISO input from the AVR is fed into pin 10 of the
100 • FEBRUARY 2005 • ELECTRONICS FOR YOU
EO SANI TH
parallel port. IC 74LS244 (IC1) acts as a buffer as well as an isolator circuit when the AVR is not in programming mode. In idle mode, all the outputs are tristated so as not to affect the operation of the target system. When the AVR’s ISP mode is selected, the lower half of IC 74LS244 is enabled, pulling the target system’s Reset line low. Once the target system is in Reset mode, the SCK, MISO and MOSI lines are no longer loaded by the peripheral circuitry, if any, on the target system. Now, it is safe to enable the upper half of 74LS244, driving the MOSI, LED and SCK lines of the dongle. The RST pin becomes high after the AVR is programmed. Glowing of LED2 indicates that the AVR is in programming mode. There are two standard connectors for in-system programming of Atmel AVR microcontroller. One is the 10pin header (dual-in-line (DIL) connec-
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CMYK
CIRCUIT
IDEAS
tor)) used on the Atmel STK kits. The other is a 6-pin header (DIL connector) used in Atmel ISPs. The two loopback connections, pin 2-to-pin 12 and pin 3-to-pin 11 of the parallel port, are used to identify the dongle. With only pin 2-to-pin 12 link, the dongle is called STK300 or AVR ISP dongle. With only pin 3-to-pin 11 link, the dongle is called STK200 or old Kanda ISP dongle. With both links in place,
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the dongle is identified as a valueadded pack dongle. Here, we’ve used an 8-pin single-inline (SIL) connector and an additional 6-pin SIL connector for the Atmel programer circuit. With the buffer and the 40-pin ZIF socket in this circuit, it can be used as a standalone programmer. The 6-pin SIL male connector is used for connection between the dongle and the AVR on the target board. Thus, another
6-line cable of about 30cm length is required for connecting this ISP adaptor (dongle) to the target circuit. If the AVR is not on the target circuit, you can insert the AVR into the ZIF socket and program it. Regulated 5V DC is required for the AVR and the associated dongle circuit, whose terminals are also provided in connector CON4. LED1 is used as the power indicator for the circuit. z
ELECTRONICS FOR YOU • FEBRUARY 2005 • 101
CIRCUIT IDEAS
ULTRA-BRIGHT LED LAMP
I VED DWI S.C.
N.S. HARISANKAR VU3NSH
T
his ultra-bright white LED lamp works on 230V AC with minimal power consumption. It can be used to illuminate VU meters, SWR meters, etc. Ultra-bright LEDs available in the market cost Rs 8 to 15. These LEDs emit a 1000-6000mCd bright white light like welding arc and work on 3 volts, 10 mA. Their maximum voltage is 3.6 volts and the current is 25 mA. Anti-static precautions should be taken when handling the LEDs. The LEDs in water-clear plastic package
Fig. 3: 46-LED combination
emit spotlight, while diffused type LEDs have a wide-angle radiation pattern. This circuit (Fig. 1) employs capacitive reactance for limiting the current flow through the LEDs on application of mains voltage to the circuit. If we use only a series resistor for limiting the current with mains operation, the limiting resistor itself will dissipate around 2 to 3 watts of power,
Fig. 1: The circuit of ultra-bright white LED lamp
Fig. 2: 16-LED combination
whereas no power is dissipated in a capacitor. The value of capacitor is calculated by using the following relationships: XC = 1/(2πfC) ohms —————(a) XC = VRMS /I ohms ———— (b) where XC is capacitive reactance in ohms, C is capacitance in farads, I is the current through the LED in amperes, f is the mains frequency in Hz, and Vrms is the input mains voltage. The 100-ohm, 2W series resistor avoids heavy ‘inrush’ current during transients. MOV at the input prevents surges or spikes, protecting the circuit. The 390-kilo-ohm, ½-watt resistor acts as a bleeder to provide discharge path for capacitor Cx when mains supply is disconnected. The zener diode at the output section prevents excess reverse voltage levels appearing across the LEDs during negative half cycles. During positive half cycle, the voltage across LEDs is limited to zener voltage.
Use AC capacitors for Cx. Filter capacitor C1 across the output provides flickerfree light. The circuit can be enclosed in a CFL round case, and thus it can be connected directly to AC bulb holder socket. A series combination of 16 LEDs (Fig. 2) gives a luminance (lux) equivalent of a 12W bulb. But if you have two series combinations of 23 LEDs in parallel (total 46 LEDs as shown in Fig. 3), it gives light equal to a 35W bulb. 15 LEDs are suitable for a tablelamp light. Diode D1 (1N4007) and capacitor C1 act as rectifying and smoothing elements to provide DC voltage to the row of LEDs. For a 16-LED row, use Cx of 0.22 µF, 630V; C1 of 22 µF, 100V; and zener of 48V, 1W. Similarly, for 23+23 LED combination use Cx of 0.47 mF, 630V; C1 of 33 µF, 150V; and zener of 69V, 1W. This circuit (inclusive of LEDs) costs Rs 200 to Rs 400.
FEBRUARY 2003
ELECTRONICS FOR YOU
CIRCUIT IDEAS
INTRUDER RADIO ALERT SYSTEM
SUN
IL KU
MAR
DAVID NASH PIOUS
C
onsider a situation where a burglar has entered your house and snapped the telephone wires, leaving you with no means of communication with the outside world. In such an emergency, you will find this intruder alarm to be very handy. It transmits a prerecorded emergency message repeatedly for reception by an FM receiver. The message containing address, geographical location, name, etc is recorded onto a chip. The prerecorded message can then be transmitted repeatedly with the help of an FM transmitter, in the hope that some noble soul will hear it and inform the police about the incident. The circuit comprises a sound recording-and-playback chip (UM5506BH). This chip consists of a 96kbit SRAM and can record up to six seconds of audio. (For details, refer ‘Mini Voice Processor’ circuit published in April 2000 issue of EFY.) After the required message has been recorded, it is passed to a low-power, VHF FM transmitter wired around BC547 and 2N2369 transistors. The range of this trans-
Fig. 2: Circuit diagram of intruder radio alert system
Fig. 1: Block diagram of the intruder radio alert system
mitter is 60 to 100 metres using a 40-70cm long wire as an antenna. The major advantage of this circuit is its low power consumption. The author operated it on 3V button cells (Maxell CR 2032, CR 2025, etc used in digital diaries). To transmit the prerecorded message, the play button is pressed repeatedly.
The transmitted message can be heard over the FM receiver. A possible modification, though it has legal complications, is to vary the coil inductance such that the transmission is on police band, thus alerting the police for quick help. Even the need of repeatedly pressing play button can be obviated by configuring an astable multiviberator (using IC 555 timer) to trigger IC UM5506BH every six seconds so that the message is played repeatedly. This circuit costs around Rs 200.
DECEMBER 2002
ELECTRONICS FOR YOU
CIRCUIT
IDEAS
MICROMOTOR CONTROLLER
V. DAVID
UMAR SUNIL K
sing this circuit, you can control the rotation of a DC micromotor simply by press-
connected between the outputs (pin 3) of IC1 and IC2. Closing switch S5 provides power to the circuit. Now, when you press switch S1 momentarily, pin 10 of IC3
tor in conjunction with switch S1. If you press switch S3 after pressing switch S1, pin 3 of IC3 goes high, while its pin 4 goes low. The motor now starts rotating in the forward direction.
ing two push-to-on switches momentarily. The circuit is built around two NE555 ICs (IC1 and IC2) and a quadNAND IC CD4011 (comprising NAND gates N1 through N4). The NE555 ICs (IC1 and IC2) are configured as inverting buffers. IC CD4011 (IC3) NAND gates are configured as bistable flipflop. The DC motor to be controlled is
goes high, while its pin 11 goes low. Since pin 10 of IC3 is connected to reset pin 4 of IC1 and IC2, the high output at pin 10 of IC3 will enable IC1 and IC2 simultaneously. When switch S2 is pressed, pin 10 of IC3 goes low, while its pin 11 goes high. The low logic at pin 10 disables both IC1 and IC2. Switches S3 and S4 are used for forward and reverse motion of the mo-
However, if you press switch S4 after pressing switch S1, the motor will rotate in reverse direction. Note. The complete kit of this circuit can be obtained from Kits‘n’Spares, 303, Dohil Chambers, 46, Nehru Place, New Delhi 110019; Phone: 011-26430523, 26449577; Website: www.kitsnspares.com; E-mail: [email protected]. z
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ELECTRONICS FOR YOU • APRIL 2005 • 63
CMYK
CIRCUIT IDEAS
THREE-COLOUR DISPLAY USING BICOLOUR LEDs
I VED DWI S.C.
PRIYANK MUDGAL
T
he circuit presented here uses bicolour LEDs to generate a display in three colours, namely, red, green, and yellowish green. Transistors T1 through T20 form a grid to which common-cathode bicolour LEDs (LED1 through LED10) are connected. Transistors T1 through T10 have their collector terminals connected to the emitter of transistor T21. Similarly, transistors T11 through T20 have their collector terminals connected to the emitter of transistor T22. The bases of each pair of transistors (i.e. T1 and T11, T2 and T12,…, T10 and T20) are tied to outputs Q0, Q1,…, Q9, respectively, of IC1 (CD4017) through 10-kiloohm resistors as shown in the figure. Positive supply to collectors of transistors T1 through T10 is controlled by transistor T21. Similarly, positive supply to collectors of transistors T11 through T20 is controlled by transistor T22. IC1 and IC2 are decade counters. Clock pulse to IC1 is provided by the oscillator circuit comprising NOR gates N1 and N2. The outputs of IC1 advance sequentially with each clock. (Any other source of squarewave pulses also serves the purpose.) IC2 is used to select the mode of display. Clock input pin 14 of IC2 is connected to Q9 output of IC1. Thus IC2 receives one pulse after every ten pulses received by IC1. When the circuit is switched on, Q0 output of IC2 is active high. Thus transistor T21 gets forward biased via diode D3 and it conducts to extend positive supply to transistors T1 through T10. Transistors T1 through T10 are forward biased sequentially by Q0 through Q9 outputs of IC1, i.e. at a time only one of these ten transistors is forward biased (on). Thus only red LED parts of bicolour LEDs light up sequentially. (Transistor T22 is not conducting at this moment.) When red LED part of LED10 glows, IC2 receives a clock pulse and its Q1 output goes high. Transistor T21 still conducts, as it is forward biased through diode D6, and next again via diode D5. Thus red LEDs complete two more glowing sequences. ELECTRONICS FOR YOU
FEBRUARY 2003
After completion of the third glowing sequence of red LEDs, when Q3 output of IC2 goes high, transistor T21 stops conducting and T22 starts conducting with the next three sequences of green LEDs of bicolour LEDs (LED1 through LED10) glowing sequentially. After completion of three sequences of green LEDs, output Q6 of IC2 goes high.
Now both transistors T21 and T22 conduct due to diodes D1 and D2. Thus both red and green LEDs in bicolour LEDs (LED1 through LED10) glow sequentially. The effect of red and green LEDs glowing together is a distinct yellowish orange colour. This sequence repeats four times. Thereafter, the whole sequence repeats, starting with red LEDs. Thus the bicolour-
CIRCUIT IDEAS LED display shows three colours—red, green, and yellowish green—one after the other. The speed of display can be controlled
by preset VR1. One can omit automatic selection of different colours by omitting IC2 and replacing connections to pins 3,
5, and 7 of IC2 with SPDT switches. (Thus diodes D3-D12 are also omitted.) This circuit costs around Rs 250.
FEBRUARY 2003
ELECTRONICS FOR YOU
CIRCUIT IDEAS
NUMBER GUESSING GAME PRIYANK MUDGAL
T
his number guessing game is quite simple. In this game the player thinks of any number between 1 and 99. Then he scans the eight groups of numbers given in the eight boxes in the table. Each group corresponds to a specific switch (indicated on the top of each group) on an 8-way DIP switch. The person scans the numbers in each box and slides the switch corresponding to a box to ‘on’ position if he finds his number in that box. After having scanned all the eight boxes and switching on the relevant DIP switches, he is required to press switch S9
I VED DWI . C . S
and the number thought of by the person is displayed on the 7-segment displays. After this, all switches on the 8-way DIP switch need to be turned off to try display of another number in a similar fashion. The circuit (Fig. 1) comprises two BCDto-7-segment decoder/driver CD4511 ICs (IC1 and IC2). IC1 generates the number for tens position and IC2 generates the number for units position. Input pins 7, 1, 2, and 6 of both the ICs are connected to ground through 1-kilo-ohm resistors. The commoncathode terminals of both the displays are connected to push-to-on switch S9.
Fig. 1: Number guessing game circuit
Fig. 2: Suggested case
Suppose you want to display 47. For this, 4 is to be displayed in tens position and 7 in units position. In order to generate 4 (binary 100) on the display (DIS1), switch S2 is to be turned on. To display 7 (binary 111) on the display (DIS2), switches S6, S7, and S8 are to be turned on. Thus to generate 47, switches S2, S6, S7, and S8 are to be turned on. The number 47 is placed in groups 6, 7, 8, and 2. So when you spot 47 in these groups, switch on the same combination of switches. On depressing switch S9, 47 appears on the display. Other numbers can be generated using the same procedure. In order to make the circuit compact, a DIP switch has been used here. As it may be difficult to turn the small switches on and off, you may use SPDT toggle switches in place of the DIP switch. The circuit can be placed inside a plastic case with appropriate cuts made for displays and switches (Fig. 2). A strip of paper containing groups of numbers can be stuck just under the 8-way DIP switch (or under the row of SPDT switches used in place of DIP switch). The proposed cabinet with front-panel layout is shown in the figure. This circuit smoothly runs on two pen torch batteries. Thus current-limiting resistors are not necessary for displays. This circuit costs around Rs 100.
Eight Groups of Numbers and Their Respective Switches Switch 1 80, 81, 82, 83, 84, 85, 86, 87, 88, 89 90, 91, 92, 93, 94, 95, 96, 97, 98, 99
Switch 2 40, 41, 42, 43, 44, 45, 46, 47, 48, 49 50, 51, 52, 53, 54, 55, 56, 57, 58, 59 60, 61, 62, 63, 64, 65, 66, 67, 68, 69 70, 71, 72, 73, 74, 75, 76, 77, 78, 79
Switch 3 20, 21, 22, 23, 24, 25, 26, 27, 28, 29 30, 31, 32, 33, 34, 35, 36, 37, 38, 39 60, 61, 62, 63, 64, 65, 66, 67, 68, 69 70, 71, 72, 73, 74, 75, 76, 77, 78, 79
Switch 4 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 30, 31, 32, 33, 34, 35, 36, 37, 38, 39 50, 51, 52, 53, 54, 55, 56, 57, 58, 59 70, 71, 72, 73, 74, 75, 76, 77, 78, 79 90, 91, 92, 93, 94, 95, 96, 97, 98, 99
Switch 5 8, 9, 18, 19, 28, 29, 38, 39 48, 49, 58, 59, 68, 69, 78, 79 88, 89, 98, 99
Switch 6 4, 5, 6, 7, 14, 15, 16, 17, 24, 25, 26, 27 34, 35, 36, 37, 44, 45, 46, 47, 54, 55 56, 57, 64, 65, 66, 67, 74, 75, 76, 77 84, 85, 86, 87, 94, 95, 96, 97
Switch 7 2, 3, 6, 7, 12, 13, 16, 17, 22, 23, 26, 27 32, 33, 36, 37, 42, 43, 46, 47, 52, 53, 56, 57, 62, 63, 66, 67, 72, 73, 76, 77, 82, 83, 86, 87, 92, 93, 96, 97
Switch 8 1, 3, 5, 7, 9, 11, 13, 15, 17, 19 21, 23, 25, 27, 29, 31, 33, 36, 39 41, 43, 45, 47, 49, 51, 53, 55, 57, 59 61, 63, 65, 67, 69, 71, 73, 75, 77, 79 81, 83, 85, 87, 89, 91, 92, 93, 95, 97, 99
ELECTRONICS FOR YOU
JANUARY 2003
CIRCUIT
IDEAS
SOLAR BUG
EO SANI TH
D. SOMNATH
H
ide this solar-powered circuit suitably and see the reaction of your friends to the chirpy sound produced by it every few minutes. In all probability, it will coax them to find out where the sound is coming from. The circuit runs off a miniature solar power panel, which can be taken out from an old calculator such as Citizen CT-500. A panel giving 1.5V to
2.5V is required. Note that the circuit can work properly from a panel as
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small as 3 cm2. If a digital voltmeter is connected across capacitor C2, a slow build-up of voltage can be observed when the panel is exposed to light. Transistors T1 and T2 form a relaxation oscillator. When C1 charges to 0.6V, transistor T1 conducts and the charge built up in C2 is discharged through the piezobuzzer to produce a short beep. While testing the circuit, the value of resistor R1 can be reduced to, say, 1 kilo-ohm. Use a good-quality buzzer to ensure that the sound produced is loud enough. z
ELECTRONICS FOR YOU • APRIL 2005 • 67
CMYK
CIRCUIT
IDEAS
STRESS METER
D. MOHAN KUMAR
T
his stress monitor lets you assess your emotional pain. If the stress is very high, it gives visual indication through a light-emitting diode (LED) display along with a
EO SANI TH
cuit. The circuit is very sensitive and detects even a minute voltage variation across the touch pads. The circuit comprises signal amplifier and analogue display sections. Voltage variations from the sensing pads are amplified by transistor BC548
Fig. 1: Circuit of the stress meter
warning beep. The gadget is small enough to be worn around the wrist. The gadget is based on the principle that the resistance of the skin varies in accordance with your emotional states. If the stress level is high the skin offers less resistance, and if the body is relaxed the skin resistance is high. The low resistance of the skin during high stress is due to an increase in the blood supply to the skin. This increases the permeability of the skin and hence the conductivity for electric current. This property of the skin is used here to measure the stress level. The touch pads of the stress meter sense the voltage variations across the touch pads and convey the same to the cir-
(T1), which is configured as a common-emitter amplifier. The base of T1 is connected to one of the touch pads through resistor R1 and to the ground rail through potmeter VR1. By varying VR1, the sensitivity of T1 can be adjusted to the desired level. Diode D1 maintains proper biasing of T1 and capacitor C1 keeps the voltage from the emitter of T1 steady. The amplified signal from transistor T1 is given to the input of IC LM3915 (IC1) through VR2. IC LM3915 is a monolithic integrated circuit that senses analogue voltage levels at its pin 5 and displays them through LEDs providing a logarithmic analogue display. It can drive up to ten LEDs one by one in the dot/
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bar mode for each increment of 125 mV in the input. Here, we’ve used only five LEDs connected at pins 14 through 18 of IC1. LED1 glows when input pin 5 of IC1 receives 150 mV. LED5 glows when the voltage rises to 650 mV and LED5 flashes and piezobuzzer PZ1 beeps when the stress level is high. Resistors R4 and R5 and capacitor C2 form the flashing elements. Resistor R3 mainFig. 2: Display panel tains the LED current at around 20 mA. Capacitor C3 should be p l a c e d close to pin Fig. 3: Self-locking straps 3 for proper functioning of the IC. Zener diode ZD1 in series with resistor R6 provides regulated 5V to the circuit. The circuit can be assembled on a small piece of perforated board. Use transparent 3mm LEDs and a small piezobuzzer for audio-visual indications. Enclose the circuit in a small plastic case with touch pads on the back side. Two self-locking straps can be used to tie the unit around your wrist. After tying the unit around your wrist (with touch pads in contact with the skin), slowly vary VR1 until LED1 glows (assuming that you are in relaxed state). Adjust VR2 if the sensitivity of IC1 is very high. The gadget is now ready for use. z
ELECTRONICS FOR YOU • SEPTEMBER 2005 • 101
CMYK
CIRCUIT
IDEAS
LITTLE DOOR GUARD
IVEDI S.C. DW
T.K. HAREENDRAN
I
f some intruder tries to open the door of your house, this circuit sounds an alarm to alert you against the attempted intrusion. The circuit (Fig. 1) uses readily available, low-cost components. For compactness, an alkaline 12V battery is used for powering the unit. Input DC supply is further regulated to a steady DC voltage of 5V by 3-pin regulator IC 7805 (IC2). Assemble the unit on a generalpurpose PCB as shown in Fig. 4 and mount the same on the door as shown in Fig. 3. Now mount a piece of mirror on the doorframe such that it is
Fig. 1: Circuit of the door guard
Fig. 2: Pin configurations of UM3561 and transistors 2N5777 and BC547
Fig. 3: Back view of the door assembly
and IC1 does not get positive supply at its pin 5. As a result, no tone is produced at its output pin 3 and the loudspeaker remains silent. Resistor R1 limits the operating current for the IR LED. When the door is opened, the absence of IR Fig. 4: Suggested enclosure with major components layout rays at phototransistor T1 forward biases npn exactly aligned with the unit. transistor T2, which provides positive Pin configurations of IC supply to IC1. Now 3-siren UM3561 and transistors sound generator IC UM3561 (IC1) gets 2N5777 and BC547 are power via resistor R5. The output shown in Fig. 2. of IC1 at pin 3 is amplified by Initially, when the Darlington-pair transistors T3 and T4 door is closed, the infrared to produce the alert tone via the (IR) beam transmitted by loudspeaker. IR LED1 is reflected (by the Rotary switch S2 is used to select mirror) back to the three preprogrammed tones of IC1. phototransistor 2N5777 (T1). IC1 produces fire engine, police and The IR beam falling on ambulance siren sounds when its pin phototransistor T1 reverse bi6 is connected to point F, P or A, reases npn transistor T2 spectively. z
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ELECTRONICS FOR YOU • SEPTEMBER 2005 • 103
CMYK
circuit
ideas
Smart Vibration Sensor
T.K. Hareendran
I
n this vibration sensor alarm circuit, initially, when power switch S1 is flipped to ‘on’ position, power indicator LED1 lights up immediately. IC LM555 (IC1), wired as a simple latch circuit with control input, is powered and R-C components R4 and C5 connected at its reset pin 4 force the latch to standby mode (with inactive low output). The circuit is driven into sleep mode. As soon as vibration is detected, MOSFET T1 is fired by the positive-
8 8 • a p r i l 2 0 0 8 • e l e c t ro n i c s f o r yo u
going pulse output from the vibration sensing mechanism built around piezo-ceramic wafer and associated components. As a result, control input pins 2 and 6 of IC1 latch are grounded. Output pin 3 of IC1 now goes high. The positive supply from output pin 3 of IC1 is extended to three-tone siren generator UM3561 (IC2) through R5, D1 and R6. Components R6 and ZD1 stabilise the input power supply of IC2 to around 3.3V. Output signals from IC2 are amplified by Darlington-pair transistors T2 and T3 to produce alert tone (police siren sound) via loud-
edi
s.c. dwiv
speaker LS1. Reset switch S1 can be used to switch off the alarm sound by resetting the latch circuit. For safety, use key-lock type switches for S1 and S2. A relay can also be connected at the output socket (SOC1) of the circuit to energise highpower beacons, emergency sirens and fence electrification units. The circuit works off 9V DC. A compact PP3-/6F22-type alkaline battery can be used to power the circuit.
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CIRCUIT IDEAS
CLAP SWITCH MOHAMMAD USMAN QURESHI
I VED DWI S.C.
ere’s a clap switch free from false triggering. To turn on/off any ap pliance, you just have to clap twice. The cir-cuit changes its output state only when you clap twice within the set time period. Here, you’ve to clap within 3 seconds. The clap sound sensed by condenser microphone is amplified by transistor T1. The amplified signal provides negative pulse
depending on the selected values of R7 and C3. This ‘on’ time (T) of IC1 can be calculated using the following relationship: T=1.1R7.C3 seconds where R7 is in ohms and C3 in microfarads. On first clap, output pin 3 of IC1 goes high and remains in this standby position for the preset time. Also, LED1 glows for this period. The output of IC1 provides supply voltage to IC2 at its pins 8 and 4.
On second clap, a negative pulse triggers IC2 and its output pin 3 goes high for a time period depending on R9 and C5. This provides a positive pulse at clock pin 14 of decade counter IC 4017 (IC3). Decade counter IC3 is wired here as a bistable. Each pulse applied at clock pin 14 changes the output state at pin 2 (Q1) of IC3 because Q2 is connected to reset pin 15. The high output at pin 2 drives transistor T2 and also energises relay RL1. LED2
to pin 2 of IC1 and IC2, triggering both the ICs. IC1, commonly used as a timer, is wired here as a monostable multivibrator. Trigging of IC1 causes pin 3 to go high and it remains high for a certain time period
Now IC2 is ready to receive the triggering signal. Resistor R10 and capacitor C7 connected to pin 4 of IC2 prevent false triggering when IC1 provides the supply voltage to IC2 at first clap.
indicates activation of relay RL1 and on/off status of the appliance. A free-wheeling diode (D1) prevents damage of T2 when relay de-energises. This circuit costs around Rs 80.
H
ELECTRONICS FOR YOU
MAY 2003
circuit
ideas
USB Power Booster
T.A. Babu
T
he USB serial bus can be configured for connecting several peripheral devices to a single PC. It is more complex than RS232, but faster and simpler for PC expansion. Since a PC can supply only a limited power to the external devices connected through its USB port, when too many devices are connected simultaneously, there is a possibility of power shortage. Therefore an external power source has to be added to power the external devices. In USB, two different types of connectors are used: type A and type B. The circuit presented here is an addon unit, designed to add more power to a USB supply line (type-A). When
edi
s.c. dwiv
power signal from the PC (+5V) is received through socket A, LED1 glows, opto-diac IC1 conducts and TRIAC1 is triggered, resulting in availability of mains supply from the primary of transformer X1. Now transformer X1 delivers 12V at its secondary, which is rectified by a bridge rectifier comprising diodes D1 through D4 and filtered by capacitor C2. Regulator 7805 is used to stabilise
the rectified DC. Capacitor C3 at the output of the regulator bypasses the ripples present in the rectified DC output. LED1 indicates the status of the USB power booster circuit. Assemble the circuit on a generalpurpose PCB and enclose in a suitable cabinet. Bring out the +5V, ground and data points in the type-A socket. Connect the data cables as assigned in the circuit and the USB power booster is ready to Fig. 2: Pin configurations of moc3021, bt136 and 5v regulator 7805 function.
Fig. 1: Circuit of the usb power booster
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e l e c t ro n i c s f o r yo u • a p r i l 2 0 0 8 • 8 7
Colour Sensor
C
olour sensor is an interesting project for hobbyists. The circuit can sense eight colours, i.e. blue, green and red (primary colours); magenta, yellow and cyan (secondary colours); and black and white. The circuit is based on the fundamentals of optics and digital electronics. The object whose colour is required to be detected should be placed in front of the system. The light rays reflected from the object will fall on the three convex lenses which are fixed in front of the three LDRs. The convex lenses are used to converge light rays. This helps to increase the sensitivity of LDRs. Blue, green and red glass plates (filters) are fixed in front of LDR1, LDR2 and LDR3 respectively. When reflected light rays from the object fall on the gadget, the coloured filter glass plates determine which of the LDRs would get triggered. The circuit makes use of only ‘AND’ gates and ‘NOT’ gates. When a primary coloured light ray falls on the system, the glass plate corresponding to that primary colour will allow that specific light to pass through. But the other two glass plates will not allow any light to pass through. Thus only one LDR will get triggered and the gate output corresponding to that LDR will become logic 1 to indicate which colour it is. Similarly, when a secondary coloured light ray falls on the system, the two primary glass plates corresponding to the mixed colour will allow that light to pass through while the remaining one will not allow any light ray to pass through it. As a result two of the LDRs get triggered and the gate output corresponding to these will become logic 1 and indicate which colour it is. When all the LDRs get triggered or remain untriggered, you will observe white and black light indications respectively. Following points may be carefully noted:
13
1. Potmeters VR1, VR2 and VR3 may be used to adjust the sensitivity of the LDRs. 2. Common ends of the LDRs should be connected to positive supply. 3. Use good quality light filters. The LDR is mounded in a tube, behind a lens, and aimed at the object.
The coloured glass filter should be fixed in front of the LDR as shown in the figure. Make three of that kind and fix them in a suitable case. Adjustments are critical and the gadget performance would depend upon its proper fabrication and use of correct filters as well as light conditions.
ELECTRONICS PROJECTS Vol. 20
COMPUTERISED MORSE CODE GENERATOR / TRANSMITTER PUNERJOT SINGH MANGAT
T
he circuit given here can be used to send telegraphic messages via computer. The message data entered through the computer keyboard is converted to corresponding Morse code and transmitted via the circuit attached to any IBM compatible computer’s printer port. Morse code pulses from the computer appearing at pin 3 of the 25-pin parallel port are routed to the base of transistor T1(CL100) which in turn switches on the audio frequency oscillator built around IC1 (NE555) for the duration of each pulse. The frequency of the oscillator can be varied by adjusting potmeters VR1 (20 kilo-ohm) and VR2 (50 kilo-ohm). The audio output from pin 3 of IC (NE555) is connected to an FM transmitter comprising transistor T2 (BF194B) and the associated components. The frequency of the transmitter can be changed with the help of trimmer capacitor VC1 or by changing the number of turns of coil L1. The FM modulated signal is coupled to a short-wire antenna via capacitor C7. The signal can be received using any readymade FM receiver tuned to the frequency of the transmitter.
As stated earlier, this circuit is connected to the parallel port of the PC. Only pins 3 and 25 of the ‘D’ connector are used. Pin 3 corresponding to data bit D1 of port 378(hex) carries the Morse Code data from the computer to the circuit while pin 25 serves as common ground. The circuit should be powered by +5 volts regulated power supply. It should be fixed inside a metal box to reduce interference. The program, written in TURBO PASCAL 7.0, accepts the message via the keyboard, converts it to corresponding Morse code and sends the code to pin 3 of the printer port. The Morse code of
various characters appears under the function ‘write(ch)’ of the program wherein ‘di’ represents a short duration pulse and ‘da’ represents a long duration pulse. The program is interactive and permits variation of speed. The program can be modified to read and transmit the text files or one can even make a TSR (terminate and-stayresident) program. It is hoped that this circuit idea would prove to be of great value to the government’s telecom department, defence services, coast guard, merchant navy and amateur radio operators as well as all those who make use of Morse code for message transmission.
PROGRAM LISTING IN TURBO PASCAL 7.0 {$M $450,0,0} uses crt,dos: label main,endpro,output,message,startmessage, speedselect,fileiput,dosshell ,start; var s:array [1..14] of string [76]: pause,x,y,i,b:integer; sl:slring[l]: ch:char: procedure color{a,b:integer); begin textcolor(a); textbackground(b); end;
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ELECTRONICS PROJECTS Vol. 19
procedure di; begin port[$378]:=2; delay(pause); port[$378]:=0; delay(pause); end; procedure da; begin port[$378]:=2; delay(pause*3); port[$378]:=0; delay(pause); end; begin pause:=100;
START: clrscr; color(11,1); gotoxy(15,4); write(‘PUNJABl UNIVERSITY PATIALA-147002‘); gotoxy(1,7); color(10,3); gotoxy(10,18); write(‘==========================’); gotoxy(10,19); write(‘Fl = Increase Speed ‘); gotoxy(10,20); write(‘F2 = Decrease Speed ‘); gotoxy(10,21); write(‘F3 = Output to Device ‘); gotoxy(10,22);
write(‘F4 = Message Input ‘); gotoxy(10,23); write(‘F5 = Dosshell ‘); gotoxy(10,24); write(‘F6 = Quit ‘); gotoxy(10,25); write(‘==========================’); color(14,1); gotoxy(25,2); write(‘PROGRAMMED BY’); gotoxy(21,3); write(‘PUNERJOT SINGH MANGAT’); color(10,3); gotoxy(26,17); write(‘CONTROLS ‘); gotoxy(35,19); write(‘SPEED’); color(10,3); gotoxy(35,20); write(pause); MAIN: window(1,1,80,25); gotoxy(2,25); color(0,7); write(‘Waiting for the command... ‘); ch:=readkey; if ch=#0 then begin ch:=readkey; if (ch=#59) or (ch=#60) then goto speedselect else if ch=#61 then goto output else if ch=#62 then goto startmessage else if ch=#63 then goto dosshell else if ch=#64 then goto endpro; end; goto main; STARTMESSAGE: begin gotoxy(2,25); write (‘ Enter the message and press ENTER KEY...’); color(12,1); window(3,2,78,15); clrscr; for x:= 1 to 14 do s[x]:=’ ‘ ; i:0;x:=1;y:=1;b:=0: end: MESSAGE: begin x:=wherex; y:=wherey; ch:=readkey; if ch=#13 then goto main; if ch=#8 then begin if x=1 then
begin if y=1 then goto message; y:=y-1; x:=76; end else x:=x-1; delete(s[y],length(s[y]),1); gotoxy(x,y); write(‘ ‘); gotoxy(x,y); goto message: end; if(x=76) and (y=14) then goto message; write(ch); s[y]:=(s[y]+ch); goto message; end; OUTPUT: begin gotoxy(2,25); write(‘ Sending output to the MorseDevice... Press any key to stop... ‘); color(12,1); window(3,2,78,15); clrscr; for i:= 1 to y do begin for x:= 1 to length(s[i]) do begin s1:=(copy(s[i],x,1)); ch:=upcase(s1[1]); delay(pause*2); write(ch); if ch=’A’ then begin di; da; end else if ch=’B’ then begin da; di; di; di; end else if ch=’C’ then begin da; di; da; di; end else if ch=’D’ then begin da; di; di; end else if ch=’E’ then begin di; end else if ch=’F’ then begin di; di; da; di; end else if ch=’G’ then begin da; da; di; end else if ch=’H’ then begin di; di; di; di; end else if ch=’I’ then begin di; di; end else if ch=’J’ then begin di; da; da; da; end else if ch=’K’ then begin da; di; da; end else if ch=’L’ then begin di; da; di; di; end else if ch=’M’ then begin da; da; end else if ch=’N’ then begin da; di; end else if ch=’O’ then begin da; da; da; end else if ch=’P’ then begin di; da; da; di; end else if ch=’Q’ then begin da; da; di; da; end else if ch=’R’ then begin di; da; di; end else if ch=’S’ then begin di; di; di; end else if ch=’T’ then begin da; end else if ch=’U’ then begin di; di; da; end else if ch=’V’ then begin di; di; di; da; end else if ch=’W’ then begin di; da; da; end else if ch=’X’ then begin da; di; di; da; end else
if ch=’Y’ then begin da; di; da; da; end else if ch==’Z’ then begin da; da; di; di; end else if ch=’1' then begin di; da; da; da; da; end else if ch=’2' then begin di; di; da; da; da; end else if ch=’3' then begin di; di; di; da; da; end else if ch=’4' then begin di; di; di; di; da; end else if ch=’5' then begin di; di; di; di; di; end else if ch=’6' then begin da; di; di; di; di; end else if ch=’7' then begin da; da; di; di; di; end else if ch=’8' then begin da; da; da; di; di; end else if ch=’9' then begin da; da; da; da; di; end else if ch=’0' then begin da; da; da; da; da; end else if ch=’.’ then begin di; da; di; da; di; da; end else if ch=’;’ then begin da; di; da; di; da; di; end else it ch=’:’ then begin da; da; da; di; di; di; end else if ch=’,’ then begin da; da; di; di; da; da; end else if ch= ” ‘ then begin di; da; di; di; da; di; end else if ch=’?’ then begin di; di; da; da; di; di; end else if ch=’-’ then begin da; di; di; di; di; da; end else if ch=’_’ then begin di; di; da; da; di; da; end else if ch=’/’ then begin da; di; di; da; di; end else if (ch=#39) or (ch=#96) then begin di; da; da; da; da; di; end else if (ch=’(‘) or (ch=’)’)then begin da; di; da; da; di; da; end else if ch=’ ‘then delay(pause*6); if key pressed then goto main; end; end; goto main; end; SPEEDSELECT; begin if(ch=#59)and(pause>50) then pause: = pause+2; if(ch= =#60) and (pause < 190) then pause: = pause - 2; color(10,3); gotoxy(35,20); write1n(pause,’ ‘); goto main; end; DOSSHELL: begin color(7,0); clrscr; write1n(‘Type EXIT to return to programme...’); swapvectors; exec(getenv(‘comspec’),”); swapvectors; goto start; end; ENDPRO: color(7,0); clrscr; end.
ELECTRONICS PROJECTS Vol. 19
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CIRCUIT
CONDENSER MIC AUDIO AMPLIFIER
IDEAS
MAR IL KU SUN
D. PRABAKARAN
T
he compact, low-cost condenser mic audio amplifier described here provides good-quality audio of 0.5 watts at 4.5 volts. It can be used as part of intercoms, walkie-talkies, low-power
transmitters, and packet radio receivers. Transistors T1 and T2 form the mic preamplifier. Resistor R1 provides the necessary bias for the condenser mic while preset VR1 functions as gain control for
ELECTRONICS FOR YOU ❚ DECEMBER 2001
varying its gain. In order to increase the audio power, the low-level audio output from the preamplifier stage is coupled via coupling capacitor C7 to the audio power amplifier built around BEL1895 IC. BEL1895 is a monolithic audio power amplifier IC designed specifically for sensitive AM radio applications that delivers 1 watt into 4 ohms at 6V power supply voltage. It exhibits low distortion and noise and operates over 3V-9V supply voltage, which makes it ideal for battery operation. A turn-on pop reduction circuit prevents thud when the power supply is switched on. Coupling capacitor C7 determines low-frequency response of the amplifier. Capacitor C9 acts as the ripple-rejection filter. Capacitor C13 couples the output available at pin 1 to the loudspeaker. R15-C13 combination acts as the damping circuit for output oscillations. Capacitor C12 provides the boot strapping function. This circuit is suitable for lowpower HAM radio transmitters to supply the necessary audio power for modulation. With simple modifications it can also be used in intercom circuits.
Contactless AC Mains Voltage Detector
T
his is a CMOS IC (CD4033) based circuit which can be used to detect presence of AC mains voltage without any electrical contact with the conductor carrying AC current/voltage. Thus it can be used to detect mains AC voltage without removing the insulation from the conductor. Just take it in the vicinity of the conductor and it would detect presence of AC voltage. If AC voltage is not present, the display would randomly show any digit (0 through 9) permanently. If mains supply is available in the conductor, the electric field would be induced into the sensing probe. Since IC used is CMOS type, its input
ELECTRONICS PROJECTS Vol. 20
impedance is extremely high and thus the induced voltage is sufficient to clock the counter IC. Thus display count advances rapidly from 0 to 9 and then repeats itself. This is the indication for presence of mains supply. Display stops advancing when the unit is taken away from the mains carrying conductor. For compactness, a 9-volt PP3 battery may be used for supply to the gadget.
N
Cordless Phone Backup
ormally the base of a cordless phone has an adaptor and the handset has Ni-Cd cells for its operation. The base unit becomes inoperative in case of power failure. Under such conditions, it is better to provide a backup using Ni-Cd cells externally. Here is a simple power supply back-up circuit which can be used with cordless phone SANYO CLT-420 or similar sets. The working is simple. When AC mains is present, Ni-Cd cells are charged through IC LM317L, which is wired as a current source. Also, diode D3 is reverse-biased, which keeps Ni-Cd cells isolated from positive rail. When AC mains goes off, the Ni-Cd cells provide supply to the cordless phone base
ELECTRONICS PROJECTS Vol. 20
unit through diode D3. A green LED is used to indicate the presence of AC mains. Each Ni-Cd cell costs around Rs 34,
and the cost of the backup unit, including the box and cells, would not exceed Rs 300. Hence the circuit is well worth the investment.
CIRCUIT IDEAS
CRYSTAL-CONTROLLED TIME-BASE GENERATOR
I VED DWI S.C.
PRATAP CHANDRA SAHU
figure. The 1kHz signal can be further divided using decade counters to generate the required time period. EFY lab note. To generate required gate for use in a frequency counter circuit, the final oscillator output needs to be followed by a toggle flip-flop. For example, a 1kHz clock, when applied to a toggle flip-
A
digital frequency counter needs a time-base generator to count the frequency with high resolution. Normally, a crystal-based oscillator with divider IC chain or a similar circuit in the form of an ASIC (application-specific IC) is used for time-base generation. Here we’ve presented a simple circuit for accurate time-base generation using the readily available 3.5795MHz crystal commonly used in telecommunication equipment. The 3.5795MHz crystal is used in conjunction with a CD4060-based crystal oscillator-cum-divider (IC1). The crystal frequency is divided by 512 by IC1, which is further divided by 7 by CD4017 (IC2). IC2 is reset as soon as its Q7 output goes high.
ELECTRONICS FOR YOU
MAY 2002
Thus the crystal frequency is divided by 3584, giving the final output frequency of around 998.8 Hz. This frequency can be trimmed to exactly 1 kHz with the help of trimmer capacitor VC1 as shown in the
flop, will generate gates with 1-sec ‘on’ period and 1-sec ‘off’ period. This circuit is estimated to cost below Rs 50.
T
DARKROOM TIMER
he timer circuit described here provides a pleasant musical tone in your darkroom at 1-second intervals. The circuit takes up very little space and can be easily converted into a metronome. Unijunction transistor (UJT) T1 functioning as a relaxation oscillator triggers the phase-shift audio oscillator circuit
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ELECTRONICS PROJECTS Vol. 22
built around transistor T2, turning it on and off. As capacitor C1 is charged through preset VR1 and resistor R1, the emitter voltage of UJT rises toward the supply voltage. When the emitter voltage becomes sufficiently positive, the emitter becomes forward biased and discharges capacitor C1 through the emitter-base 1 (B1) junction and resistor R2. The voltage drop across R2 forward biases transistor T2 and turns it on. As capacitor C1 becomes discharged, the current through resistor R2 drops and transistor T2 is cut off. A tone signal is generated by transistor T2 and R-C coupled phase-shift oscillator. Part of the signal taken from the collector of tran-
sistor T2 is coupled to a small speaker through a transistor-radio type output transformer. The 22-kilo-ohm value of resistor R3 represents a compromise between tone duration and intensity. You can use resistors having a value anywhere between 10 kilo-ohms and 25 kilo-ohms for different durations and intensities of the output signals. Since the unijunction transistor is functioning as the oscillator trigger, changing the values of one or more components in the UJT circuit will change the rate of the tone burst. The tone frequency can be varied by changing the value of any or more of capacitors C2 through C4 and resistors R5 and R6 in the phase-shift network. The primary winding of transformer X1 can be tuned for a slight increase in the output, using capacitor values between 0.05 and 0.25 μF for C5 by trial-and-error method. Tone pulses should begin about ten seconds after the unit is turned on. After a minute or so, adjust preset VR1 for 1-second beats by comparing the timing of the beats with the seconds needle on your wristwatch.
DIGITAL DICE WITH NUMERIC DISPLAY
T
he circuit described here is that of a digital dice with numeric display. Timer IC 555 wired as an astable multivibrator produces pulses at about 48 kHz rate. These pulses are fed to pin 14 of the decade counter IC 7490. The oscillator is activated by depression of switch S1. Using different connections tor pins 2,
binary output pins of the counter IC2 are connected to corresponding input pins of 4-bit binary adder IC3 (7483) which is wired to give binary output equal to binary input+1. Thus the output of the dice ranges from 1 to 6. For obtaining other dice ranges, reset pins 2 and 3 connections may be made as per Table I. The binary summation outputs from
3 (reset to zero inputs Ro(1) and Ro(2)) and the binary output pins 12, 9, 8 and 11 of IC7490, various count ranges can be set. For the given circuit the count range is set as 0 to 5 by connecting QB and QC outputs to Ro(1) and Ro(2) inputs, respectively. At the count of 6, QB and QC outputs of IC2 go high and counter is reset. The
IC 7483 are connected to IC4 (7447) which is a BCD to 7-segment decoder/driver. The output from IC4 is connected to a 7-segment common-anode LED display (LTS542). When switch S1 is depressed, the LED (D1) glows and the number displayed at the 7-segment display changes at a rate
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ELECTRONICS PROJECTS Vol. 19
Dice range 1 to 2 1 to 3 1 to 4 1 to 5 1 to 6 1 to 8 1 to 9
TABLE I Connect pin 2 to pin 9 pin 9 pin 8 pin 8 pin 8 pin 11 pin 11
Connect pin 3 to +5V pin 12 +5V pin 12 pin 9 +5V pin 12
of about 48,000 times per second. As soon as the switch is released, the last (latest) number remains on display. Thus the circuit performs the function of a random number generator with the displayed number lying within the selected (wired) range.
DIGITAL MAINS VOLTAGE INDICATOR
C
ontinuous monitoring of the mains voltage is required in many applications such as manual voltage stabilisers and motor pumps. An analogue voltmeter, though cheap, has many disadvantages as it has moving parts and is sensitive to vibrations. The solidstate voltmeter circuit described here indicates the mains voltage with a resolution that is comparable to that of a general-purpose analogue voltmeter. The status of the mains voltage is available in the form of an LED bar graph. Presets VR1 through VR16 are used
CD4029B (counter). The counter clocked by NE555 timer-based astable multivibrator generates 4-bit binary address for multiplexer-demultiplexer pair of CD4067B and CD4514B. The voltage from the wipers of presets are multiplexed by CD4067B and the output from pin 1 of CD4067B is fed to the non-inverting input of comparator A2 (half of op-amp LM358) after being buffered by A1 (the other half of IC2). The unregulated voltage sensed from rectifier output is fed to the inverting input of comparator A2.
from comparator A2 inhibits the decoder (CD4514) that is used to decode the output of IC4029 and drive the LEDs. This ensures that the LEDs of the bar graph are ‘on’ up to the sensed voltage-level proportional to the mains voltage. The initial adjustment of each of the presets can be done by feeding a known AC voltage through an auto-transformer and then adjusting the corresponding preset to ensure that only those LEDs that are up to the applied voltage glow. (EFY note. It is advisable to use additional transformer, rectifier, filter, and
The output of comparator A2 is low until the sensed voltage is greater than the reference input applied at the noninverting pins of comparator A2 via buffer A1. When the sensed voltage goes below the reference voltage, the output of comparator A2 goes high. The high output
regulator arrangements for obtaining a regulated supply for the functioning of the circuit so that performance of the circuit is not affected even when the mains voltage falls as low as 50V or goes as high as 280V. During Lab testing regulated 12volt supply for circuit operation was used.)
+
to set the DC voltages corresponding to the 16 voltage levels over the 50-250V range as marked on LED1 through LED16, respectively, in the figure. The LED bar graph is multiplexed from the bottom to the top with the help of ICs CD4067B (16-channel multiplexer) and
ELECTRONICS PROJECTS Vol. 22
Digital Switching System
T
his circuit can control any one out of 16 devices with the help of two push-to-on switches. An up/down
Before using the circuit, press switch S1 to reset the circuit. Now the circuit is ready to receive the input clock. By press-
ing on the appropriate triac and the corresponding LED to indicate the selected channel.
counter acts as a master-controller for the system. A visual indication in the form of LEDs is also available. IC1 (74LS193) is a presettable up/ down counter. IC2 and IC3 (74LS154) (1 of 16 decoder/demultiplexer) perform different functions, i.e. IC2 is used to indicate the channel number while IC3 switches on the selected channel.
ing switch S2 once, the counter advances by one count. Thus, each pressing of switch S2 enables the counter to advance by one count. Likewise, by pressing switch S3 the counter counts downwards. The counter provides BCD output. This BCD output is used as address input for IC2 and IC3 to switch one (desired channel) out of sixteen channels by turn-
The outputs of IC3 are passed through inverter gates (IC4 through IC6) because IC3 provides negative going pulses while for driving the triacs we need positive-going pulses. The high output of inverter gates turn on the npn transistors to drive the triacs. Diodes connected in series with triac gates serve to provide unidirectional current for the gate-drive.
ELECTRONICS PROJECTS Vol. 20
DIGITAL SPEEDOMETER NARENDRA WADHWANI
T
his instrument displays the speed of the vehicle in kmph. An opaque disc is mounted on the spindle attached to the front wheel of the vehicle. The disc has ten equidistant holes on its periphery. On one side of the disc an infrared LED is fixed and on the opposite side of the disc, in line with
the IR LED, a phototransistor is mounted. IC LM324 is wired as a comparator. When a hole appears between the IR LED and phototransistor, the phototransistor conducts. Hence the voltage at collector of the phototransistor and inverting input of LM324 go ‘low’,
and thus output of LM324 becomes logic ‘high’. So rotation of the speedometer cable results in a pulse (square wave) at the output of LM324. The frequency of this waveform is proportional to the speed. Let ‘N’ be the number of pulses in time ‘t’ seconds and numerically equal to the number of kilometres per hour (kmph). For a vehicle such as LML Vespa, with a wheel circumference of 1.38 metres, and number of pulses equal to 10 per revolution, we get the relationship: N pulses = N kmph t =
Nx1000 metres per second 3600x1.38
=
Nx1000x10 pulses per second 3600x1.38
Therefore, time ‘t’ in seconds = 0.4968 second. As shown in the timing diagram, at t=0, output of astable flip-flop IC1(a) i.e. ½556 goes low and triggers monostable multivibrator IC1(b) i.e. ½556. Pulse width of monostable IC1(b) = 0.5068 sec. For IC1(a), t(on) = 0.51 sec. and t(off)= 0.01 sec. The outputs of IC1(a) and IC1(b), and the signal from the transducer section are ANDed. The number of pulses counted during the gating period (0.4968 sec.) is the speed N in kmph (kilometres per hour). At the end of the gating period, output ‘B’ of monostable IC1(b) goes low and B goes high. The rising edge of B is used to enable the quad ‘D’ flip-flops IC6 and IC7. At this instant, i.e. at t=0.5068 sec., the number (speed) N will be latched corresponding to the ‘D’ flip-flops and displayed. At t=0.52 sec., output of astable flip-flop IC1(a) goes low and remains low for 0.01 sec. This waveform is inverted and applied to the reset terminals of all counters (active high). Thus the counters are reset and
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counting begins afresh at t=0.53 sec. up to the time t=0.52+0.2068 sec. However the ‘D’ flip-flops are not enabled and the
previous speed is displayed. The new speed is displayed at t=0.52 + 0.5068 sec. In this way the speed will be updated every 0.52 sec. This speedometer can measure up to 99 kmph with a resolution of 1 kmph. The range can be increased up to 999 kmph by adding another stage consisting of one each of ICs 7490, 74175, 7447 and a 7-segment display. The voltage supply required for the operation of the circuit is derived from the vehicle power supply (12V).
Yamaha, whose circumference of wheel = 1.8353m, can be obtained in a similar fashion. The gating period will simply vary in direct proportion to the wheel diameter. It will be 0.6607 sec. for Yamaha. The same speedometer can be used for other vehicles by making similar calculations. In all the calculations it has been assumed that the speedometer cable makes one revolution for every revolution of the wheel of the vehicles. Note that on/off periods of the waveforms have to be precise. High quality
The calculations shown above are for LML Vespa and Kinetic Honda. The calculations for using this speedometer for
multiturn pots and low temperature coefficient components should be used in the timer ICs.
ELECTRONICS PROJECTS Vol. 19
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CIRCUIT
IDEAS
DTMF PROXIMITY DETECTOR DTMF-based IR transmitter and receiver pair can be used to realise a proximity detector. The circuit presented here enables you to detect any object capable of reflecting the IR beam and moving in front of the IR LED photodetector pair up to a distance of about 12 cm from it.
A
column 1 (pin 12) get connected together via transistor T2 after a power-on delay (determined by capacitor C1 and resistors R1 and R16 in the base circuit of the transistor) to generate DTMF tone (combination of 697 Hz and 1209 Hz) corresponding to keypad digit “1” continuously. LED 2 is used to indicate the tone
from an object, falls on photodetector diode D1. (The photodetector is to be shielded from direct IR light transmission path of IR LED1 by using any opaque partition so that it receives only the reflected IR light.) On detection of the signal by photodetector, it is coupled to DTMF decoder IC2 through emitter-follower transistor T1. When the valid tone pair is detected by the decoder, its StD pin 15 (shorted to TOE pin 10) goes ‘high’. The detection of
The circuit uses the commonly available telephony ICs such as dial-tone generator 91214B/91215B (IC1) and DTMF decoder CM8870 (IC2) in conjunction with infrared LED (IR LED1), photodiode D1, and other components as shown in the figure. A properly regulated 5V DC power supply is required for operation of the circuit. The transmitter part is configured around dialer IC1. Its row 1 (pin 15) and
output from IC3. This tone output is amplified by Darlington transistor pair of T3 and T4 to drive IR LED1 via variable resistor VR1 in series with fixed 10-ohm resistor R14. Thus IR LED1 produces tone-modulated IR light. Variable resistor VR1 controls the emission level to vary the transmission range. LED 3 indicates that transmission is taking place. A part of modulated IR light signal transmitted by IR LED1, after reflection
the object in proximity of IR transmitterreceiver combination is indicated by LED1. The active-high logic output pulse (terminated at connector CON1, in the figure) can be used to switch on/off any device (such as a siren via a latch and relay driver) or it can be used to clock a counter, etc. This DTMF proximity detector finds applications in burglar alarms, object counter and tachometers, etc.
K.S. SANKAR
RUP
ANJA
ELECTRONICS FOR YOU ❚ JUNE 2001
NA
ECONOMICAL PUMP CONTROLLER
T
quick response, no wear and tear, and he automatic pump controller no mechanical failures. The circuit eliminates the need for any diagram is shown in Fig.2. The device manual switching of pumps inperformed satisfactorily on a test run stalled for the purpose of pumping wain conjunction with a 0.5 HP motor ter from a reservoir to an overhead tank (refer Fig. 1). It automatically switches on the pump when the water level in the tank falls below a certain low level L), provided the water level in the reservoir is above a certain level (R). Subsequently, as the water level in the lank rises to an upper l e v e l ( M ) , Fig. 2: Circuit diagram of pump controller. the pump is and pump. switched off automatically. The pump The sensors used in the circuit can is turned on again only when the water be any two conducting probes, preferably level again falls below level L in the resistant to electrolytic corrosion. For tank, provided the level in the reserinstance, in the simplest case, a properly voir is above R. This automated action sealed audio jack can be used to work as continues. the sensor. The circuit is designed to ‘overlook’ The circuit can also be used as a the transient oscillations of the water constant fluid level maintainer. For this level which would otherwise cause the purpose the probes M and L are brought logic to change its state rapidly and very close to each other to ensure that the unnecessarily. The circuit uses a single fluid level is maintained within the M and CMOS chip (CD4001) for logic processL levels. ing. The advantage of this system is that No use of any moving electroit can be used in tanks/reservoirs of any mechanical parts in the water-level capacity whatsoever. However, the circuit sensor has been made. This ensures
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cannot be used for purely non-conducting fluids. For non-conducting fluids, some modifications need to be made in the fluidlevel sensors. The circuit can however be kept intact.
Fig. 1: Block diagram of pump controller.
Electrical Equipment Control Using PC P.V. Vinod Kumar
H
ere is a novel idea for using the printer port of a PC, for control application using software and some interface hardware. The interface circuit along with the given software can be used with the printer port of any PC for controlling up to eight equipment. The interface circuit shown in the figure is drawn
for only one device, being controlled by D0 bit at pin 2 of the 25-pin parallel port. Identical circuits for the remaining data
ELECTRONICS PROJECTS Vol. 20
179
Electronic cardlock system
T
he circuit presented here can be used as a lock for important electronic/electrical appliances. When card is inserted inside its mechanism, depending upon the position of punched hole on the card, a particular appliance would be switched on. The card is inserted just like a floppy disk inside the disk drive. This card should be rectangular in shape with only one punched hole on it. The circuit uses eight photo-transistors (T1 through T8). When there is no card in the lock, light from incandescent lamp L1 (40-watt, 230V) falls on all the photo- transistor detectors. Transistor T8 is used as enable detector for IC1 (74LS244). When light is incident on it, it conducts and its collector voltage goes low. This makes transistor T16 to cut-off, and its collector voltage goes high. This logic high on its collector terminal will inhibit IC1 as long as light is present on phototransistor T8. IC1 will get enabled only when the card is completely inserted inside the lock mechanism. This arrangement ensures that only the selected appliance is switched on and prevents false operation of the system. You can make these cards using a black, opaque plastic sheet. A small rectangular notch is made on this card to indicate proper direction for insertion of the card. If an attempt is made to insert the card wrongly, it will not go completely inside the mechanism and the system will not be enabled. When card for any appliance (say appliance 1) is completely inserted in the mechanism, the light will fall only on photo-transistor T1. So only T1 will
be on and other photo-transistors will be in off state. When transistor T1 is on, its collector voltage falls, making transistor T9 to cut-off. As a result, collector voltage of transistor T9 as also pin 2 of IC1 go logic high. This causes
as buffer with Schmitt trigger. All outputs (Q1 through Q7) of this IC are connected to IC2 (ULN2003) which is used as relay driver. IC2 consists of seven highcurrent relay drivers having integral diodes. External free-wheeling diodes are therefore
pin 18 (output Q1) also to go high, switching LED1 on. Simultaneously, output Q1 is connected to pin 1 of IC2 (ULN2003) for driving the relay corresponding to appliance 1. Similarly, if card for appliance 2 is inserted, only output pin 16 (Q2) of IC1 will go highmaking LED2 on while at the same time energising relay for appliance 2 via ULN2003. The same is true for other cases/appliances also. The time during which card is present inside the mechanism, the system generates musical tone. This is achieved with the help of diodes D1 through D7 which provide a wired-OR connection at their common-cathode junction. When any of the outputs of IC1 is logic high, the commoncathode junction of diodes D1 through D7 also goes logic high, enabling IC3 (UM66) to generate a musical tone. In this circuit IC1 (74LS244) is used
not required. When an input of this IC is made logic high, the corresponding output will go logic low and relay connected to that pin gets energised. This switches on a specific appliance and the corresponding LED. Once a specific card is inserted to switch on a specific relay, that relay gets latched through its second pair of contacts. Thus even when the card is removed, the specific appliance remains on. The same holds true for all other relays/appliances as well. The only way to deenergise a latched relay after removal of the corresponding card is to switch off the corresponding switch (S1 through S7) which would cut-off the supply to the desired relay. The +5V and +12V supplies can be obtained with conventional arrangement using a step-down transformer followed by rectifier, filter and regulator (using 7805 and 7812 etc).
ELECTRONICS PROJECTS Vol. 22
13
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ELECTRONICS PROJECTS Vol. 22
Electronic Scoring Game
Y
ou can play this game alone or with your friends. The circuit comprises a timer IC, two decade counters and a display driver along with a 7-segment display. The game is simple. As stated above, it is a scoring game and the competitor who scores 100 points rapidly (in short steps) is the winner. For scoring, one has the option of pressing either switch S2 or S3. Switch S2, when pressed, makes the counter count in the forward direction, while switch S3 helps to count downwards. Before starting a fresh game, and for that matter even a fresh move, you must press switch S1 to reset the circuit. Thereafter, press any of the two switches, i.e. S2 or S3. On pressing switch S2 or S3, the counter’s BCD outputs change very rapidly and when you release the switch, the last number remains latched at the output of IC2. The latched BCD number is input to BCD to 7-segment decoder/ driver IC3 which drives a commonanode display DIS1. However, you can read this number only when you press switch S4. The sequence of operations for playing the game between, say two players ‘X’ and
ELECTRONICS PROJECTS Vol. 20
‘Y’, is summarised below: 1. Player ‘X’ starts by momentary pressing of reset switch S1 followed by pressing and releasing of either switch S2 or S3. Thereafter he presses switch S4 to read the display (score) and notes down this number (say X1) manually. 2. Player ‘Y’ also starts by momentary pressing of switch S1 followed by pressing of switch S2 or S3 and then notes down his score (say Y1), after pressing switch S4, exactly in the same fashion as done by the first player. 3. Player ‘X’ again presses switch S1 and repeats the steps shown in step 1
above and notes down his new score (say, X2). He adds up this score to his previous score. The same procedure is repeated by player ‘Y’ in his turn. 4. The game carries on until the score attained by one of the two players totals up to or exceeds 100, to be declared as the winner. Several players can participate in this game, with each getting a chance to score during his own turn. The circuit may be assembled using a multipurpose board. Fix the display (LEDs and 7-segment display) on top of the cabinet along with the three switches. The supply voltage for the circuit is 5V.
Fastest Finger First Indicator p. rajesh bhat
Q
uiz-type game shows are increasingly becoming popular on television these days. In such games, fastest finger first indicators (FFFIs) are used to test the player’s reaction time. The player’s designated number is displayed with an audio alarm when the player presses his entry button. The circuit presented here determines as
The combinational circuitry comprising dual 4-input NAND gates of IC3 (7420) locks out subsequent entries by producing the appropriate latch-disable signal. Priority encoder IC4 (74147) encodes the active-low input condition into the corresponding binary coded decimal (BCD) number output. The outputs of IC4 after inversion by inverter gates inside hex
quency can be varied with the help of preset VR1. Logic 0 state at one of the outputs of IC2 produces logic 1 input condition at pin 4 of IC7, thereby enabling the audio oscillator. IC7 needs +12V DC supply for sufficient alarm level. The remaining circuit operates on regulated +5V DC supply, which is obtained using IC1 (7805).
to which of the four contestants first pressed the button and locks out the remaining three entries. Simultaneously, an audio alarm and the correct decimal number display of the corresponding contestant are activated. When a contestant presses his switch, the corresponding output of latch IC2 (7475) changes its logic state from 1 to 0.
inverter 74LS04 (IC5) are coupled to BCDto-7-segment decoder/display driver IC6 (7447). The output of IC6 drives commonanode 7-segment LED display (DIS.1, FND507 or LT542). The audio alarm generator comprises clock oscillator IC7 (555), whose output drives a loudspeaker. The oscillator fre-
Once the organiser identifies the contestant who pressed the switch first, he disables the audio alarm and at the same time forces the digital display to ‘0’ by pressing reset pushbutton S5. With a slight modification, this circuit can accommodate more than four contestants.
ELECTRONICS PROJECTS Vol. 22
179
C I R C U I T
I D E A S
UNIVERSAL HIGH-RESISTANCE VOLTMETER YOGESH KATARIA
T
he full-scale deflection of the universal high-input-resistance voltmeter circuit shown in the figure
depends on the function switch position as follows: (a) 5V DC on position 1 (b) 5V AC rms in position 2 (c) 5V peak AC in position 3 (d) 5V AC peak-to-peak in position 4 The circuit is basically a voltage-tocurrent converter. The design procedure is as follows:
I VED DWI S.C.
Calculate RI according to the application from one of the following equations: (a) DC voltmeter: RIA = full-scale EDC/IFS (b) RMS AC voltmeter (sine wave only): RIB = 0.9 full-scale ERMS/ IFS (c) Peak reading voltmeter (sine wave only): RIC = 0.636 fullscale EPK/IFS (d) Peak-to-peak AC voltmeter (sine wave only): RID = 0.318 full-scale EPK-TO-PK / IFS The term IFS in the above equations refers to meter’s fullscale deflection current rating in amperes. It must be noted that neither meter resistance nor diode voltage drops affects meter current. Note: The results obtained during practical testing of the circuit in EFY lab are tabulated in Tables I through IV. A high-input-resistance op-amp, a bridge rectifier, a microammeter, and a few other discrete components are all that are required to realise this versatile circuit. This circuit can be used for measurement of DC, AC RMS, AC peak, or AC peak-to-peak voltage by simply chang-
ELECTRONICS FOR YOU ❚ FEBRUARY 2000
TABLE I Position 1 of Function Switch Edc input 5.00V 4.00V 3.00V 2.00V 1.00V
Meter Current 44 µA 34 µA 24 µA 14 µA 4 µA
TABLE II Position 2 of Function Switch Erms input 5V 4V 3V 2V 1V
Meter Current 46 µA 36 µA 26 µA 18 µA 10 µA
TABLE III Position 3 of Function Switch EPk input 5V peak 4V peak 3V peak 2V peak 1V peak
Meter Current 46 µA 36 µA 26 µA 16 µA 6 µA
TABLE IV Position 4 of Function Switch EPk-To-Pk 5V peak to peak 4V peak to peak 3V peak to peak 2V peak to peak 1V peak to peak
Meter Current 46 µA 36 µA 26 µA 16 µA 7 µA
ing the value of the resistor connected between the inverting input terminal of the op-amp and ground. The voltage to be measured is connected to non-inverting input of the op-amp.
CIRCUIT IDEAS
FM BOOSTER
MAR IL KU SUN
PRADEEP G.
H
ere is a low-cost circuit of an FM booster that can be used to listen to programmes from distant FM
ELECTRONICS FOR YOU
FEBRUARY 2002
stations clearly. The circuit comprises a common-emitter tuned RF preamplifier wired around VHF/UHF transistor
2SC2570. (Only C2570 is annotated on the transistor body.) Assemble the circuit on a good-quality PCB (preferably, glass-epoxy). Adjust input/output trimmers (VC1/VC2) for maximum gain. Input coil L1 consists of four turns of 20SWG enamelled copper wire (slightly space wound) over 5mm diameter former. It is tapped at the first turn from ground lead side. Coil L2 is similar to L1, but has only three turns. Pin configuration of transistor 2SC2570 is shown in the figure.
CCIIRRC UC IUT IITD EIADS E A S
line voltage is more than 15 volts, i.e. when the handset is placed on the cradle. Once the transistor pair of TI and T2 starts conducting, melody generator IC1
minals of pnp transistor T1) develops enough voltage to forward bias transistor T1 and it starts conducting. As a consequence, output voltage at the collector of transistor T1 sustains forward biasing of transistor T2, even if switch S1 is released. This latching action keeps both transistors T1 and T2 in conduction as long as the output of the bridge rectifier is greater than 15 volts. If the handset is now lifted off-hook, the rectifier output drops to about 9 volts and hence latching action ceases and the
gets the supply and is activated. The music is coupled to the telephone lines via capacitor C2, resistor R1, and the bridge rectifier. With the handset off-hook after a ring, momentary depression of switch S1 causes forward biasing of transistor T2. Meanwhile, if the handset is placed on the cradle, the current passing through R1 (connected across the emitter and base ter-
circuit automatically switches off. (EFY lab note. The value of resistor R2 determines the current through resistor R1 to develop adequate voltage (greater than 0.65 volts) for conduction of transistor T1. Hence it may be test selected between 33 kilo-ohms and 100 kilo-ohms to obtain instant latching.) The total cost of this circuit is around Rs 50.
MUSIC-ON-HOLD FOR TELEPHONES
EDI DWIV S.C.
SIBIN K. ZACHARIAH
H
ere is a simple circuit for musicon-hold with automatic shut off facility. During telephone conversation if you are reminded of some urgent work, momentarily push switch S1 until red LED1 glows, keep the telephone handset on the cradle, and attend to the work on hand. A soft music is generated and passed into the telephone lines while the other-end subscriber holds. When you return, you can simply pick up the handset again and continue with the conversation. The glowing of LED1, while the music is generated, indicates that the telephone is in hold position. As soon as the handset is picked up, LED1 is turned off and the music stops. Normally, the voltage across telephone lines is about 50 volts. When we pick up the receiver (handset), it drops to about 9 volts. The minimum voltage required to activate this circuit is about 15 volts. If the voltage is less than 15 volts, the circuit automatically switches off. However, initially both transistors T1 and T2 are cut off. The transistor pair of T1 and T2 performs switching and latching action when switch S1 is momentarily pressed, provided the
ELECTRONICS FOR YOU
APRIL 2002
FIRE ALARM
W
ith the onset of summer, chances of fire accidents increase. Such fire accidents can be prevented if timely alarms are available. The circuit presented here warns the user against such fire accidents. The circuit should be placed in fire-prone areas such as a kitchen. Everyone is aware that when anything catches fire, smoke is produced. When this smoke passes between a bulb and an LDR, the amount of light falling on the LDR decreases. This causes the resistance of LDR to increase and the voltage at pin 2 of IC 555 goes below 1/3 Vcc. thus triggering IC 555 which is used here in bistable mode. As a result the voltage of pin 3 goes high. This high voltage (approximately +9V) completes the supply to the COB (chip-on-board). Different COBs are available in the market to generate different sounds. How-
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ever, one may select a COB which generates sound such as ‘aag lag gai hai’. The signal generated by COB is amplified by an audio amplifier. In this circuit, the audio power amplifier is wired around IC2 TDA 2002. The sensitivity of the circuit depends on the distance between bulb and LDR
as well as setting of preset VR1. Thus by placing the bulb and the LDR at appropriate distances, one may vary preset VR1 to get optimum sensitivity. Reset switch S1 is provided in the circuit to switch off the alarm after the fire has been noticed by the user.
PROGRAMMABLE DOOR-BELL WITH FLASHING LEDS
I
C1 (NE555) is used here as a clock generator. It is configured as an astable multivibrator whose frequency can be adjusted with the help of potmeter VR1. The clock pulses obtained from IC1 are fed to pin 14 of IC2 (CD4017) which is a well-known decade counter. Here LEDS have been connected in a rather unusual way. The LEDs flash sequentially from Q0 to Q9. Five presets
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of 100k each are connected to each pair of LEDs. IC3 here works as tune generator and the Darlington pair comprising transistors BC547B and SL100B is used to amplify its output. The frequency of IC3 is adjusted by potmeter VR2. Each 100k preset (VR3 through VR7) is adjusted for a different tune depending on individual choice. The 10-LED display
is assembled in such a way that the first vertical column has orange LED1 through LED5 and the second parallel column has green LED10 through LED6, as shown in the figure. The circuit can be easily assembled on a veroboard. Any well-filtered 9V, 250mA DC power supply is suitable. Primary of the supply transformer may be connected to the bell AC outlet points.
H
Frequency Measurements Using PC
ere is a simple technique for measuring frequencies over quite a wide frequency range and with acceptable accuracy limits using a PC. It follows the basic technique of measuring low frequencies, i.e. at low frequency, period is measured for a complete wave and frequency is calculated from the measured time-period. Cascaded binary counters are used for converting the high-frequency signals into low-frequency signals. The parallel port of a computer is used for data input from binary counters. This data is used for measuring time and calculating the frequency of the signal. The block diagram shows the basic connections of the counters and parallel port pin numbers on 25-pin ‘D’ connector of a PC (control register 379 Hex is used for input). External hardware is used only for converting the higher frequency signals into low frequency signals. Thus, the major role in frequency-measurement is played by the software. The PC generates a time-interrupt at a frequency of 18.21 Hz, i.e. after every 54.92 millisecond. Software uses this time-interrupt as a time-reference. The control register of the PC’s parallel port is read and the data is stored continuously in an array for approximately 54.9 ms using a loop. This stored data is then analysed bit-wise. Initially, the higher-order bit (MSB or the seventh-bit) of every array element is scanned for the presence of a complete square wave. If it is found, its time period is measured and if not then the second-highest order bit (sixth bit) is scanned. This operation is performed till the third bit and if no full square wave is still found, an error message is generated which indicates that either there is an error in reading or the frequency signal is lower than 19 Hz. Lower three bits of the control register are not used. When a wave is found,
along with its time-period and frequency components, its measurement precision in percentage is also calculated and displayed. Number of data taken in 54.9 ms is also displayed. As stated above, the lower starting range is about 19 Hz. Data is read for approximately 54.9 ms. Thus, the lowest possible frequency that can be measured is 1/.0549 Hz. Lower frequency range depends only on the sampling time and is practically fixed at 19 Hz (18.2 Hz, to be precise). Upper frequency range depends on factors such as value of the MOD counter used and the operating frequency range of the counter IC. If MOD-N counter is used (where N is an integer), upper limit (UL) of frequency is
given by UL=19xN5 Hz. Thus for MOD 16 counters UL≈20 MHz, and for MOD 10 counters UL≈1.9 MHz. Care should be taken to ensure that this upper limit is within the operating frequency range of counter IC used. Precision of measurement is a machine-dependent parameter. High-speed machines will have better precision compared to others. Basically, precision depends directly upon the number of data read in a standard time. Precision of measurement varies inversely as the value of MOD counter used. Precision is high when MOD 10 counters are used in place of MOD 16 counters, but this will restrict the upper limit of frequency measurement and vice-versa.
ELECTRONICS PROJECTS Vol. 20
FLUID LEVEL DETECTOR
H
ere is a simple but versatile circuit of fluid level detector which can be used for various applications at home and in industry. Circuit is built around 2-input NAND Schmitt trigger gates N1 and N2. Gate N1 is configured as an oscillator operating at around 1 kHz frequency. When the fluid level reaches the probe’s level, the oscillations are coupled to the diode detector stage comprising diodes D1 and D2. capacitor C4 and resistor R2. The positive voltage developed across capacitor C4 and resistor R2 combination is applied to Schmitt NAND gate N2 which is used here as a buffer/driver. The output of gate N2 is connected to opto-coupler MCT2E. The output across pins 4 and 5 of the opto-coupler can
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be suitably interfaced to any external circuit for indication purposes or driving any load as desired. Use of opto-coupler ensures complete isolation of the load from the fluid level
detector circuit. Since high frequency AC is used for the electrodes, there is no corrosion of the electrodes which is normally observed with DC being applied to the electrodes.
CIRCUIT
IDEAS
GENERATION OF 1-SEC. PULSES SPACED 5-SEC. APART RUP
ANJA
NA
PRAVEEN SHANKER
T
his circuit using a dual-timer NE556 can produce 1Hz pulses spaced 5 seconds apart, either manually or automatically. IC NE556 comprises two independent NE555 timers in a single package. It is used to produce two separate pulses of different pulse widths, where one pulse initiates the activation of the second pulse. The first half of the NE556 is wired for 5-second pulse output. When slide switch S2 is in position ‘a’, the first timer is set for manual operation, i.e. by press-
ing switch S1 momentarily you can generate a single pulse of 5second duration. When switch S2 is kept in ‘b’ position, i.e. pins 6 and 2 are shorted, timer 1 in NE556 triggers by itself. The output of the first timer is connected to trigger pin 8 of second timer, which, in turn, is connected to a potential divider comprising resistors R4 and R5. Resistor R1, preset VR1, resistor R2, preset VR2, and capacitors C2 and C5 are the components determining time period. Presets VR1 and VR2 permit trim-
ELECTRONICS FOR YOU ❚ FEBRUARY 2001
ming of the 5-second and 1-second pulse width of respective sections. When switch S2 is in position ‘a’ and switch S1 is pressed momentarily, the output at pin 5 goes high for about 5 seconds. The trailing (falling) edge of this 5second pulse is used to trigger the second timer via 0.1µF capacitor C6. This action results in momentarily pulling down of pin 8 towards the ground potential, i.e. ‘low’. (Otherwise pin 8 is at 1/2 Vcc and triggers at/below 1/3 Vcc level.) When the second timer is triggered at the trailing edge of 5-second pulse, it generates a 1second wide pulse. When switch S2 is on position ‘b’, switch S1 is disconnected, while pin 6 is connected to pin 2. When capacitor C is charged, it is discharged through pin 2 until it reaches 1/3Vcc potential, at which it is retriggered since trigger pin 6 is also connected here. Thus timer 1 is retriggered after every 5-second period (corresponding to 0.2Hz frequency). The second timer is triggered as before to produce a 1-second pulse in synchronism with the trailing edge of 5-second pulse. This circuit is important wherever a pulse is needed at regular intervals; for instance, in ‘Versatile Digital Frequency Counter Cum Clock’ construction project published in EFY Oct. ’97, one may use this circuit in place of CD4060-based circuit. For the digital clock function, however, pin 8 and 12 are to be shorted after removal of 0.1µF capacitor and 10-kilo-ohm resistors R4 and R5.
Handy Zener Diode Tester
H
ere is a handy zener diode tester which tests zener diodes with breakdown voltages extending up to 120 volts. The main advantage of this circuit is that it works with a voltage as low as 6V DC and consumes less than 8 mA current. The circuit can be fitted in a 9V battery box. Two-third of the box may be used for
four 1.5V batteries and the remaining onethird is sufficient for accommodating this circuit. In this circuit a commonly available transformer with 230V AC primary to 9-09V, 500mA secondary is used in reverse to achieve higher AC voltage across 230V AC terminals. Transistor T1 (BC547) is configured as an oscillator and driver to obtain re-
quired AC voltage across transformer’s 230V AC terminals. This AC voltage is converted to DC by diode D1 and filter capacitor C2 and is used to test the zener diodes. R3 is used as a series current limiting resistor. After assembling the circuit, check DC voltage across points A and B without connecting any zener diode. Now switch S1 on. The DC voltage across A-B should vary from 10V to 120V by adjusting potmeter VR1 (10k). If every thing is all right, the circuit is ready for use. For testing a zener diode of unknown value, connect it across points A and B with cathode towards A. Adjust potmeter VR1 so as to obtain the maximum DC voltage across A and B. Note down this zener value corresponding to DC voltage reading on the digital multimeter. When testing zener diode of value less than 3.3V, the meter shows less voltage instead of the actual zener value. However, correct reading is obtained for zener diodes of value above 5.8V with a tolerance of ±10 per cent. In case zener diode shorts, the multimeter shows 0 volts.
ELECTRONICS PROJECTS Vol. 20
HOUSE SECURITY SYSTEM MALAY BANERJEE
H
ere is a low-cost, invisible laser circuit to protect your house from thieves or trespassers. A laser pointer torch, which is easily available in the market, can be used to operate this device. The block diagram of the unit shown in Fig. 1 depicts the overall arrangement for providing security to a house. A laser torch powered by 3V powersupply is used for generating a laser beam. A combination of plain mirrors
M1 through M6 is used to direct the laser beam around the house to form a net. The laser beam is directed to finally fall on an LDR that forms part of the receiver unit as shown in Fig. 2. Any interruption of the beam by a thief/ trespasser will result into energisation of the alarm. The 3V power-supply circuit is a conventional full-wave rectifier-filter circuit. Any alarm unit that operates on 230V AC can be connected at the output. The receiver unit comprises two identical
step-down transformers (X1 and X2), two 6V relays (RL1 and RL2), an LDR, a transistor, and a few other passive components. When switches S1 and S2 are activated, transformer X1, followed by a full-wave rectifier and smoothing capacitor C1, drives relay RL1 through the laser switch. The laser beam should be aimed continuously on LDR. As long as the laser beam falls on LDR, transistor T1 remains forward biased and relay RL1 is thus in de-energised condition. When a person crosses the line of laser beam, relay RL1 turns on and transformer X2
gets power supply and RL2 energises. In this condition, the laser beam will have no effect on LDR and the alarm will continue to operate as long as switch S2 is on. When the torch is switched on, the pointed laser beam is reflected from a definite point/ place on the periphery of the house. Making use of a set of properly oriented mirrors one can form an invisible net of laser rays as shown in the block diagram. The final ray should fall on LDR of the circuit. Note. LDR should be kept in a long pipe to protect it from other sources of light, and its total distance from the source may be kept limited to 500 metres. ELECTRONICS PROJECTS Vol. 22
119
CIRCUIT
IDEAS
IC Controlled Emergency Light with Charger
AINA R. R
A.P.S. DHILLON
T
he circuit shown here is that of the IC controlled emergency light. Its main features are: automatic switching-on of the light on mains failure and battery charger with overcharge protection. When mains is absent, relay RL2 is in deenergised state, feeding battery supply to inverter section via its N/ C contacts and switch S1. The inverter section comprises IC2 (NE555) which is used in stable mode to produce sharp pulses at the rate of 50 Hz for driving the MOSFETs. The output of IC3 is fed to gate of MOSFET (T4) directly while it is applied to MOSFET (T3) gate after inversion by transistor T2. Thus the power amplifier built around MOSFETs T3 and T4 functions in push-pull mode. The output across secondary of transformer X2 can easily drive a 230-volt, 20-watt fluorescent tube. In case light is not required to be on during mains failure, simply flip switch S1 to off position.
Battery overcharge preventer circuit is built around IC1 (LM308). Its non-
inverting pin is held at a reference voltage of approximately 6.9 volts which is obtained using diode D5 (1N4148) and 6.2-volt zener D6. The inverting pin of IC1 is connected to the positive terminal of battery. Thus when mains supply is present, IC1 comparator output is high, unless battery voltage exceeds 6.9 volts. So transistor T1 is normally forward biased, which energises relay RL1. In this state the battery remains on charge via N/O contacts of relay RL1 and current limiting resistor R2. When battery voltage exceeds 6.9 volts (overcharged condition), IC1 output goes low and relay RL1 gets deenergised, and thus stops further charging of battery. MOSFETs T3 and T4 may be mounted on suitable heat sinks.
INTELLIGENT SWITCH
T
his intelligent switch circuit enables automatic, switching on of an emergency light system during darkness in the event of mains failure. The mains power failure condition is detected by the section consisting of mains step-down transformer X1 followed by bridge rectifier comprising diodes D1 through D4 and smoothing capacitor C1. If the mains is available then it causes energisation of relay RL1 which has two sets of changeover contacts. The light/darkness condition is detected by the circuit comprising phototransistor FPT100/2N5777 followed by Darlington pair comprising transistors T2 and T3. However, this section will function only when mains supply is not available (i.e. when relay RL1 is in de-energised state) since battery supply (negative lead) path gets completed via lower N/C contact of relay RL1. During daylight, photo transistor conducts and places transistor T2 base near ground potential. Thus Darlington pair remains cut-off and relay RL2 Period
Conditions
[ [
remains de-energised. However, during darkness, photo transistor is cut-off and therefore transistor T2 receives forward base bias via resistor R1 (connected to positive rail), as resistor R2 is no more grounded (via photo-transistor T1). As a result, relay RL2 gets energised. Thus it would be observed that when mains is absent (relay RL1 de-energised) and it is dark (relay RL2 energised), the switch
Switch status
During daylight (when mains is present (when mains is absent) During night darkness
200
] ]
(when mains is present) (when mains is absent)
ELECTRONICS PROJECTS Vol. 19
intelligent switch is ‘off ’. intelligent switch is ‘off ’. intelligent switch is ‘on’.
output path is complete. In any other condition switch output path would get broken. The switch output terminals can be used (in series with supply) to control a lighting system directly or indirectly through another contactor/heavy-duty relay depending upon the load. The working of the intelligent switch is summarised in the table.
CIRCUIT
INVISIBLE BROKEN WIRE DETECTOR
IDEAS
EDI DWIV S.C.
K. UDHAYA KUMARAN, VU3GTH
P
ortable loads such as video cameras, halogen flood lights, electrical irons, hand drillers, grinders, and cutters are powered by connecting long 2- or 3-core cables to the mains plug. Due to prolonged usage, the power cord wires are subjected to mechanical strain and stress, which can lead to internal snapping of wires at any point. In such a case most people go for replacing the core/cable, as finding the exact loca-
tion of a broken wire is difficult. In 3-core cables, it appears almost impossible to detect a broken wire and the point of break without physically disturbing all the three wires that are concealed in a PVC jacket. The circuit presented here can easily and quickly detect a broken/faulty wire and its breakage point in 1-core, 2-core, and 3-core cables without physically disturbing wires. It is built using hex inverter CMOS CD4069. Gates N3 and N4 are used as a pulse generator that oscillates at around 1000 Hz in audio range.
The frequency is determined by timing components comprising resistors R3 and R4, and capacitor C1. Gates N1 and N2 are used to sense the presence of 230V AC field around the live wire and buffer weak AC voltage picked from the test probe. The voltage at output pin 10 of gate N2 can enable or inhibit the oscillator circuit. When the test probe is away from any high-voltage AC field, output pin 10 of gate N2 remains low. As a result, diode D3 conducts and inhibits the oscillator circuit from oscillating. Simultaneously, the output of gate N3 at pin 6 goes ‘low’ to cut off transistor T1. As a result, LED1 goes off. When the test probe is moved closer to 230V AC, 50Hz mains live wire, during every positive halfcycle, output pin 10 of gate N2 goes high. Thus during every positive half-cycle of the mains frequency, the oscillator circuit is allowed to oscillate at around 1 kHz, making red LED (LED1) to blink. (Due to the persistence of vision, the LED appears to be glowing continuously.) This type of blinking reduces consumption of the current from button cells used for power supply. A 3V DC supply is sufficient for powering the whole circuit. AG13 or LR44 type button cells, which are also used inside laser pointers or in LED-based continuity testers, can be used for the circuit. The circuit consumes 3 mA during the sensing of AC mains voltage.
ELECTRONICS FOR YOU ❚ AUGUST 2001
For audio-visual indication, one may use a small buzzer (usually built inside quartz alarm time pieces) in parallel with one small (3mm) LCD in place of LED1 and resistor R5. In such a case, the current consumption of the circuit will be around 7 mA. Alternatively, one may use two 1.5V R6- or AA-type batteries. Using this gadget, one can also quickly detect fused small filament bulbs in serial loops powered by 230V AC mains. The whole circuit can be accommodated in a small PVC pipe and used as a handy broken-wire detector. Before detecting broken faulty wires, take out any connected load and find out the faulty wire first by continuity method using any multimeter or continuity tester. Then connect 230V AC mains live wire at one end of the faulty wire, leaving the other end free. Connect neutral terminal of the mains AC to the remaining wires at one end. However, if any of the remaining wires is also found to be faulty, then both ends of these wires are connected to neutral. For single-wire testing, connecting neutral only to the live wire at one end is sufficient to detect the breakage point. In this circuit, a 5cm (2-inch) long, thick, single-strand wire is used as the test probe. To detect the breakage point, turn on switch S1 and slowly move the test probe closer to the faulty wire, beginning with the input point of the live wire and proceeding towards its other end. LED1 starts glowing during the presence of AC voltage in faulty wire. When the breakage point is reached, LED1 immediately extinguishes due to the non-availability of mains AC voltage. The point where LED1 is turned off is the exact broken-wire point. While testing a broken 3-core rounded cable wire, bend the probe’s edge in the form of ‘J’ to increase its sensitivity and move the bent edge of the test probe closer over the cable. During testing avoid any strong electric field close to the circuit to avoid false detection.
CIRCUIT IDEAS
LASER TORCH-BASED VOICE TRANSMITTER AND RECEIVER
SAN
I THE
O
PRADEEP G.
U
sing this circuit you can communicate with your neighbours wirelessly. Instead of RF signals, light from a laser torch is used as the carrier in the circuit. The laser torch can transmit light up to a distance of about 500 metres. The phototransistor of the receiver must be accurately oriented towards the laser beam from the torch. If there is any obstruction in the path of the laser beam, no sound will be heard from the receiver. The transmitter circuit (Fig. 1) comprises condenser microphone transistor amplifier BC548 (T1) followed by an opamp stage built around µA741 (IC1). The gain of the op-amp can be controlled with the help of 1-mega-ohm potmeter VR1. The AF output from IC1 is coupled to the base of transistor BD139 (T2), which, in turn, modulates the laser beam. The transmitter uses 9V power supply. However, the 3-volt laser torch (after removal of its battery) can be directly connected to the circuit—with the body of the torch connected to the emitter of BD139 and the spring-loaded lead protruding from inside the torch to circuit ground. The receiver circuit (Fig. 2) uses an npn phototransistor as the light sensor that is followed by a two-stage transistor preamplifier and LM386-based audio power amplifier. The receiver does not need any complicated alignment. Just keep the phototransistor oriented towards the remote transmitter’s laser point and adjust the volume control for a clear sound. To avoid 50Hz hum noise in the speaker, keep the phototransistor away
from AC light sources such as bulbs. The reflected sunlight, however, does not
cause any problem. But the sensor should not directly face the sun.
JANUARY 2002
ELECTRONICS FOR YOU
CIRCUIT IDEAS
MOBILE PHONE BATTERY CHARGER
I THE SAN
O
T.K. HAREENDRAN
M
obile phone chargers available in the market are quite expensive. The circuit presented here comes as a low-cost alternative to charge mobile telephones/battery packs with a rating of
ELECTRONICS FOR YOU
JANUARY 2002
7.2 volts, such as Nokia 6110/6150. The 220-240V AC mains supply is downconverted to 9V AC by transformer X1. The transformer output is rectified by diodes D1 through D4 wired in bridge
configuration and the positive DC supply is directly connected to the charger’s output contact, while the negative terminal is connected through current limiting resistor R2. LED2 works as a power indicator with resistor R1 serving as the current limiter and LED3 indicates the charging status. During the charging period, about 3 volts drop occurs across resistor R2, which turns on LED3 through resistor R3. An external DC supply source (for instance, from a vehicle battery) can also be used to energise the charger, where resistor R4, after polarity protection diode D5, limits the input current to a safe value. The 3-terminal positive voltage regulator LM7806 (IC1) provides a constant voltage output of 7.8V DC since LED1 connected between the common terminal (pin 2) and ground rail of IC1 raises the output voltage to 7.8V DC. LED1 also serves as a power indicator for the external DC supply. After constructing the circuit on a veroboard, enclose it in a suitable cabinet. A small heat sink is recommended for IC1.
CIRCUIT
IDEAS
Simple Low-Cost Digital Code Lock any digital code lock circuits have been published in this magazine. In those circuits a set of switches (conforming to code) are pressed one by one within the specified time to open the lock. In some other circuits, custom-built ICs are used and positive and negative logic pulses are
M
An essential property of this electronic code lock is that it works in monostable mode, i.e. once triggered, the output becomes high and remains so for a period of time, governed by the timing components, before returing to the quiescent low state. In this circuit, timer IC 555 with 8 pins is used. The
R4, and on releasing these two switches, capacitor C3 starts discharging through resistor R4. Capacitor C3 and resistor R4 are so selected that it takes about five seconds to fully discharge C3. Depressing switches S1 and S8 in unison, within five seconds of releasing the switches SA and SC, pulls pin 2 to ground and IC 555 is triggered. The capacitor C1 starts charging through resistor R1. As a result, the output (pin 3) goes high for five seconds (i.e. the charging time T of the capacitor C1 to the threshold voltage, which is calculated by the relation T=1.1 R1 x C1 seconds). Within these five seconds, switches SA and SC are to be pressed momentarily once again, followed by the depression of last code-switch pair S3-S4.
keyed in sequence as per the code by two switches to open the lock. A low-cost digital code lock circuit is presented in this article. Here the keying-in code is rather unique. Six switches are to be pressed to open the lock, but only two switches at a time. Thus a total of three sets of switches have to be pressed in a particular sequence. (Of these three sets, one set is repeated.) The salient features of this circuit are: 1. Use of 16 switches, which suggests that there is a microprocessor inside. 2. Elimination of power amplifier transistor to energise the relay. 3. Low cost and small PCB size.
IC is inexpensive and easily available. Its pin 2 is the triggering input pin which, when held below 1/3 of the supply voltage, drives the output to high state. The threshold pin 6, when held higher than 2/3 of the supply voltage, drives the output to low state. By applying a low-going pulse to the reset pin 4, the output at pin 3 can be brought to the quiescent low level. Thus the reset pin 4 should be held high for normal operation of the IC. Three sets of switches SA-SC, S1S8 and S3-S4 are pressed, in that order, to open the lock. On pressing the switches SA and SC simultaneously, capacitor C3 charges through the potential divider comprising resistors R3 and
These switches connect the relay to output pin 3 and the relay is energised. The contacts of the relay close and the solenoid pulls in the latch (forming part of a lock) and the lock opens. The remaining switches are connected between reset pin 4 and ground. If any one of these switches is pressed, the IC is reset and the output goes to its quiescent low state. Possibilities of pressing these reset switches are more when a code breaker tries to open the lock. LED D5 indicates the presence of power supply while resistor R5 is a current limiting resistor. The given circuit can be recoded easily by rearranging connections to the switches as desired by the user.
A. JEYABAL
RUP
ANJA
ELECTRONICS FOR YOU n JULY '99
NA
CIRCUIT
IDEAS
Electronic Jam RAJESH K.P.
T
his jam circuit can be used in quiz contests wherein any participant who presses his button (switch) before the other contestants, gets the first chance to answer a question. The circuit given here permits up to eight contestants with each one allotted a distinct number (1 to 8). The display will show the number of the contestant pressing his button before the others. Simultaneously, a buzzer will also sound. Both, the display as well as the buzzer have to be reset manually using a common reset switch. Initially, when reset switch S9 is momentarily pressed and released, all outputs of 74LS373 (IC1) transparent latch go ‘high’ since all the input data lines are returned to Vcc via resistors R1
N ILLO . DH A.P.S
through R8. All eight outputs of IC1 are connected to inputs of priority encoder 74LS147 (IC2) as well as 8-input NAND gate 74LS30 (IC3). The output of IC3 thus becomes logic 0 which, after inversion by NAND gate N2, is applied to latch-enable pin 11 of IC1. With all input pins of IC2 being logic 1, its BCD output is 0000, which is applied to 7segment decoder/driver 74LS47 (IC6) after inversion by hex inverter gates inside 74LS04 (IC5). Thus, on reset the display shows 0. When any one of the push-to-on switches—S1 through S8—is pressed, the corresponding output line of IC1 is latched at logic 0 level and the display indicates the number associated with the specific switch. At the same time,
ELECTRONICS FOR YOU n JUNE '99
output pin 8 of IC3 becomes high, which causes outputs of both gates N1 and N2 to go to logic 0 state. Logic 0 output of gate N2 inhibits IC1, and thus pressing of any other switch S1 through S8 has no effect. Thus, the contestant who presses his switch first, jams the display to show only his number. In the unlikely event of simultaneous pressing (within few nano-seconds difference) of more than one switch, the higher priority number (switch no.) will be displayed. Simultaneously, the logic 0 output of gate N1 drives the buzzer via pnp transistor BC158 (T1). The buzzer as well the display can be reset (to show 0) by momentary pressing of reset switch S9 so that next round may start. Lab Note: The original circuit sent by the author has been modified as it did not jam the display, and a higher number switch (higher priority), even when pressed later, was able to change the displayed number.
circuit
ideas
Long-Range IR Transmitter
EFY Lab
M
ost of the IR remotes work reliably within a range of 5 metres. The circuit complexity increases if you design the IR transmitter for reliable operation over a longer range, say, 10 metres. To double the range from 5 metres to 10 metres, you need to increase the transmitted power
IR laser pointer as the IR signal source. The laser pointer is readily available in the market. However, with a very narrow beam from the laser pointer, you have to take extra care, lest a small jerk to the gadget may change the beam orientation and cause loss of contact. Here is a simple circuit that will give you a pretty long range. It uses three infrared transmitting LEDs (IR1
Fig. 1: Circuit of the long-range ir transmitter
Fig. 2: Pin configurations of bc547/557 and bS170
w w w. e f y m ag . co m
four times. If you wish to realise a highly directional IR beam (very narrow beam), you can suitably use an
through IR3) in series to increase the radiated power. Further, to increase the directivity and so also the power density, you may assemble the IR LEDs inside the reflector of a torch. For increasing the circuit efficiency, a MOSFET (BS170) has been used, which acts as a switch and thus re-
edi
s.c. dwiv
duces the power loss that would result if a transistor were used. To avoid any dip during its ‘on’/‘off’ operations, a 100μF reservoir capacitor C2 is used across the battery supply. Its advantage will be more obvious when the IR transmitter is powered by ordinary batteries. Capacitor C2 supplies extra charge during ‘switching on’ operations. As the MOSFET exhibits large capacitance across gate-source terminals, a special drive arrangement has been made using npn-pnp Darlington pair of BC547 and BC557 (as emitter followers), to avoid distortion of the gate drive input. Data (CMOS-compatible) to be transmitted is used for modulating the 38 kHz frequency generated by CD4047 (IC1). However, in the circuit shown here, tactile switch S1 has been used for modulating and transmitting the IR signal. Assemble the circuit on a general-purpose PCB. Use switch S2 for power ‘on’/‘off’ control. Commercially available IR receiver modules (e.g., TSOP1738) could be used for efficient reception of the transmitted IR signals.
e l e c t ro n i c s f o r yo u • S e p t e m b e r 2 0 0 8 • 1 0 9
Long-range FM Transmitter
S
everal circuits for constructing FM transmitters have been published in EFY. The power output of most of these circuits were very low because no power amplifier stages were incorporated. The transmitter circuit described here has an extra RF power amplifier stage, after the oscillator stage, to raise the power output to 200-250 milliwatts. With a good matching 50-ohm ground—plane antenna or multi-element Yagi antenna, this transmitter can provide reasonably good signal strength up to a distance of about 2 kilometres. The circuit built around transistor T1 (BF494) is a basic low-power variable-frequency VHF oscillator. A varicap diode circuit is included to change the frequency of the transmitter and to provide frequency modulation by audio signals. The output of the oscillator is about 50 milliwatts. Transistor T2 (2N3866) forms a VHF-class A power amplifier. It boosts the oscillator signals’ power four to five times. Thus, 200-250 milliwatts of power is generated at the collector of transistor T2. For better results, assemble the circuit on a good-quality glass epoxy board and house the transmitter inside an aluminium case. Shield the oscillator stage using
an aluminium sheet. Coil winding details are given below: L1 – 4 turns of 20 SWG wire close wound over 8mm diameter plastic former. L2 – 2 turns of 24 SWG wire near top end of L1. (Note: No core (i.e. air core) is used for the above coils) L3 – 7 turns of 24 SWG wire close wound with 3mm diameter air core. L4 – 7 turns of 24 SWG wire-wound on a ferrite bead (as choke) Potentiometer VR1 is used to set the centre frequency whereas potentiometer
VR2 is used for power control. For humfree operation, operate the transmitter on a 12V rechargeable battery pack of 10 x 1.2-volt Ni-Cd cells. Transistor T2 must be mounted on a heat sink. Do not switch on the transmitter without a matching antenna. Adjust both trimmers (VC1 and VC2) for maximum transmission power. Adjust potentiometer VR1 to set the centre frequency near 100 MHz. This transmitter should only be used for educational purposes. Regular transmission using such a transmitter without a licence is illegal in India.
ELECTRONICS PROJECTS Vol. 20
LOW-COST PCO BILLING METER
T
he circuit presented here can be used in PCOs for displaying the actual bill. The comparative disadvantages of the presented circuit are as follows: 1. The calculator used along with this circuit is required to be switched ‘on’ manually before making a call. 2. Certain manual entries have to be made in the calculator; for example, for a pulse rate of Rs 1.26, number 1.26 is
well as upon the impedance of telephone instrument). Handset is normally lifted either for dialing or in response to a ring. In the circuit shown in Fig. 1, when the handset is off-hook, the optocoupler MCT2E (IC1) conducts and forward biases transistor T1, which, in turn, forward biases transistor T2 and energises relay RL1. In energised condition of relay, the upper set of relay contacts connects the positive supply rail
optocoupler IC3. The output of this optocoupler is used to bridge the ‘=’ button on a calculator (such as Taksun make), which has the effect of pressing the ‘=’ button of the calculator. Considering that pulse rate for a specific town/time/day happens to be Rs 1.26 per pulse, then before maturity of the call one enters 1.26 followed by pressing of ‘+’ key twice. Now, if a total of ten billing pulses have been received
to be entered after switching ‘on’ the calculator followed by pressing of ‘+’ button twice. However, possibility exists for automating these two functions by using additional circuitry. In telephony, on-hook condition is represented by existance of 48V to 52V across the line. Similarly, the off-hook condition is represented by the line voltage dropping to a level of 8V to 10V (depending upon the length of the local lead line (local loop) from telephone exchange to the subscriber’s premises as
to PLL (phaselocked loop) IC2 (LM567) pin 4, while the lower set of relay contacts couples the positive telephone lead to input pin 3 of LM567 via capacitor C1 and resistor R3. The negative telephone lead is permanently capacitively coupled to ground via capacitor C2. As soon as call matures, 16kHz tone pulses would be pumped into the telephone line by the telephone exchange at suitable intervals. This interval depends on the pulse rate of the place called and also the time of the day and whether it’s a working-day or holiday. On receipt of 16kHz pulse, output pin 8 of IC LM567 (which is tuned for centre frequency of 16 kHz) goes ‘low’ for the duration of the pulse. The output of IC2 is coupled via transistor T3 to
from exchange for the duration of the call, then on completion of the call, the calculator display would show 12.60. The telephone operator has to bill the customer Rs 14.60 (Rs 12.60 towards call charges plus Rs 2.00 towards service charges). For tuning of the PLL circuit around IC2, lift the handset and inject 16kHz tone across the line input points. Tune IC2 to centre frequency of 16 kHz with the help of preset VR1. Proper tuning of the PLL will cause LED (D6) to glow even with a very lowamplitude 16kHz tone. EFY Lab note. Arrangement used for simulating a 16kHz pulsed tone is shown in Fig. 2. Push-to-on switch is used for generation of fixed-duration pulse for modulating and switching on a 16kHz oscillator. For more details regarding pulse rates, pulse codes, etc, readers are advised to go through the tariff rates and pulse code information given in the beginning pages of telephone directories, such as MTNL, Delhi directory, Vol. I.
188
ELECTRONICS PROJECTS Vol. 21
Low-cost Transistorised Intercom
S
everal intercom circuits have appeared in EFY using integrated circuits. The circuit described here uses three easily available transistors only. Even a beginner can easily assemble it on a piece of veroboard. The circuit comprises a 3-stage resistor-capacitor coupled amplifier. When ring button S2 is pressed, the amplifier circuit formed around transistors T1 and T2 gets converted into an asymmetrical astable multivib-rator generating ring signals. These ring signals are amplified by transistor T3 to drive the speaker of earpiece. Current consumption of this intercom is 10 to 15 mA only. Thus a 9-volt PP3
battery would have a long life, when used in this circuit.
For making a two-way intercom, two identical units, as shown in figure, are
required to be used. Output of one amplifier unit goes to speaker of the other unit, and vice versa. For single-battery operation, join corresponding supply and ground terminals of both the units together. The complete circuit, along with microphone and earpiece etc, can be housed inside the plastic body of a toy cellphone, which is easily available in the market. Suggested cellphone cabinet, with the position of switches, speakers and mike etc is shown.
ELECTRONICS PROJECTS Vol. 20
Low Current, High Voltage Power Supply
A
high voltage power supply is a very useful source which can be effectively used in many applications like biasing of gas-discharge tubes and radiation detectors etc. Such a power supply could also be used for protection of property by charging of fences. Here the current requirement is of the order of a few microamps. In such an application, high voltage would essentially exist between a ‘live’ wire and ground. When this ‘live’ wire is touched, the discharge occurs via body resistance and it gives a non-lethal but deterrent shock to an intruder. The circuit is built around a transistorised blocking oscillator. An important element in this circuit is the transformer. It can be fabricated using easily available ferrite core. Two ‘E’ sections of the core are joined face-to-face after the enamelled copper wire wound on former is placed in it. The details of the transformer windings are given in the Table. In this configuration, the primary winding and the feedback winding are arranged such that a sustained oscillations are ensured once the supply is switched on. The waveform’s duty cycle is asymmetrical, but it is not very important in this application. Please note that if the oscillations do not occur at the ‘switch-on’ time, the transformer winding terminals of the feedback or the primary winding (but not both) should be reversed. The primary oscillations amplitude is
ELECTRONICS PROJECTS Vol. 20
Table Details of the Transformer Windings Windings No. of Standard wire turns gauge (SWG) Primary 50 31 Feedback 12 31 Secondary 1650 41
about 24V(p-p). This gets further amplified due to the large step-up ratio of the transformer and we get about 800V(p-p) across the secondary. A simple series voltage multiplier (known as Cockroft-Walton circuit) is used to boost up this voltage in steps to give a final DC voltage of about 2 kV. The output voltage, however, is not very well regulated. But if there is a constant load, the final voltage can be
adjusted by varying the supply voltage. The present configuration gives 2 kV for an input DC voltage of 15 V. Though higher voltages could be achieved by increasing input supply, one word of caution is necessary: that the component ratings have to be kept in mind. If the ratings are exceeded then there will be electrical discharges and breakdowns, which will damage the device.
CIRCUIT
IDEAS
LUGGAGE SECURITY SYSTEM EDI DWIV S.C.
DHURJATI SINHA
W
hile travelling by a train or bus, we generally lock our luggage using a chain-and-lock arrangement. But, still we are under tension, apprehending that somebody may cut the chain and steal our luggage. Here is a simple circuit to alarm you when somebody tries to cut the chain. Transistor T1 enables supply to the sound generator chip when the base current starts flowing through it. When the wire (thin enameled copper wire of 30 to 40 SWG, used for winding transformers) loop around the chain is broken by somebody, the base of transistor T1, which was earlier tied to positive rail, gets opened. As a result, tran-
sistor T1 gets forward biased to extend the positive supply to the alarm circuit. In idle mode, the power consumption of the circuit is minimum and thus it can be used for hundreds of travel hours. To enable generation of different
86
alarm sounds, connections to pin 1 and 6 may be made as per the table. Select 1 Select 2 Sound effect (Pin6) (Pin1) X X Police siren VDD X Fire-engine siren VSS X Ambulance siren “-” VDD Machine-gun sound Note: X = no connection; “-” = do not care
MAGIC LIGHTS
T
he circuit as shown in the figure employs 14 bi-colour (red and green) LEDs having three termi-
emits green light. And when positive voltage is simultaneously applied to its pins 1 and 3, it emits amber light. The circuit can
BCD to 7-segment latch/decodor/driver ICs. Thus we obtain a total of 14 segment outputs from each of the IC pairs
nals each. Different dancing colour patterns are produced using this circuit since each LED can produce three different colours. The middle terminal (pin 2) of the LEDs is the common cathode pin which is grounded. When a positive voltage is applied to pin 1, it emits red light. Similarly, when positive voltage is applied to pin 3. it
be used for decorative lights. IC1 (555) is used in astable mode to generate clock signal for IC2 and IC3 (CD4518) which are dual BCD counters. Both counters of each of these ICs have been cascaded to obtain 8 outputs from each. The outputs from IC2 and IC3 are connected to IC4 through IC7 which are
consisting of IC4 plus IC5 and IC6 plus IC7. While outputs from former pair are connected to pin No. 1 of all the 14 bi-colour LEDs via current limiting resistors, the ouputs of the latter pair are similarly connected to pin No.3 of all the bi-colour LEDs to get a magical dancing lights effect.
ELECTRONICS PROJECTS Vol. 19
187
Magnetic Proximity Switch
H
ere is an interesting circuit for a magnetic proximity switch which can be used in various applica-
tions. The circuit, consists of a reed switch at its heart. When a magnet is brought in the vicinity of the sensor (reed switch), its contacts close to control the rest of the switching circuit. In place of the reed switch, one may, as well, use a general-purpose electromagnetic reed relay (by making use of the reed switch contacts) as the sensor, if required. These tiny reed relays are easily available as they are widely used in telecom products. The reed switch or relay to be used with this circuit should be the ‘normally open’ type. When a magnet is brought/placed in the vicinity of the sensor element for a moment, the contacts of the reed switch close to trigger timer IC1 wired in monostable mode. As
a consequence its output at pin 3 goes high for a short duration and supplies clock to the clock input (pin 3) of IC2 (CD4013—dual D-type flip-flop). LED D2 is used as a response indicator. This CMOS IC2 consists of two independent flip-flops though here only one
is used. Note that the flip-flop is wired in toggle mode with data input (pin 5) connected to the Q (pin 2) output. On receipt of clock pulse, the Q output at pin 1 changes from low to high state and due to this the relay driver transistor T1 gets forward-biased. As a result the relay RL1 is energised.
ELECTRONICS PROJECTS Vol. 20
CIRCUIT IDEAS
DING-DONG BELL PRAVEEN SHANKER
T
his simple and cost-effective door bell circuit is based on IC 8021-2 from Formox Semiconductors
ELECTRONICS FOR YOU
MARCH 2002
EO I TH SAN
(Website address: fortech@mantramail. com). It is an 8-pin DIP IC whose only four pins, as shown in the circuit, have been used. The IC has an in-built circuitry to produce dingdong sound each time its pin 3 is pulled low. The sound is stored in the IC as bits, as in a ROM. The sound output from the IC can’t however drive a speaker directly, as this puts strain on the device. Therefore a complemen-
tary-pair, two-transistor amplifier is used to amplify the sound to a fair level of audiblity. You may either use a piezo tweeter or an 8-ohm, 500mW speaker at the output. During the standby period, the IC consumes nominal current of a few microamperes only. Therefore switch S1 may be kept closed. Each time switch S2 is pressed, ding dong sound is produced twice. If you try to press switch S2 a second time when the first ding dong sound is still being produed, it has no effect whatever and the two ding-dong bell sounds will be invariably produced. The circuit costs no more than Rs 35 and the IC 8021-2 used in the circuit is readily available for around Rs 15 in the market.
C I R CC UI RICTU I IT DI ED EAASS
IR REMOTE SWITCH K.S. SANKAR
I
magine the convenience of selecting TV channels using your remote and then pointing the same remote to your switchboard to switch on/off the fan or the tubelight. Here is a simple circuit to remotely switch on/off any electrical device through a relay using the normal TV/ VCR/VCP/VCD remote control unit. It
EDI DWIV S.C.
works up to a distance of about 10 metres. The circuit is built around a 3-pin IR IC receiver (Siemens SFH-506-38 or equivalent) that can detect 38kHz burst frequency generated by a TV remote. (This IR receiver module has been covered earlier in many projects published in EFY.) The output pin of IR sensor goes low
when it detects IR light, triggering the monostable (1-second) built around timer NE555. The output of the mono toggles the J-K flip flop, whose Q output drives the relay through SL100 npn transistor (T1). LED2, LED3, and LED4 are used to display the status of each output stage during circuit operation. Back-EMF diode D5 is used for protection. Transistor T1 is configured as an open-collector output device to drive the relay rated at 12V DC. The circuit draws the power from voltage regulator 7805. Capacitor C5 is soldered close to the IR sensor’s pins to avoid noise and false triggering. Capacitor C3 and resistor R3 also avoid false triggering of monostable NE555. The monostable acts as a 1-second hysterisis unit to restrict the flipflop from getting retriggered within one second. To activate any other 12V logic device, use the output across the relay coil terminals.
MARCH 2002
ELECTRONICS FOR YOU
CIRCUIT
IDEAS
Wiper Speed Controller A
AT Y. K
ARIA
PRADEEP G.
continuously working wiper in a car may prove to be a nuisance, especially when it is not raining heavily. By using the circuit described here one can vary sweeping rate of the wiper from once a second to once in ten seconds. The circuit comprises two timer NE555 ICs, one CD4017 decade counter, one TIP32 driver transistor, a 2N3055 power transistor (or TIP3055) and a few other discrete components. Timer IC1 is configured as a mono- stable multivibrator which produces a pulse when one presses switch S1 momentarily. This pulse acts as a clock pulse
for the decade counter (IC2) which advances by one count on each successive clock pulse or the push of switch S1. Ten presets (VR1 through VR10), set for different values by trial and error, are used at the ten outputs of IC2. But since only one output of IC2 is high at a time, only one preset (at selected output) effectively comes in series with
ELECTRONICS FOR YOU n MARCH '99
timing resistors R4 and R5 connected in the circuit of timer IC3 which functions in astable mode. As presets VR1 through VR10 are set for different values, different time periods (or frequencies) for astable multivibrator IC3 can be selected. The output of IC3 is applied to pnp driver transistor T1 (TIP32) for driving the final power transistor T2 (2N3055) which in turn drives the wiper motor at the selected sweep speed. The power supply for the wiper motor as well as the circuit is tapped from the vehicle’s battery itself. The duration of monostable multivibrator IC1 is set for a nearly one second period.
Miniature Strobe Light
S
trobe lights are widely used by disco lovers to create wonderful visual effects in disco halls and auditoria. The circuit of a battery operated portable miniature strobe light, which can be constructed using readily available inexpensive components, is described here. For convenience and simplicity, an ordinary neon lamp is used here in place of the conventional Xenon tube. The whole gadget can thus be easily accommodated in a small cabinet, such as a mains adaptor cover, with a suitable reflector for neon lamp to give a proper look. Since current requirement of this circuit is very small, it may be powered by two medium-size dry cells (3V) or Ni-Cd cells (2.4V). Transistors T1 and T2 in the circuit form a complimentary-pair amplifier. When switch S1 is momentarily depressed, the circuit oscillates because of the positive feedback provided via resistor R2 and capacitor C1 to the base of transistor T1. The sharp pulses in the secondary windings induce a high voltage in primary windings of transformer X1, which in fact is a line driver transformer (used in reverse) generally used in 36cm TV sets.
High voltage pulses induced in primary side are rectified by diode D1 and rapidly charge reservoir capacitor C2 to nearly 300V DC. When switch S1 is released, capacitor C2 holds the voltage level for a finite period while capacitor C3 charges slowly through resistor R3. When voltage across capacitor C3 becomes high enough, neon strikes and the capacitor rapidly discharges through the lamp. When voltage across capacitor C3 falls below the extinguishing potential of
neon lamp, it goes off and capacitor C3 starts charging again. This cycle keeps on repeating for a short time, based on the reservoir capacitor C2’s value. Precautions. The neon lamp flasher section of this circuit carries dangerously high voltages. All precautions should therefore be taken for protection. Before any repair work, discharge capacitor C2 using a short length of wire with a 100k resistor connected in series.
ELECTRONICS PROJECTS Vol. 20
Musical ‘Touch’ Bell Sukant Kumar Behara
H
ere is a musical call bell that can be operated by just bridging the gap between the touchplates with one’s fingertips. Thus there is no need for a mechanical ‘on’/‘off’ switch because the touch-plates act as a switch. Other features include low cost and low power consumption. The bell can work on 1.5V or 3V, using one or two pencil cells, and can be used in homes and offices. Two transistors are used for sensing the finger touch and switching on a melody IC. Transistor BC548 is npn type while
transistor BC558 is pnp type. The emitter of transistor BC548 is shorted to the ground, while that of transistor BC558 is connected to the positive terminal. The collector of transistor BC548 is connected to the base of BC558. The base of BC548 is connected to the washer (as shown in the figure). The collector of BC558 is connected to pin 2 of musical IC UM66, and pin 3 of IC UM66 is shorted to the ground. The output from pin 1 is connected to a transistor amplifier comprising BEL187 transistor for feeding the
loudspeaker. One end of 2.2-ohm resistor R1 is connected to the positive rail and the other to a screw (as shown in the figure). The complete circuit is connected to a single pencil cell of 1.5V. When the touch-plate gap is bridged with a finger, the emitter-collector junction of transistor BC548 starts conducting. Simultaneously, the emitter-baser junction of transistor BC558 also starts conducting. As a result, the collector of transistor BC558 is pulled towards the positive rail, which thus activates melody generator IC1 (UM66). The output of IC1 is amplified by transistor BEL187 and fed to the speaker. So we hear a musical note just by touching the touch points. The washer’s inner diameter should be 1 to 2 mm greater than that of the screwhead. The washer could be fixed in the position by using an adhesive, while the screw can be easily driven in a wooden piece used for mounting the touch-plate. The use of brass washer and screw is recommended for easy solder-ability.
Readers’ comments: The circuit starts ringing (without touching the screw) when connected to 3V. On disconnecting points 1 and 2 (kept open), I still received the ring. Why so? A. Vaidhyanathan
Pollachi The author, Sukant Kumar Behara, replies: You can rectify this snag by changing transistor T1 (BC148) with a new one. On touching the base of transistor T1, its
emitter-collector junction starts conducting. But you’ve mentioned that even without touching the base of transistor T1, the bell starts ringing, which means that the emitter-collector junction of the transistor has got shorted internally.
ELECTRONICS PROJECTS Vol. 22
PARALLEL TELEPHONE WITH SECRECY
O
ften a need arises for connection of two telephone instruments in parallel to one line. But it creates quite a few problems in their proper performance, such as over loading and overhearing of the conversation by an undesired person. In order to eliminate all such problems and get a clear reception, a simple scheme is presented here (Fig. 1). This system will enable the incoming ring to be heard at both the telephones. The DPDT switch, installed with each of the parallel telephones, connects you to the line in one position of the switch and
disconnects you in the other position of the switch. At any one time, only one telephone is connected to the line. To receive a call at the instrument, which is not connected to the line, you just have to flip the
toggle switch at your telephone end to receive the call and have a conversation. As soon as the position of the toggle switch is changed, the line gets transferred to the other telephone instrument. Mount one DPDT toggle switch, one telephone ringer, and one telephone terminal box on two wooden electrical switchboards, as shown in Fig. 3. Interconnect the boards using a 4-pair telephone cable as per Fig. 1. The system is ready to use. Ensure that the two lower leads of switch S2 are connected to switch S1 after reversal, as shown in the figure. Lab. Note: The external ringer for the project as shown in Fig. 2, was designed/fabricated at EFY Lab.
ELECTRONICS PROJECTS Vol. 21
181
circuit
ideas
PC Multimedia Speakers
T.K. Hareendran
T
his circuit of multimedia speakers for PCs has single-chipbased design, low-voltage power supply, compatibility with USB power, easy heat-sinking, low cost, high flexibility and wide temperature tolerance. At the heart of the circuit is IC TDA2822M. This IC is, in fact, mono-
age down to 1.8 volts and minimum output power of around 450 mW/ channel with 4-ohm loudspeaker at 5V DC supply input. An ideal power amplifier can be simply defined as a circuit that can deliver audio power into external loads without generating significant signal distortion and without consuming excessive quiescent current. This circuit is powered by 5V DC
Fig. 1: Circuit for PC multimedia speaker
lithic type in 8-lead mini DIP package. It is intended for use as a dual audio power amplifier in battery-powered sound players. Specifications of TDA2822M are low quiescent current, low crossover distortion, supply volt-
supply available from the USB port of the PC. When power switch S1 is flipped to ‘on’ position, 5V power supply is extended to the circuit and power-indicator red LED1 lights up instantly. Resistor R1 is a current surge
9 0 • au g u s t 2 0 0 8 • e l e c t ro n i c s f o r yo u
edi
s.c. dwiv
limiter and capacitors C1 and C4 act as buffers. Working of the circuit is simple. Audio signals from the PC auFig. 2: Pin configuration dio socket/headphone of TDA2822M socket are fed to the amplifier circuit through components R2 and C2 (left channel), and R3 and C3 (right channel). Potmeter VR1 works as the volume controller for left (L) channel and potmeter VR2 works for right (R) channel. Pin 7 of TDA2822M receives the left-channel sound signals and pin 6 receives the right-channel signals through VR1 and RIGHT VR2, respectively. Amplified signals for driving the left and right loudspeakers are available at pins 1 and 3 of IC1, respectively. Components R5 and C8, and R6 and C10 form the traditional zobel network. Assemble the circuit on a medium-size, general-purpose PCB and enclose in a suitable cabinet. It is advisable to use a socket for IC TDA2822M. The external connections should be made using suitably screened wires for better result.
w w w. e f y m ag . co m
CIRCUIT
IDEAS
PC TEMPERATURE ALARM
RAJ K. GORKHALI
I
f your PC overheats, it could damage its expensive components. Here’s a circuit that warns you of your PC getting heated. Today’s computers contain most of the circuitry on just a few chips and reduced power consumption is a byproduct of this LSI and VSLI approach. Some PCs still have power supplies that are capable of supplying around 200W, but few PCs actually consume power to this extent. On the other hand, apart from some portable and small desktop computers that use the latest micro-power components, most PCs still consume significant amount of power and generate certain amount of heat. The temperature inside the average PC starts to rise well above the ambient temperature soon after it is switched on. Some of the larger integrated circuits become quite hot and if the temperature inside the PC rises
too high, these devices may not be able to dissipate heat fast enough. This, in turn, could lead to failure of devices and eventually of the PC. Various means to combat overheating are available, ranging from simple temperature alarms to devices like temperature-activated fans to keep the microprocessor cool. Here is a temperature alarm that activates an audio ‘beeper’ if the temperature inside the PC exceeds a preset threshold. This temperature is useradjustable and can be anywhere between 0°C and 100°C. The unit is in the form of a small PC expansion card, which you simply need to plug into any available slot of the host PC. It is powered from the PC and consumes only about 12 mA. The sensor (LM35) used here provides a substantial amount of on-chip signal conditioning, including amplification, level shifting and phase inversion. As a result, it provides an output of 10 mV per degree centigrade
IVEDI S.C. DW
rise in temperature. It caters to a temperature measurement range of 0°C to 100°C, which corresponds to 0V to 1V of voltage. The voltage-detector stage compares the output voltage of the temperature sensor with the preset reference voltage. The output of the comparator goes high if the output potential from the sensor exceeds the reference voltage. When this happens, the voltage comparator enables a lowfrequency oscillator, which, in turn, activates the audio oscillator. The output of the audio oscillator is connected to a loudspeaker (LS1), which sounds a simple ‘beep-beep’ alarm. The reference voltage determines the temperature at which the alarm is activated. Fig. 1 shows the circuit of the PC temperature alarm and Fig. 2 shows the pin configuration of sensor LM35. IC LM35 (IC1) is an easy-to-use temperature sensor. It is basically a three-terminal device (two supply leads plus the output) that operates
Fig. 1: Circuit for PC temperature alarm
92 • SEPTEMBER 2007 • ELECTRONICS FOR YOU
WWW.EFYMAG.COM
CIRCUIT
IDEAS
over a wide supply range of 4 to 20V. It consumes only 56 µA at 5V and generates insignificant heat. IC2 is an operational amplifier used here as a voltage comparator. VR1 provides a reference voltage that can be set anywhere from 0V to approximately 1V, which matches the output voltage range of IC1. This reference voltage is applied to the inverting input of IC2 and the output of IC1 is coupled to the non-inverting input. Consequently, the output of IC2 is low if the output of IC1 is below the reference voltage, or high if the output of IC1 exceeds the reference voltage. The low-frequency oscillator IC3 is a standard 555 astable multivibrator circuit. It is gated via the reset input at pin 4, which holds output pin 3 low when IC3 is gated ‘off’ (when the output of IC2 is low). This prevents IC4 from oscillating. IC4 is another 555
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astable multivibrator circuit, gated via its reset input. It has an operating frequency of approximately 2.5 kHz. When IC3 is activated, its output proFig. 2: Pin details of LM35 vides a square wave of 1 Hz. This is used to trigger IC4, which gives an audio output of 2.5 kHz in bursts. It is connected to loudspeaker LS1 to generate alarm. The alarm circuit can be fitted into any spare expansion slot of the PC, but be careful to fit it the right way round. Before setting VR1 to a suitable threshold temperature, decide what that temperature should be. The technical specification in your computer’s manual might be of help here. If we assume that the room tem-
perature will not normally exceed 25oC, the temperature of the interior of the computer would be up to 35oC. Unless you have good reason to use a different threshold temperature, VR1 should be set for a wiper potential of 350 mV. Trial-and-error method can be used in the absence of test equipment to enable VR1, but it would be a bit time-consuming. There is a slight complication in that the computer’s outer casing must be at least partially removed to provide access to VR1. Once VR1 has been adjusted, the outer casing must be put back into place so that the interior of the computer can warm up in the normal way. You must therefore allow time for the temperature inside the computer to rise back to its normal operating level each time VR1 is readjusted.
ELECTRONICS FOR YOU • SEPTEMBER 2007 • 93
PC-Based 7-Segment Rolling Display
It
is very interesting and con venient to be able to control everything while sitting at your PC terminal. Here, a simple hardware circuit and software is used to interface a 7-segment based rolling display. The printer port of a PC provides a set of points with some acting as input lines and some others as output lines. Some lines are open collector type which can be used as input lines. The circuit given here can be used for interfacing with any type of PC’s printer port. The 25-pin parallel port connector at the back of a PC is a combination of three ports. The address varies from 378H-37AH. The 7 lines of port 378H (pins 2 through 8) are used in this circuit to output the code for segment display through IC1. The remaining one line of port 378H (pin 9) and four lines of port 37AH (pins 1, 14, 16, 17) are used to enable the display digits (one a time) through IC2. The bits D0, D1 and D3 of port 37AH connected to pins 1, 14 and 17 of ‘D’ connector are inverted by the computer before application to the pins while data bit D2 is not inverted. Therefore to get a logic high at any of former three pins, we must send logic 0 output to the corresponding pin of port 37AH. Another important concept illustrated by the project is the time division multiplexing. Note that all the five 7-segment displays share a common data bus. The
PC places the 7-segment code for the first digit/character on the data bus and enables only the first 7-segment display. After delay of a few milliseconds, the 7-segment code for the digit/character is replaced by that of the next charter/digit, but this time only second display digit is enabled. After the display of all characters/ digits in this way, the cycle repeats itself
over and over again. Because of this repetition at a fairly high rate, there is an illusion that all the digits/characters are continuously being displayed. DISP1 is to be physically placed as the least significant digit. IC1 (74LS244) is an octal buffer which is primarily used to increase the driving capability. It has two groups of four buff-
P r o g r a m /*DISP.C*** PC BASED ROLLING DISPLAY */ /* P.R.DESHMUKH*/ #include #include #include #define PORTA 0x378 #define PORTB 0x37a void main() { int dno[6]={0x0a,0x09,0x0f,0x03,0x80}; /* code for “hallo”*/ int m[5]={0x76,0x77,0x38,0x38,0x3f }; /*code for the selection of display*/ int f,j;
clrscr(); for(f=200;f<=500;f+=100) { sound(f ); delay(100); } nosound(); while (!kbhit()) { for (j=0;j<=4;j++) { outportb(PORTA,m[j]); if(j<=3) { outportb(PORTB,dno[j]);
delay(300); } else { outportb(PORTB,0x0b); outportb(PORTA,m[j]); outportb(PORTA ,(m[j] || ( 0x80))); delay(300); } } } }
ELECTRONICS PROJECTS Vol. 20
ers with non-inverted tri-state outputs. The buffer is controlled by two active low enable lines. IC2 (75492) can drive a maximum of six 7-segment displays. (For driving up to seven common-cathode displays one may use ULN2003 described in the previous circuit idea.) The program for rolling display is given in the listing DISP.C above.
ELECTRONICS PROJECTS Vol. 20
Whatever the message/characters to be displayed (here five characters have been displayed), these are separated and stored in an array. Then these are decoded. Decoding software is very simple. Just replace the desired character with the binary equivalent of the display code. The display code is a byte that
has the appropriate bits turned on. For example, to display character ‘L’, the segments to be turned on are f, e and d. This is equivalent to 111000 binary or 38 hex. Please note that only limited characters can be formed using 7-segment display. Characters such as M, N and K cannot be formed properly.
PHONE BROADCASTER
H
ere is a simple yet very useful circuit which can be used to eavesdrop on a telephone conversation. The circuit can also be used as a wireless telephone amplifier. One important feature of this circuit is that the circuit derives its power directly from the active telephone lines, and thus avoids use of any external battery or other power supplies. This not only saves a lot of space but also money. It consumes very low current from telephone lines without disturbing its performance. The circuit is very tiny and can be built using a single-IC type veroboard that can be easily fitted inside a telephone connection box of 3.75 cm x 5 cm. The circuit consists of two sections, namely, automatic switching section and FM transmitter section. Automatic switching section comprises resistors R1 to R3, preset VR1, transistors T1 and T2, zener D2, and diode D1. Resistor R1, along with preset VR1, works as a voltage divider. When voltage across the telephone lines is 48V DC, the voltage available at wiper of preset VR1 ranges from 0 to 32V (adjustable). The switching voltage of the circuit depends on zener
breakdown voltage (here 24V) and switching voltage of the transistor T1 (0.7V). Thus, if we adjust preset VR1 to get over 24.7 volts, it will cause the zener to breakdown and transistor T1 to conduct. As a result collector of transistor T1 will get pulled towards negative supply, to cut off transistor T2. At this stage, if you lift the handset of the telephone, the line voltage drops to about 11V and transistor T1 is cut off. As a result, transistor T2 gets forward biased through resistor R2, to provide a DC path for transistor T3 used in the following FM transmitter section. The low-power FM transmitter section comprises oscillator transistor T3, coil L1, and a few other components. Transis-
tor T3 works as a common-emitter RF oscillator, with transistor T2 serving as an electronic ‘on’/‘off’ switch. The audio signal available across the telephone lines automatically modulates oscillator frequency via transistor T2 along with its series biasing resistor R3. The modulated RF signal is fed to the antenna. The telephone conversation can be heard on an FM receiver remotely when it is tuned to FM transmitter frequency. Lab Note: During testing of the circuit it was observed that the telephone used was giving an engaged tone when dialed by any subscriber. Addition of resistor R5 and capacitor C6 was found necessary for rectification of the fault.
ELECTRONICS PROJECTS Vol. 21
167
POWER-SUPPLY FAILURE ALARM
M
ost of the power-supply failure indicator circuits need a separate power-supply for themselves. But the alarm circuit presented here needs no additional supply source. It employs an electrolytic capacitor to store adequate charge, to feed power to the alarm circuit which sounds an alarm for a reasonable duration when the mains supply fails. During the presence of mains power supply, the rectified mains voltage is stepped down to a required low level. A zener is used to limit the filtered voltage to 15-volt level. Mains presence is indicated by an LED. The low-level DC is used for charging capacitor C3 and reverse biasing switching transistor T1. Thus, transistor T1 remains cut-off as long as the mains supply is present. As soon as the mains power fails, the charge stored in the capacitor acts as a power-supply source for transistor T1. Since, in the absence of mains supply, the base of transistor is pulled
182
ELECTRONICS PROJECTS Vol. 21
‘low’ via resistor R8, it conducts and sounds the buzzer (alarm) to give a warning of the power-failure. With the value of C3 as shown, a goodquality buzzer would sound for about a minute. By increasing or decreasing the value of capacitor C3, this time can be altered to serve one’s need. Assembly is quite easy. The values of the components are not critical. If the alarm circuit is powered from any external DC power-supply source, the mainssupply section up to points ‘P’ and ‘M’ can
be omitted from the circuit. Following points may be noted: 1. At a higher DC voltage level, transistor T1 (BC558) may pass some collector-to-emitter leakage current, causing a continuous murmuring sound from the buzzer. In that case, replace it with some low-gain transistor. 2. Piezo buzzer must be a continuous tone version, with built-in oscillator. To save space, one may use five smallsized 1000μF capacitors (in parallel) in place of bulky high-value capacitor C3.
PRECISION AMPLIFIER WITH DIGITAL CONTROL
T
his circuit is similar to the preceding circuit of the attenuator. Gain of up to 100 can be achieved
in this configuration, which is useful for signal conditioning of low output of transducers in millivolt range. The gain selection resistors R3 to R6 can be selected by the user and can be anywhere from 1 kilo-ohm to 1 meg-ohm. Trimpots can be used for obtaining any value of gain required by the user. The resistor values shown in the circuit are for decade gains suitable for an autoranging DPM. Resistor R1 and capacitor C1 reduce ripple in the input and also snub transients. Zeners Z1 and Z2 limit the input to ±4.7V, while the input current is limited by resistor R1. Capacitors C2 and C3 are the power supply decoupling capacitors. Op-amp IC1 is used to increase the input impedance so that very low inputs are not loaded on measurement. The user can terminate the inputs with resistance
ELECTRONICS PROJECTS Vol. 22
of his choice (such as 10 meg-ohm or 1 meg-ohm) to avoid floating of the inputs when no measurement is being made.
Truth Table (Control Input vs Gain) X,Y (On-switch (2) (1) Gain Pair) B A (Av.) X0,Y0 0 0 1/10 X1,Y1 0 1 1 X2,Y2 1 0 10 X3,Y3 1 1 100
IC5 is used as an inverting buffer to restore polarity of the input while IC4 is used as buffer at the output of CD4052, because loading it by resistance of value less than 1 meg-ohm will cause an error. An alternative is to make R9=R10=1 megohm and do away with IC4, though this may not be an ideal method. Gains greater than 100 may not be practical because even at gain value of 100 itself, a 100μV offset will work out to be around 10 mV at the output (100μV x 100). This can be trimmed using the offset null option in the OP07, connecting a
trimpot between pins 1 and 8, and connecting wiper to +5V supply rails. For better performance, use ICL7650 (not pin-
compatible) in place of OP07 and use ±7.5V instead of ±5V supply. Eight steps for gain or attenuation can be added by using two CD4051 and pin 6 inhibit on CD4051/52. More steps can be added by cascading many CD4051, or CD4052, or CD4053 ICs, as pin 6 works like a chip select. Some extended applications of this circuit are given below. 1. Error correction in transducer amplifiers by correcting gain. 2. Autoranging in DMM. 3. Sensor selection or input type selection in process control. 4. Digitally preset power supplies or electronic loads. 5. Programmable precision mV or mA sources. 6. PC or microcontroller or microprocessor based instruments. 7. Data loggers and scanners.
W
PRECISION ATTENUATOR WITH DIGITAL CONTROL
hen instruments are designed, an analogue front-end is essential. Further, as most equipment have digital or microcontroller interface, the analogue circuit needs to have digital control/access. The circuit of a programmable attenu-
testing or trials, use 1 per cent 100ppm MFR resistors. The expected errors will be around 1 per cent. To keep parts count (hence cost) to a minimum, the common or ground is used as the positive input terminal and one end of resistor R1 as the negative.
Since ¼W resistors can withstand up to 250V, resistors R1 and R2 in series are used for 1 meg-ohm with 500V (max) input limit. These resistors additionally limit the input current as well. Diodes D1 and D2 clamp the voltage across input of op-amp to ±0.5V, thereby protecting the Truth Table (Control input VS attenuation) X,Y (ON-switch (2) (1) Gain Pair) B A (Attenuation) X0,Y0 0 0 1/1000 X1,Y1 0 1 1/100 X2,Y2 1 0 1/10 X3,Y3 1 1 1
ator with digital control is described here, where digital control can be a remote dip switch, or CMOS logic outputs of a decade counter (having binary equivalent weight of 1, 2, 4, and 8, respectively), or I/O port of a microcontroller like 80C31. The heart of this circuit is the popular OP07 op-amp with ultra-low offset in the inverting configuration. A dual, 4channel CMOS analogue multiplexer switch CD4052 enables the change in gain. An innovative feature of the circuit is that the ‘on’ resistance (around 100 ohms) of CD4052 switch is bypassed so that no error is introduced by its use. Resistors R1 to R6 used in the circuit should be of 0.1 per cent tolerance, 50 ppm (parts per million) if you use 3½digit DPM, i.e. ±1999 counts (approx. 11 bits). But for 4½-digit DPM (approx. 14 bits), you may need to have trimpots (e.g. replace 1k-ohm resistor R6 by a fixed 900ohm resistor in series with a 200-ohm trimpot) to replace R3, R4, R5, and R6 gain selection resistors for proper calibration to required accuracy. However, for
This is so because the op-amp inverts the polarity as it is used in inverting configuration. This does not matter as the equipment will be isolated by the power supply transformer and all polarities are relative. In case you want the common to be the negative, you will have to add some stages (IC4 and IC5 circuitry shown in precision amplifier circuit described later). The OP07 pinout is based on standard single op-amp 741. Any other op-amp like CA3140, TLO71, or LF351 can be used but with offset errors in excess of 1 per cent, which is not tolerable in precision instrumentation. The OP07 has equivalent ICs like μA741 and LM607 having ultra-low offset voltage (<100μV), low input bias current (<10nA), and high input impedance (>100M), which are the key requirements for a good instrumentation op-amp for use with DC inputs. The following design considerations should be kept in mind: (a) Input: 500V max
op-amp. (b) Output The output can be connected to a 7107/ 7135-based DPM or any other analogueto-digital converter or op-amp stage. Use a buffer at the output if the output has to be loaded by a load less than 1 meg-ohm. Use an inverting buffer if input leads have to have polarity where ground is the inverting terminal. (For details, see next circuit.) (c) CD4052 CMOS switch The on-resistance (100-ohm approx.) comes in series with the op-amp output source resistance, which produces no error at output. Caution. The circuit does not isolate, it only attenuates. When high voltage is present at its input, do not touch any part of the circuit. (d) Digital control options (i) A and B can be controlled by I/O port of a microcontroller like 80C31 so that the controller can control gain. (ii) A and B can be given to counters like 4029/4518 to scroll gain digitally. (iii) A and B can be connected to DIP switch. (iv) A and B can be connected to a thumbwheel switch. Notes. 1. Digital input logic 0 is 0V and logic 1 is 5V. 2. All resistors are metal film resistors (MFR) with 1% tolerance, unless specified otherwise. 3. C2 and C3 are ceramic disk capacitors of 0.1μF = 100nF value.
ELECTRONICS PROJECTS Vol. 22
PRECISION DIGITAL AC POWER CONTROLLER
S
CRs and Triacs are extensively used in modern electronic power controllers—in which power is controlled by means of phase angle variation of the conduction period. Controlling the phase angle can be made simple and easy if we set different firing times corresponding to different firing angles. The design given here is a synchronised programmable timer which achieves this objective. The following equation for a sinewave shows how firing time and the phase angle are related to each other: θ = 2πft or θ∝t Here, θ is the angle described by a sinewave in time t (seconds), while f is the frequency of sinewave in Hz. Time period T (in seconds) of a sinewave is equal to the reciprocal of its
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ELECTRONICS PROJECTS Vol. 21
frequency, i.e. T = 1/f. The above equation indicates that if one divides the angle described during one complete cycle of the sinewave (2π = 360o) into equal parts, then time period T of the wave will be divided into identical equal parts. Thus, it becomes fairly easy to set the different programmable timings synchronised with the AC mains sinewave at zero crossing. The main advantage of such an arrangement, as already mentioned earlier, is that only the firing time has to be programmed to set different firing angles. It is to be noted that the more precise the timer, the more precise will be the power being controlled. In this circuit, the time period of mains waveform is divided into 20 equal parts. So, there is a time interval of 1 ms between two consecutive steps. The sampling voltage is unfiltered full-wave and is obtained from the diode bridge at the output of the power transformer. The timer is reset at every zero crossing of full wave and
set again instantly for the next delay time. This arrangement helps the timer to be set for every half of mains wave—when the positive half of the mains waveform starts building up, the timer is set for that half and as it begins to cross zero, it gets reset and set again for negative half, when the negative half begins to build up. The process is repeated. Here, instead of using two zero crossing detectors—one for each half of mains wave—a single detector is used to perform both the functions. This is possible because the sampling wave for negative half is inverted by the rectifier diode bridge. The 18V AC from power transformer is fed to the four diodes in bridge configuration, followed by the filter capacitor which is again followed by a threeterminal voltage regulator IC LM7812. The voltage so obtained drives the circuit. The unfiltered voltage is isolated from the filter capacitor by a diode and is fed to zener diode D8, which acts as a clipper to clip voltage above 6 volts. This voltage is fed to the base of transistor T1, which is wired as zero crossing detector. When base voltage reaches the threshold, it conducts. It thus sup-
plies a narrow positive pulse which resets the timer at every zero crossing. A 32.768kHz crystal is used to get stable output of nearly 1 kHz (1,024Hz) frequency after five stages of binary division by an oscillator-cum-divider IC CD4060. The 32.768kHz crystal is used because it can be found in unused quartz clocks and is readily available in the market. But use of a 1kHz crystal using a quad-NAND IC
CD4093 as clock generator, as shown in Fig. 2, is better as it provides the exact time interval required. In that case, CD4060 oscillator/divider is not required. The CD4017B counter-cum-decoder IC then divides this 1kHz signal into ten equal intervals, which are programmed via the single-pole, 10-way rotary switch. Once the delayed output reaches the desired time interval, the corresponding output of CD4017 inhibits the counter CD4017 (via
pole of rotary switch and diode D6) and fires the Triac. Transistor T2 here acts as a driver transistor. The reset pin of 4017 is connected to zero crossing detector output to reset it at every zero crossing. (The load-current waveforms for a few positions of the rotary switch, as observed at EFY Lab, are shown in Fig. 3.) The circuit can be used as power controller in lighting equipment, hot-air oven, universal single-phase AC motor, heater, etc.
ELECTRONICS PROJECTS Vol. 21
175
Precision 1Hz Clock Generator using Chip-on-Board
U
sually the circuits for generation of 1Hz clock for applications in digital clock and counter circuits make use of ICs in conjunction with a crystal and trimmer capacitors, etc. However, similar or better accuracy can be achieved using a chip-on-board (COB) device found inside a digital clock, which is readily available in the market for Rs 15-20. This COB consists of IC, capacitors and quartz crystal, etc which are mounted on its surface. It works on 1.4 volt DC source. This COB can be used to derive 1Hz clock.
clock pulse has a very low amplitude of the order of a few milli-volts which cannot be used to drive the digital circuits directly. This low-level voltage is amplified several times by op-amp IC CA3140. The op-amp CA3140 is connected in a non-inverting mode, and its gain is set by resistors R4 and R3. Capacitor C2 reduces the AC gain and unwanted stray pick-up and thus improves stability of the circuit. The input impedance of IC CA3140 is very high and thus there is no drop
Resistor R1, capacitor C3, diodes D1 and D2 shown in the circuit convert 5V DC into 1.4V DC. A ½Hz clock is available at terminals A and B with a phase difference of 90o. The two outputs, are combined using capacitors C1 and C2 to obtain a complete 1Hz clock. This 1Hz
at the input when 1Hz clock signal of low level is connected across its input terminals from the COB. Amplified 1Hz clock pulse is available at its output pin 6, which is further amplified by transistors T1 and T2 to drive the digital clocks and timers.
ELECTRONICS PROJECTS Vol. 20
Preset VR1 is offset null control used to adjust proper 1Hz pulse at the output terminal ‘E’. Connect one LED in series with 220-ohm resistor between the terminal ‘E’ and ground and adjust preset VR1 till the LED blinks once every second. When using the COB, affix the same on a general-purpose PCB using rubber based adhesive and solder the terminals neatly using thin single-strand wire. Lab Note: The COBs used in different watches may differ somewhat in their configuration. But by trial-anderror one can always find out the appropriate points corresponding to points A, B, C and D. Figure of a second COB used by EFY Lab is shown alongside. The points A and B (on the COB used by us) were observed to have complementary 1Hz outputs and hence anyone (only) could be used as input to opamp CA3140.
Programmable LED Indicator
A
lthough IC CD4017 is a decade counter, it can be used in a variety of ways. In this circuit it has been used to program a bicolour LED indicator in 10 different modes which can be selected with a single push-button switch. IC3(555) is used in astable mode to generate square wave and transistor T1 is used to obtain its complementary waveform. IC2 CD4081 is a quad 2-input AND gate. These AND gates and the diode matrix form the logical part of the circuit. IC4 (555) is configured as a monostable flipflop which provides a single clock pulse to IC1 CD4017 for changing the mode by depression of push-to-on switch S1. The use of IC4 avoids switch debouncing problem which causes multiple makes/breaks. TABLE I Mode Operation 0 1 2 3 4 5 6 7 8 9
Off Red ON Green ON Blinking Green-Yellow-Green-Yellow... Blinking Red Blinking Yellow Blinking Green Yellow ON Blinking Red-Yellow-Red-Yellow... Blinking Red-Green-Red-Green...
Switch S2 is included for resetting the circuit. Instead of just one bicolour LED you can use an array of bicolour LEDs in conjunction with two driver transistors. The bicolour (RED and GREEN) LED has three legs. The middle terminal (pin2) LED is the common cathode pin which is grounded when a positive voltage is ap-
plied to pin1, it emits red light. Similarly, when positive voltage is applied to pin3, it emits green light. And when positive voltage is simultaneously applied to its pin1 and 3, it emits yellowish light. Power supply used is +5V regulated. Various modes of this circuit are summerised in Table I.
Outputs of IC1 can also be selected through a 10-way rotary switch connected to Vcc. Now IC1 can be eliminated. Different indications can be activated for different functions of a device. Construction is very easy and total cost of this circuit is less than Rs 60. Current consumption of the circuit is less than 100mA.
ELECTRONICS PROJECTS Vol. 22
PROTECTION FOR YOUR ELECTRICAL APPLIANCES
H
ere is a very low-cost circuit to save your electrically operated appliances, such as TV, tape recorder, refrigerator, and other instruments during sudden tripping and resumption of mains supply. Appliances like refrigerators and air-conditioners are more prone to damage due to such conditions. The simple circuit given here switches off the mains supply to the load as soon as the power N trips. The supply can be resumed only by manual intervention. Thus, the supply may be switched on only after it has stabilised. The circuit comprises a step-down transformer followed by a full-wave rectifier and smoothing capacitor C1 which acts as P a supply source for relay H
RL1. Initially, when the circuit is switched on, the power supply path to the stepdown transformer X1 as well as the load is incomplete, as the relay is in de-energised state. To energise the relay, press switch S1 for a short duration. This completes the path for the supply to transformer X1 as also the load via closed contacts of switch S1. Meanwhile, the supply to relay becomes available and it gets energised to provide
a parallel path for the supply to the transformer as well as the load. If there is any interruption in the power supply, the supply to the transformer is not available and the relay de-energises. Thus, once the supply is interrupted even for a brief period, the relay is de-energised and you have to press switch S1 momentarily (when the supply resumes) to make it available to the load. Very-short-duration (say, 1 to 5 milliseconds) interruptions or fluctuations will not affect the circuit because of presence of largevalue capacitor which has to discharge via the relay coil. Thus the circuit provides suitable P safety against erratic power supply N conditions. H
ELECTRONICS PROJECTS Vol. 21
155
RAMP CONTROLLED LIGHT
T
he circuit described here can be used for controlling a decorative lamp from zero intensity to maximum intensity in a specified time. Typical applications are for controlling Christmas lamps and serial lampsets etc. The brightness of the lamps is controlled by a continuously running ramp voltage generated by a timer. The circuit features a triac (BT136) controlled by pulses from a UJT (unijunction transistor) relaxation oscillator. Pedestal voltage control is employed for controlling the firing angle. Pedestal voltage is derived from a ramp generator which sets the time period of intensity control. X1 secondary (sec. 2) provides the power supply for the ramp generator section. The 555 timer circuit is configured as astable multivibrator which provides rectangular pulses having required time period which are converted to a positive going ramp by sweep generator transistor T1. This is coupled to the base of transistor T2. The time period of control can be altered by modifying the sweep generator and 555 timer sections. X1 is a step-down transformer having two secondaries. Any centre-tapped
step-down transformer will serve the purpose. However, it is advisable to have a higher voltage rating (9V or 12V) for X1 secondary (sec. 1) for extended control range. Zener diode D12 generates the required trapezoidal waveform for the UJT oscillator from the bridge rectifier output.
Transistor T2 controls the charging time of capacitor C5, thus providing the pedestal voltage control. The pulses generated by the UJT oscillator are coupled to the gate of triac through a pulse transformer (X2). A ferrite core transformer with 1:1 ratio can also be used for X2.
ELECTRONICS PROJECTS Vol. 19
177
CIRCUIT IDEAS
SOLIDSTATE SWITCH FOR DC-OPERATED GADGETS
EDI DWIV . C . S
PRAVEEN SHANKER
T
his solidstate DC switch can be assembled using just three transistors and some passive components. It can be used to switch on one gadget while switching off the second gadget with momentary operation of switch. To reverse the operation, you just have to momentarily depress another switch. The circuit operates over 6V-15V DC supply voltage. It uses positive feedback
from transistor T2 to transistor T1 to keep this transistor pair in latched state (on/ off), while the state of the third transistor stage is the complement of transistor T2’s conduction state. Initially when switch S3 is closed, both transistors T1 and T2 are off, as no forward bias is available to these, while the base of transistor T3 is effectively grounded via resistors R8 and R6 (shunted by the load of the first gadget). As a result, transistor T3 is forward biased and gadget 2 gets the supply. This is indicated by glowing of LED2. When switch S1 is momentarily depressed, T1 gets the base drive and it grounds the base of transistor T2 via resistor R4.
Hence transistor T2 (pnp) also conducts. The positive voltage available at the collector of transistor T2 is fed back to the base of transistor T1 via resistor R3. Hence a latch is formed and transistor T2 (as also transistor T1) continues to conduct, which activates gadget 1 and LED1 glows. Conduction of transistor T2 causes its collector to be pulled towards positive rail. Since the collector of T2 is connected to the base of pnp transistor T3, it causes transistor T3 to cut off, switching off the supply to gadget 2) as well as extinguishing LED2. This status is maintained until switch S2 is momentarily pressed. Depression of switch S2 effectively grounds the base of transistor T1, which cuts off and thus virtually opens the base-emitter circuit of transistor T2 and thus cutting it off. This is the same condition as was obtained initially. This condition can be reversed by momentarily pressing switch S1 as explained earlier. EFY lab note. During testing, it was noticed that for proper operation of the circuit, gadget 1 must draw a current of more than 100 mA (i.e. the resistance of gadget 1 must be less than 220 ohms) to sustain the latched ‘on’ state. But this stipulation is not applicable for gadget 2. A maximum current of 275 mA could be drawn by any gadget. The total cost of this circuit is around Rs 30.
APRIL 2002
ELECTRONICS FOR YOU
Running Message Display
L
ight emitting diodes are advantageous due to their smaller size, low current consumption and catchy colours they emit. Here is a running message display circuit wherein the letters formed by LED arrangement light up progressively. Once all the letters of the message have been lit up, the circuit gets reset. The circuit is built around Johnson decade counter CD4017BC (IC2). One of the IC CD4017BE’s features is its provision of ten fully decoded outputs, making
clock pulse. The timer NE555 (IC1) is wired as a 1Hz astable multivibrator which clocks the IC2 for sequencing operations. On reset, output pin 3 goes high and drives transistor T7 to ‘on’ state. The output of transistor T7 is connected to letter ‘W’ of the LED word array (all LEDs of a letter array are connected in parallel) and thus letter ‘W’ is illuminated. On arrival of first clock pulse, pin 3 goes low and pin 2 goes high. Transistor T6 conducts and letter ‘E’ lights up. The preceding letter ‘W’
the complete word becomes visible. On the following clock pulse, pin 6 goes to logic 1 and resets the circuit, and the sequence repeats itself. The frequency of sequencing operations is controlled with the help of potmeter VR1. The display can be fixed on a veroboard of suitable size and connected to ground of a common supply (of 6V to 9V) while the anodes of LEDs are to be connected to emitters of transistors T1 through T7 as shown in the circuit. The above circuit is very versatile and
the IC ideal for use in a whole range of sequencing operations. In the circuit only one of the outputs remains high and the other outputs switch to high state successively on the arrival of each
also remains lighted because of forward biasing of transistor T7 via diode D21. In a similar fashion, on the arrival of each successive pulse, the other letters of the display are also illuminated and finally
can be wired with a large number of LEDs to make an LED fashion jewellery of any design. With two circuits connected in a similar manner, multiplexing of LEDs can be done to give a moving display effect.
ELECTRONICS PROJECTS Vol. 20
circuit
ideas
Security System Switcher
T.K. Hareendran
A
n audio signal can be used as a form of input to control any security system. For example, an automatic security camera can be configured to respond to a knock on the door. The circuit described here allows the security system to automati-
of small signal preamplifier built around transistor T1. Biasing resistor R1 determines to a large extent the microphone sensitivity. A microphone usually has an internal FET which requires a bias voltage to operate. The sound picked up by the microphone is amplified and fed to input pin 2 of IC1 (LMC555) wired in monostable
Indicator LED1 is provided to display the relay activity. Any AC/DC operated security gadget is activated or deactivated through a security switch. Thus, the security switch of the gadget
configuration. IC2 (CD4538B) is a dual, precision monostable multivibrator with independent trigger and reset controls. The output of IC1 is connected to the first trigger input pin 4 of IC2(A) through switch S1. If an intruder opens or breaks the door, IC1 is triggered by sound signals; the timer output pin 3 of IC1 goes high and enables first monostable multi vibrator IC2(A). IC2(A) provides a time period of around 5 to 125 seconds, which is adjusted with preset VR1. Another monostable multivibrator IC2(B) also provides a time period of around 25 to 600 seconds, which is adjusted with preset VR2. The output of IC2(B) is used to energise relay RL1.
is connected in the n/o contacts of the relay. you can also operate highpower beacons, sirens or hooters in place of the security switch for any AC/DC operated security gadget. Assemble the circuit on a general-purpose PCB and enclose it in a cabinet as shown in Fig. 2 along with 5V adaptor for powering the circuit. Connect the security switch according to the circuit diagram and use appropriate AC/DC power supply required to operate the security gadget. Warning! All relevant electrical safety precautions should be taken when connecting mains power supply to the relay contacts. With the help of single pole double throw (SPDT) switch S1, internal or external trigger input (active high signal) can be selected.
edi
s.c. dwiv
Fig. 1: Security system switcher
+5V ADAPTOR FOR POWER SUPPLY
CONNECTOR FOR SECURITY GADGET
Fig. 2: Proposed cabinet
cally switch on when a master switch is in on state. It uses a transducer to detect intruders and a 5V regulated DC power supply provides power to the circuit. As shown in Fig. 1, a condenser microphone is connected to the input
1 4 0 • J a n ua ry 2 0 1 0 • e l e c t ro n i c s f o r yo u
w w w. e f y m ag . co m
Self-switching Power Supply
O
ne of the main features of the regulated power supply circuit being presented is that though fixed-voltage regulator LM7805 is used in the circuit, its output voltage is variable. This is achieved by connecting a potentiometer between common terminal of regulator IC and ground. For every 100-ohm increment in the in-circuit value of the resistance of potentiometer VR1, the output voltage increases by 1 volt. Thus, the output varies from 3.7V to 8.7V (taking into account 1.3-volt drop across diodes D1 and D2). Another important feature of the supply is that it switches itself off when no load is connected across its output terminals. This is achieved with the help of transistors T1 and T2, diodes D1 and D2, and capacitor C2. When a load is connected at the output, potential drop across diodes D1 and D2 (approximately 1.3V) is sufficient for transistors T2 and T1 to conduct. As a result, the relay gets energised and remains in that state as long as the load remains connected. At the same time, capacitor C2 gets charged to around 7-8 volt potential through transistor T2. But when the load is disconnected, transistor T2 is cut off. However, capacitor C2 is still charged and it starts discharging through base of transistor T1. After some time (which is basically determined by value of C2), relay RL1 is de-energised,
which switches off the mains input to primary of transformer X1. To resume the power again, switch S1 should be pressed momentarily. Higher the value of capacitor C2, more will be the delay in switching off the power supply on disconnection of the load, and vice versa. Though in the prototype a transformer with a secondary voltage of 12V-0V, 250mA was used, it can nevertheless be changed as per user’s
requirement (up to 30V maximum and 1-ampere current rating). For drawing more than 300mA current, the regulator IC must be fitted with a small heat sink over a mica insulator. When the transformer’s secondary voltage increases beyond 12 v olt s (R MS), p ot ent iom et er V R 1 must be redimensioned. Also, the relay voltage rating should be redetermined.
ELECTRONICS PROJECTS Vol. 20
SENSITIVE TEMPERATURE SWITCH
T
his temperature switch has a high sensitivity and is quite reliable. Here, in place of a single transistor, a Darlington pair has been used for switching. At normal room temperature germinum diode D1 (0A79 or equivalent) has a back resistance value of about 10 kiloohms. As a result Darlington pair, comprising transistors T1 and T2. conducts and keeps the anode terminal of diode D2 at ground potential. Consequently, transistor T3 does not get base bias and thus relay RL1 is not activated. But when temperature increases. the back resistance of diode D1 decreases
sharply, which results in cutting off of Darlington pair and forward biasing of transistor T3 via resistor R2 and diode D2. As a result, relay RL1 energises and switches on the alarm. Potmeter VR1 may be adjusted for required sensitivity. This simple circuit can be used as an overheat indicator, fire alarm, or it can be used in a constant temperature circuit for switching on a fan etc. The circuit can be easily assembled on a piece of veroboard. Diode
sensor D1 must be of germanium type and not silicon.
ELECTRONICS PROJECTS Vol. 19
189
CIRCUIT
IDEAS
Charge Monitor for 12V Rechargeable Lead-acid Battery
RUP
ANJA
NA
SIDDHARTH SINGH AND SRINIVAS REDDY PINGLE
A
battery is a vital element of any battery-backed system. In many cases the battery is more expensive than the system it is backing up. Hence we need to adopt all practical measures to conserve battery life. As per manufacturer’s data sheets, a 12V rechargeable lead-acid battery should be operated within 10.1V and 13.8V. When the battery charges higher than 13.8V it is said to be overcharged, and when it discharges below 10.1V it can be deeply discharged. A single event of overcharge or deep discharge can bring down the charge-holding capacity of a battery by 15 to 20 per cent. It is therefore necessary for all concerned to monitor the charge level of
their batteries continuously. But, in practice, many of the battery users are unable to do so because of non-availability of reasonably-priced monitoring equipment. The circuit idea presented here will fill this void by providing a circuit for monitoring the charge level of lead-acid batteries continuously. The circuit possesses two vital features: • First, it reduces the requirement of human attention by about 85 per cent. • Second, it is a highly accurate and sophisticated method. Input from the battery under test is applied to LM3914 IC. This applied voltage is ranked anywhere between 0 and 10, depending upon its magnitude. The lower reference voltage of 10.1V is ranked
ELECTRONICS FOR YOU n SEPTEMBER '99
‘0’ and the upper voltage of 13.8V is ranked as ‘10.’ (Outputs 9 and 10 are logically ORed in this circuit.) This calibration of reference voltages is explained later. IC 74LS147 is a decimal-to-BCD priority encoder which converts the output of LM3914 into its BCD complement. The true BCD is obtained by using the hex inverter 74LS04. This BCD output is displayed as a decimal digit after conversion using IC5 (74LS247), which is a BCD-to-seven-segment decoder/driver. The seven-segment LED display (LTS542) is used because it is easy to read compared to a bar graph or, for that matter, an analogue meter. The charge status of the battery can be quickly calculated from the display. For instance, if the display shows 4, it means that the battery is charged to 40 per cent of its maximum value of 13.8V. The use of digital principles enables us to employ a buzzer that sounds whenever there is an overcharge or deep discharge, or there is a need to conserve battery charge. A buzzer is wired in the circuit such that it sounds whenever battery-charge falls to ten per cent. At this point it is recommended that unnecessary load be switched off and the remaining charge be conserved for more important purposes. Another simple combinational logic
CIRCUIT circuit can also be designed that will sound the buzzer when the display shows 9. Further charging should be stopped at this point in order to prevent overcharge. The circuit is powered by the battery under test, via a voltage regulator IC. The circuit takes about 100 mA for its operation. For calibrating the upper and lower reference levels, a digital multimeter
and a variable regulated power supply source are required. For calibrating the lower reference voltage, follow the steps given below: • Set the output of power supply source to 10.1V. • Connect the power supply source in place of the battery. • Now the display will show some reading. At this point vary preset VR2 until the reading on the display just
ELECTRONICS FOR YOU n SEPTEMBER '99
IDEAS
changes from 1 to 0. The higher reference voltage is calibrated similarly by setting the power supply to 13.8V and varying preset VR1 until reading on the display just changes from 8 to 9. After the calibration is completed, the circuit may be housed in a suitable enclosure. The cost of all the components, including the enclosure, would be around Rs 200.
INFRARED ELECTRONIC SHOOTING GAME T.K. HAREENDRAN
J
ust trigger an infrared electronic gun and there goes one invisible bullet hitting the bull’s eye, if timed properly. The circuit is very simple, inex-
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ELECTRONICS PROJECTS Vol. 19
pensive and easy to construct. The game offers hours of fun and excitement. The target screen consists of a number of LEDs moving rapidly in a circular fashion. All the LEDs are red except one—the real target located in centre of the screen which is green. When a shot is fired by triggering the gun, all LEDs go off except one. If it happens to be the target (green LED) then you have made a hit which is indicated by lighting up of another green LED accompanied by a pleasing musical tone. After a short delay the game restarts automatically. Infrared gun (transmitter) for this electronic game is built around IC1 timer (NE555) wired as an astable multivibra-
tor with a centre frequency of about 35 kHz. The frequency is determined by the timing components comprising resistors R1 and R2 and capacitor C2. When push-to-on trigger switch S1 is pressed, the astable multivibrator starts modulating the infrared beam with short pulses (See output waveform). The whole circuit can be enclosed in a toy gun for giving it a professional look as illustrated in the figure. The infrared LED has to be fitted with a suitable reflector to ensure good sensitivity. When power switch S2 in the receiver is turned on, astable multivibra-tor wired around IC3 (NE555) generates clock pulses which are fed to clock input (pin 14) of decade counter IC4 (CD4017B). This IC has ten outputs, and each one goes high sequentially on the rising edge of successive clock pulse. As a result, LEDs connected to the output appear to move from one to the other rapidly.
You would notice that only nine outputs are used for driving LEDs. The tenth output (Q9) at pin 11 is connected to reset pin 15. When gun is fired, infrared bursts are received by the integrated infrared module and its output at pin 2 goes low. The resulting falling edge triggers monostable
IC2 and its output (pin 3) goes high. This makes clock enable (CE) pin 13 of IC4 to go high (normally held at a low potential via resistor R8) and it starts counting. When mono pulse ends, and if the last lit LED happens to be the target LED then both inputs of NAND gate N1 become high. As a result, the output of gate N2 also goes high. This in turn switches on transistor T2; thereby the ‘HIT’ LED lights up and the buzzer also sounds. At the end of the mono pulse period (about 5 sees), decided by resistor R5 and capacitor C5, the mono IC2 is again ready to receive another trigger pulse. Assembly and component layout is not very critical. The circuit may be assembled on a veroboard using IC sockets. A well regulated power supply is required for powering the unit. In place of the IR transmitter it is also possible to make use of the remote control used for TVS or VCPs/VCRs.
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short wave AM TRANSMITTER RAJESH KAMBOJ
T
he main feature of this transmitter is that it is free from the LC (inductor, capacitor) tuned circuit and operates on a fixed frequency of 12 MHz which is extremely stable. An LC based tuned circuit is inherently unstable due to drift of resonant frequency on account of temperature and humidity variations. The circuit is very simple and uses only a few components. The figure shows the complete circuit diagram of the transmitter. Resistors R1 and R2 are used for DC biasing of transistor T1. The capacitor C1 provides coupling between the condenser microphone and the base of transistor T1. Similarly, resistors R3, R4 and R5 provide DC biasing to transistor T2. The oscillator section is a combination of transistor T2, crystal XTAL, capacitor C2, C3 and resistors R3, R4 and R5. The crystal is excited by a portion of energy from the collector of transistor T2 through the feedback capacitor C2. The crystal vibrates at its fundamental frequency and the oscillations occurring due to the crystal are applied to the base of transistor T2 across resistor R4. In this way, continuous undamped oscillations are obtained. Any crystal having
the frequency in short wave range can be substituted in this circuit, although the operation was tested with a 12 MHz crystal. Transistor T1 serves three functions: 1. It provides the DC path for extending +VCC supply to transistor T2. 2. It amplifies the audio signals obtained from condenser microphone. 3. It injects the audio signal into the high frequency carrier signal for modulation. The condenser microphone converts the voice message into the electrical signal which is amplified by transistor T1. This amplified audio signal modulates the carrier frequency generated by transistor T2. The amplitude modulated output is obtained at the collector of transistor T2 and is transmitted by a
loop antenna into space in the form of electromagnetic waves. The antenna can be tuned to a particular frequency varying trimmer C5 and also by changing the length of ferrite rod into the coil. The transmitted signals can be received on any short wave receiver without distortion and noise. The range of this transmitter is 25 to 30 metres and can be extended further if the length of the antenna wire is suitably increased along with proper matching.
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SHORTWAVE TRANSMITTER
T
his transmitter circuit operates in shortwave HF band (6 MHz to 15 MHz), and can be used for shortrange communication and for educational purposes.
capacitance of gang condenser, the frequency of oscillation can be changed. The carrier RF signal from the oscillator is inductively coupled through the secondary of transformer X1 to the next RF am-
be directly connected to its input without any amplification. The transmitter’s stability is governed by the quality of the tuned circuit components as well as the degree of regu-
The circuit consists of a mic amplifier, a variable frequency oscillator, and modulation amplifier stages. Transistor T1 (BF195) is used as a simple RF oscillator. Resistors R6 and R7 determine base bias, while resistor R9 is used for stability. Feedback is provided by 150pF capacitor C11 to sustain oscillations. The primary of shortwave oscillator coil and variable condenser VC1 (365pF, 1/2J gang) form the frequency determining network. By varying the coil inductance or the
plifier-cum-modulation stage built around transistor T2 that is operated in class ‘A’ mode. Audio signal from the audio amplifier built around IC BEL1895 is coupled to the emitter of transistor 2N2222 (T2) for RF modulation. IC BEL1895 is a monolithic audio power amplifier designed for sensitive AM radio applications. It can deliver 1W power to 4 ohms at 9V power supply, with low distortion and noise characteristics. Since the amplifier’s voltage gain is of the order of 600, the signal from condenser mic can
lation of the supply voltage. A 9V regulated power supply is required. RF output to the aerial contains harmonics, because transistor T2 doesn’t have tuned coil in its collector circuit. However, for short-range communication, this does not create any problem. The harmonic content of the output may be reduced by means of a high-Q L-C filter or resonant L-C traps tuned to each of the prominent harmonics. The power output of this transmitter is about 100 milliwatts.
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ELECTRONICS PROJECTS Vol. 23
Simple Analogue-todigital Converter
N
ormally analogue-to-digital converter (ADC) needs interfacing through a microprocessor to convert analogue data into digital format. This requires additional hardware and necessary software, resulting in increased complexity and hence the total cost. The circuit of A-to-D converter shown
ELECTRONICS PROJECTS Vol. 20
here is configured around ADC 0808, avoiding the use of a microprocessor. The ADC 0808 is an 8-bit A-to-D converter, having data output lines D0-D7. It works on the principle of successive approximation. It has a total of eight analogue input channels, out of which any one can be selected using address lines A, B and C. Here, in this case, input channel IN0 is selected by grounding A, B and C address lines. Usually the control signals EOC (end of conversion), SC (start conversion), ALE (address latch enable) and OE (output enable) are interfaced by means of a microprocessor. However, the circuit shown here is built to operate in its continuous mode without using any microprocessor. Therefore the input control signals ALE and OE, being active-high, are tied to Vcc (+5 volts). The input control signal SC, being active-low, initiates start of conversion at falling edge of the pulse, whereas the output signal EOC becomes high after completion of conversion (digiti-
sation). This EOC output is coupled to SC input, where falling edge of EOC output acts as SC input to direct the ADC to start the next conversion. As the conversion starts, EOC signal goes high. At next clock pulse EOC output again goes low, and hence SC is enabled to start the next conversion. Thus, it provides continuous 8-bit digital output corresponding to instantaneous value of analogue input. The maximum level of analogue input voltage should be appropriately scaled down below positive reference (+5V) level. The ADC 0808 IC requires clock signal of typically 550 kHz, which can be easily derived from an astable multivibrator constructed using 7404 inverter gates. In order to visualise the digital output, the row of eight LEDs (LED1 through LED8) have been used, wherein each LED is connected to respective data output lines D0 through D7. Since ADC works in the continuous mode, it displays digital output as soon as analogue input is applied. The decimal equivalent digital output value D for a given analogue input voltage Vin can be calculated from the relationship
SIMPLE ELECTRONIC CODE LOCK
T
he circuit diagram of a simple electronic code lock is shown in figure. A 9-digit code number is used to operate the code lock. When power supply to the circuit is turned on, a positive pulse is applied to the RESET pin (pin 15) through capacitor C1. Thus, the first output terminal Q1 (pin 3) of the decade counter IC (CD 4017) will be high and all other outputs (Q2 to Q10) will be low. To shift the high state from Q1 to Q2, a positive pulse must be applied at the clock input terminal (pin 14) of IC1. This is possible only by pressing the push-to-on switch S1 momentarily. On pressing switch S1, the high state shifts from Q1 to Q2. Now, to change the high state from Q2 to Q3, apply another positive pulse at pin 14, which is possible only by pressing switch S2. Similarly, the high state can be shifted up to the tenth output (Q10) by pressing the switches S1 through S9 sequentially in that order. When Q10 (pin 11) is high, transistor T1 conducts and energises relay RL1. The relay can be used to switch ‘on’ power to any electrical appliance. Diodes D1 through D9 are provided to prevent damage/malfunctioning of the IC when two switches corresponding to ‘high’ and ‘low’ output terminals are pressed simultaneously. Capacitor C2 and resistor R3 are provided to prevent noise during switching action. Switch S10 is used to reset the circuit manually. Switches S1 to S10
can be mounted on a keyboard panel, and any number or letter can be used to mark them. Switch S10 is also placed together with other switches so that any stranger trying to operate the lock frequently presses the switch S10, thereby resetting the circuit many times. Thus, he is never able to turn the relay ‘on’. If necessary, two or three switches can
be connected in parallel with S10 and placed on the keyboard panel for more safety. A 12V power supply is used for the circuit. The circuit is very simple and can be easily assembled on a general-purpose PCB. The code number can be easily changed by changing the connections to switches (S1 to S9).
ELECTRONICS PROJECTS Vol. 21
169
T
SIMPLE INTERCOM CIRCUIT
he circuit of a two-position intercom is presented here. This circuit is very simple yet it functions quite satisfactorily. The circuit does not involve any complicated switching. The switches S1/S2 must be fixed in such a way
ELECTRONICS PROJECTS Vol. 22
that when the handset is resting on the cradle, the switch is OFF and when it is taken off the cradle, the switch turns on. Both the sets used are identical in construction. When one set (say, party 1) is switched on, the other set’s (party 2’s) bell
the two sets. Only three wires are required to connect the two sets if separate battery is used in each set. However, if the battery is common for the two sets, it requires four wires for interconnections. The circuit can be easily assembled on
energises. When party 2 turns on his own set, his bell automatically stops and he can talk to party 1 via his microphone. One can substitute the BEL 1895 IC based amplifier and bell circuit with any other low power amplifier and bell circuit. The block diagram clarifies the connection of
a general-purpose PCB. Intercom cases are also available in the market which may be used for giving it a professional outlook.
simple sensitive remote control tester
H
ere is a handy gadget for testing of infrared (IR) based remote control transmitters used for TVs and VCRs etc. The IR signals from a remote control transmitter are sensed by the IR sensor module in the tester and its output at pin 2 goes low. This in turn switches on transistor T1 and causes LED1 to blink. At the same time, the buzzer beeps at the same rate as the incoming signals from the remote control transmitter. The pressing of different buttons on the remote control will result in different pulse rates which would change the rate at which the LED blinks or the buzzer beeps. When no signal is sensed by the sensor module, output pin 2 of the sensor goes high and, as a result, transistor T1 switches off and hence LED1 and buzzer BZ1 go off. This circuit requires 5V regulated power supply which can be obtained from 9V eliminator and connected to the circuit through a jack. Capacitor C1 smoothes DC input while capacitor C2 suppresses any spikes appearing in the input supply. Proper grounding of the metal case will ensure that the electromagnetic emissions which are produced by tube-lights
and electronic ballasts etc (which lie within the bandwidth of receiver circuit) and repeats the steps shown in step 1 above and notes down his new score (say, X2). He adds up this score to his previous score. The same procedure is repeated by player ‘Y’ in his turn. 4. The game carries on until the score attained by one of the two players totals up to or exceeds 100, to be declared as the winner. Several players can participate in this game, with each getting a chance to score
during his own turn. The circuit may be assembled using a multipurpose board. Fix the display (LEDs and 7-segment display) on top of the cabinet along with the three switches. The supply voltage for the circuit is 5V. are effectively grounded and do not interfere with the functioning of the circuit. The proposed layout of the box containing the circuit is shown in the figure. The 9-volt DC supply from the eliminator can be fed into the jack using a banana-type plug.
ELECTRONICS PROJECTS Vol. 22
15
simple sensitive remote control tester
H
ere is a handy gadget for testing of infrared (IR) based remote control transmitters used for TVs and VCRs etc. The IR signals from a remote control transmitter are sensed by the IR sensor module in the tester and its output at pin 2 goes low. This in turn switches on transistor T1 and causes LED1 to blink. At the same time, the buzzer beeps at the same rate as the incoming signals from the remote control transmitter. The pressing of different buttons on the remote control will result in different pulse rates which would change the rate at which the LED blinks or the buzzer beeps. When no signal is sensed by the sensor module, output pin 2 of the sensor goes high and, as a result, transistor T1 switches off and hence LED1 and buzzer BZ1 go off. This circuit requires 5V regulated power supply which can be obtained from 9V eliminator and connected to the circuit through a jack. Capacitor C1 smoothes DC input while capacitor C2 suppresses any spikes appearing in the input supply. Proper grounding of the metal case will ensure that the electromagnetic emissions which are produced by tube-lights
and electronic ballasts etc (which lie within the bandwidth of receiver circuit) and repeats the steps shown in step 1 above and notes down his new score (say, X2). He adds up this score to his previous score. The same procedure is repeated by player ‘Y’ in his turn. 4. The game carries on until the score attained by one of the two players totals up to or exceeds 100, to be declared as the winner. Several players can participate in this game, with each getting a chance to score
during his own turn. The circuit may be assembled using a multipurpose board. Fix the display (LEDs and 7-segment display) on top of the cabinet along with the three switches. The supply voltage for the circuit is 5V. are effectively grounded and do not interfere with the functioning of the circuit. The proposed layout of the box containing the circuit is shown in the figure. The 9-volt DC supply from the eliminator can be fed into the jack using a banana-type plug.
ELECTRONICS PROJECTS Vol. 22
15
CIRCUIT
SIMPLE TELEPHONE RING TONE GENERATOR
IDEAS
MAR IL KU N U S
K. UDHAYA KUMARAN, VU3GTH ere is a simple telephone ring tone generator circuit designed using only a few components. It produces simulated telephone ring tone and needs only DC voltage (4.5V DC to 12V DC). One may use this circuit in ordinary intercom or phone-type intercom.
H
tor (CMOS IC CD4060B) is used to generate three types of pulses, which are available from pin 1 (O11), pin 3 (O13), and pin 14 (O7), respectively. Preset VR1 is adjusted to obtain 0.3125Hz pulses (1.6second ‘low’ followed by 1.6-second ‘high’) at pin 3 of IC1. At the same time, pulses available from pin 1 will be of 1.25 Hz
The sound is quite loud when this circuit is operated on +12V DC power supply. However, the volume of ring sound is adjustable. The commonly available 14-stage binary ripple counter with built-in oscilla-
(0.4-second ‘low’, 0.4-second ‘high’) and 20 Hz at pin 14. The three output pins of IC1 are connected to base terminals of transistors T1, T2, and T3 through resistors R1, R2, and R3, respectively. Transistors T1 through T3 are cas-
ELECTRONICS FOR YOU ❚ JULY 2001
caded in such a way that the positive voltage available at the emitter of transistor T1 is extended to the collector of transistor T3 when the outputs of all the three stages are low. As a result, transistors T1 through T3 are forward biased for 0.4, 1.6, and 0.025 seconds, respectively and reverse biased for similar durations. Using a built-in oscillator-type piezobuzzer produces around 1kHz tone. In this circuit, the piezo-buzzer is turned ‘on’ and ‘off’ at 20 Hz for ring tone sound by transistor T3. 20Hz pulses are available at the collector of transistor T3 for 0.4-second duration. After a time interval of 0.4 second, 20Hz pulses become again available for another 0.4-second duration. This is followed by two seconds of nosound interval. Thereafter the pulse pattern repeats itself. Refer the figure that indicates waveforms available at various points including the collector of transistor T3. Preset VR2 can be used for adjusting the amplitude of the ring tone.
Sleep-switch cum Wake-up Timer
H
ere is a sleep-switch circuit that can be easily converted into a wake-up timer. A dual-mode time setting makes the system versatile. The circuit is low-cost and can function as a precise timer. The heartbeat produced by IC1 is a sharp 1Hz square wave signal having a duty cycle of 50 per cent. This is achieved by using a 4.194304MHz crystal in combination with discrete components around it. The 1Hz output of IC1 is connected to IC2 as well as one of the terminals of switch S1. IC2 is configured as divideby-6 counter while IC3 further divides the output of IC2 by ten to produce one-minute output at its pin 12. This is brought to the second terminal of twoway switch S1 to help select either the ‘minutes’ or the ‘seconds’ mode of operation for IC4.
IC7 pair provides tens output since IC6 clock input pin is connected to D output pin of IC4. Rotary switches S2 and S3 can be set to select any time between either 0 to 99 seconds or 0 to 99 minutes, depending upon the position of mode switch S1. Switches S2 and S3 could also be replaced by thumb-wheel type switches or 10-position DIP switches with one of their side terminals shorted together to serve as a pole. Please note that IC5 and IC7 (74145) have active low outputs. The outputs from switches S2 and S3 are input to a two-input OR gate inside IC8 (7432) to obtain active low output on completion of the set time delay to deactivate relay RL1 through relay driver transistor T1 (normally conducting) when set time is reached. When transistor T1 cuts off, its collector goes high to reset oscillator IC1,
The BCD outputs of IC4 and IC6 are converted to seven-segment outputs by IC9 and IC10 to drive the units and tens displays respectively for indicating elapsed time continuously. The relay contacts (normally open and normally closed) can be suitably used to energise or de-energise an alarm after the preset delay. It can thus be used as wake-up alarm or sleep timer. If you want to de-energise the relay, say after 30 minutes, then set switch S1 to minutes mode, S2 to 0 and S3 to 3, and then switch on the supply to the circuit. After 30 minutes the outputs at poles of switches S2 and S3 will go low and so also the output of OR gate (IC8). As a result, transistor T1 will be cut-off to de-energise the relay. One can easily add 0-99 hours capability by cascading two counters similar
The decade counter IC4 provides binary output as it counts up the input pulses and IC5 decodes/converts them to 1-of-10 outputs (units). Similarly, the IC6-
and thus count at output of IC4 and IC6 gets locked. For resetting or restarting, the power supply to the circuit should be switched off and then switched on again.
to the minutes counter section comprising IC2 and IC3. Input clock for hours counter would be the minutes clock available at pin 12 of IC3.
ELECTRONICS PROJECTS Vol. 20
Smart Phone Light The circuit shown here is used to switch on a lamp when the telephone rings, provided that the ambient light is insufficient.
The circuit can be implemented using just two ICs. A light dependent resistance (LDR), with about 5 kilo-ohms resistance ELECTRONICS PROJECTS Vol. 20
in the ambient light and greather than 100 kilo-ohms in darkness, is at the heart of the circuit. The circuit is fully isolated from the
power failure or load shedding also. The light switches off automatically after a programmable time period. If required, the lamp lighting period can
phone lines and it draws current only when the phone rings. The lamp can be battery powered to provide light during
be extended by simply pressing a pushbutton switch (S1). The first part of the circuit functions as a ring detector. When telephone is onhook, around 48V DC is present across the TIP and RING terminals. The diode in the opto-coupler is ‘off’ during this condition and it draws practically no current from the telephone lines. The optocoupler also isolates the circuit from the telephone lines. Transistor in the opto-coupler is normally ‘off’ and a voltage of +5V is present at the ring indicator line B. When telephone rings, an AC voltage of around 70-80V AC present across the telephone lines turns on the diode inside the opto-coupler (IC2), which in turn switches on transistor inside the optocoupler. The voltage at its collector drops to a low level during ringing to trigger IC3 74LS123(A) monostable flip-flop. The other opto-coupler (IC1) is used to detect the ambient light condition. When there is sufficient light, LDR has a low resistance of about 5 kilo-ohms and the transistor inside the opto-coupler is in ‘on’ state. When there is insufficient light available, the resistance of LDR increases to a few mega-ohms and the transistor switches to ‘off’ state. Thus
the DC voltage present at the collector of transistor of the opto-coupler is normally low and it jumps to 5V when there is no light or insufficient light. The 74LS123 retriggerable monostable multivibrator IC is used to generate a programmable pulse-width. The first monostable 74LS123(A) generates a pulse from the trigger input available during ringing, provided its pin 2 input (marked B) is logic high (i.e. during darkness). It remains high for
the programmed duration and switches back to 0V at the end of the pulse period. This high-to-low transition (trailing edge) is used to trigger the second monostable flip-flop 74LS123(B) in the same package. Output of the second monostable is used to control a relay. The lamp being controlled via the N/O contacts of the relay gets switched ‘on.’ The ‘on’ period can be extended by simply pressing pushbutton switch S1. If
nobody attends the phone, the light turns off automatically after the specific time period equal to the pulse-width of the second flip-flop. The light sensitivity of LDR can be changed by changing resistance R3 connected at collector of the transistor in light monitor circuit. Similarly, switch-on period of the lamp can be controlled by changing capacitor C3’s value in the second 74123(B) monostable circuit.
ELECTRONICS PROJECTS Vol. 20
SOLIDSTATE AUTO POWER-OFF ON MAINS FAILURE SUNISH ISAAC
A
good number of circuits which turn off the power to the load on mains failure have already appeared in EFY. This circuit is different because it consumes no power when the load is off. Further, the circuit is completely solidstate as it does not use any relay or other electromechanical devices. Here resistor R1, diode D1, capacitor C1 and zener D2 are used to develop a mains-derived 9V DC supply which can be fed to the triac gate. Switches S1 and S2 are used to accomplish ‘off ’ and ‘on’ operations respectively. When switch S2 is momentarily depressed the neutral line gets extended to MT1 of triac TR1. This in turn makes the DC voltage available to the transistor circuit. Capacitor C2 and resistor R5 provide the initial base drive to transistor T2 at power-on. By the time capacitor C2 is fully charged, transistor T1 is driven into saturation via transistor T2 and resistor R3. The collector of transistor T1 is connected to the base of transistor T2 through resistor R4, and collector of transistor T2 is connected to base of transistor T1 via
196
ELECTRONICS PROJECTS Vol. 19
resistor R3, i.e. there exists a mutual symbiosis between transistors T1 and T2. As long as transistor T1 is on, it provides the necessary gate current to the triac through resistor R7. Capacitor C3 prevents false operation of the circuit. The load can be switched off either by pressing switch S1 or by turning off the power to the circuit. When switch S1 is pressed, transistor T1 is no more in saturation and thus it turns off the triac and
the load. Since the rectifier circuit cannot get neutral line when the triac is off, it consumes absolutely no power when the load is off. The idea can be suitably adopted for timer applications where the power to the timer circuit needs to be turned off after the set time delay. An opto-coupler can replace switch S1 for using the output of a low-voltage control circuit for switching off.
Song Number Display Prabhash K.P.
H
ere’s a circuit to display the song number in an audio system for quick reference to songs. It also serves the purpose of an extra visual indicator in modern audio systems. When the power is switched on, the power-on-reset circuit comprising 3.3k resistor R20 and 1µF, 25V capacitor C6 resets the counters, showing ‘00’ in the display. One can also reset the display to zero at any time by pressing reset switch S1. When the first song starts playing, the output pins of IC1 (KA2281) go low and capacitor C5 starts charging. This forward biases transistor T1 and hence the input to IC3 at pin 1 goes to high state. As a
result, the output of the counter goes to the next state, showing 01 on the display. The counter remains in this state until the song is completed. During the time gap before the next song starts playing, capacitor C5 discharges. After discharging of capacitor C5, the input to IC3 becomes low again. When the song starts, the process described above is repeated and the display shows 02. You can adjust VR3 to change the time gap setting. This must be set such that the circuit doesn’t respond to short gaps, if any, within a song and responds only to long gaps between different songs. Transistor T2 helps in gap-delay adjustment. The intensity of LED11 dimin-
ishes when a song is completed and the counter is ready to accept the next pulse. Connect the input to the preamp output or equaliser output of the audio system. Adjust VR1 and VR2 to get the correct audio-level indication. If you are already using KA2281 for audio-level indication, just connect diodes D1 and D2 as shown in this circuit. Note that the counter counts the songs by detecting the gaps. Therefore any long gap within a song may cause false triggering and the display will also be incremented. However, as this is very unlikely to happen, the circuit shows the correct song number almost all the time.
ELECTRONICS PROJECTS Vol. 24
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CIRCUIT
SPELLER EFFECT SIGN DISPLAY
IDEAS
EDI DWIV S.C.
VIJAYA KUMAR P. he circuit described here uses lowcost and easily available IC CD4017 to produce a speller type light display. In such displays, each letter of the sign sequentially lights up, one after the other, until all letters are glowing. After a few seconds, the letters switch off and the cycle repeats. This circuit provides a maximum of nine channels and therefore can be used to spell a word or sign having up to nine characters. Timer IC1 (555) is configured in
T
CD4017 is a decade counter having ten outputs, of which one output is high for each clock pulse. However, this produces running lights effect. To change this sequence to get the speller effect, pnp transistors T1 through T9 are wired as shown in the figure. Nine triacs (triac 1 through triac 9) are used to drive 230V bulbs. (In place of 230V bulbs, miniature lamps connected in series in the form of characters or letters can also be used, provided the voltage drop across the series
astable mode to produce clock signal for triggering IC2 (CD4017). Speed of switching on the display can be controlled by varying preset VR1.
combination is 230 volts.) When any of the outputs of IC2 goes high, the corresponding transistor connected to the output goes off. When Q0 is
ELECTRONICS FOR YOU ❚ NOVEMBER 2001
high, transistor T1 goes off and its output at the collector goes low. Since the emitter of transistor T2 is connected to the collector of transistor T1, and collector and emitter terminals of transistors T1 through T9 are connected in series, all transistors next to transistor T1, i.e. transistors T2 through T9, do not get supply and hence all their outputs go low. Next, when Q1 output goes high, transistor T2 goes off. Thus outputs of transistors T2 through T9 remain low. Since Q0 output at this instant is low, transistor T1 is forward biased and its output goes high to light up the first character. Similarly, when Q2 output goes high, Q0 and Q1 outputs are low and therefore outputs of transistors T1 and T2 go high to light up the first and second characters. This process continues until all transistors turn on, making all the characters
to light up. The cycle repeats endlessly, producing the speller type light effect.
circuit
ideas
Staircase Light With Auto Switch-Off
Raj K. Gorkhali
W
e are all familiar with the electrical wiring arrangement that connects an electrical bulb with two switches: one at the bottom of a staircase and the other at the top. Wiring is done such that either of the two switches can be used to switch the bulb on or off. In such a wiring arrangement, while climbing up the staircase which is in dark, the switch located at the bottom of the staircase is used to switch on the light. After you have climbed the staircase, you use the switch located there to switch off the light. The circuit presented here is an electronic-cum-electrical arrangement to get a similar facility as provided by the hard-wired electrical system, but you need to operate the switch only once. Whereas in the hard-wired arrangement if you forget to switch off the light once you have traversed the staircase, light would remain ‘on,’ wasting energy. In this circuit also, we have two
edi
s.c. dwiv
micro-switches—one located at the top and the other located at the bottom of the staircase—that can be pushed and released easily during climb-up from the bottom of the staircase or climbdown from the top of the staircase. With every push and release of either of the two switches, bulb L1 lights up for a preset time period of, say, 40 seconds, which is considered adequate for climbing up or going down the staircase. The bulb goes off automatically after the set 40 seconds. You can change this ‘on’ time by changing the values of resistor R7 and/or capacitor C4 depending upon your requirement. Switches S1 and S2 are the two micro-switches, which provide low inputs to the respective de-bouncing circuits. Each de-bouncing circuit is built around two NAND gates connected back to back. The de-bouncing circuits ensure a clean, bounce-free pulse at the output every time the micro-switch is pressed and released. The outputs from the two de-bouncing circuits are ORed using diodes D1 and D2 (1N4001). So every time you press and release either
of the micro-switches, you get a positive-going pulse at the junction of the cathodes of diodes D1 and D2. These pulses are used to trigger the monostable circuit built around timer IC2. On the trailing edge of the pulse, the output of the monostable goes high for a time period of 40 seconds. This drives relay-driver transistor 2N2222 (T1) wired as a switch. Relay RL1 gets energised and closes N/O contacts of the relay, wired in series with the mains and the bulb (L1). Bulb L1 switches off when the relay gets de-energised after 40-second pulse period. Free-wheeling diode D4 (1N4001) protects transistor T1 against transients during relay switch-off operation. The circuit operates off a 9V battery, which gets connected to the circuit through ‘on’/‘off’ switch S3. You can also use regulated 9V power supply. Assemble the circuit on a generalpurpose PCB and house in a small box. Connect micro-switches S1 and S2 near top and bottom of the staircase through flexible wires and bulb in the middle of the staircase.
7
w w w. e f y m ag . co m
e l e c t ro n i c s f o r yo u • M ay 2 0 0 8 • 7 1
CIRCUIT
IDEAS
STEREO TAPE HEAD PREAMPLIFIER FOR PC SOUND CARD MAR IL KU SUN
T.K. HAREENDRAN ere is a stereo tape head preamplifier circuit for your PC sound card that can playback your favourite audio cassette through the PC. Audio signals from this circuit can be di-
H
The amplified and equalised signals available at output pins 3 and 6 of IC1 are coupled to the inputs of line amplifier circuit built around transistors T1 (via capacitor C5, potmeter VR1, resistor R8, and
rectly connected to the stereo-input (lineinput) socket of the PC sound card for further processing. The circuit is built around a popular stereo head preamp IC LA3161. Weak electrical signals from the playback heads are fed to pins 1 and 8 of IC1 via DC decoupling capacitors C1 and C6, respectively. Components between pins 2 and 3 and pins 6 and 7 provide adequate equalisation to the signals for a normal tape playback.
capacitor C12) and T2 (via capacitor C10, potmeter VR2, resistor R19, and capacitor C16), respectively. Left and right playback levels can be adjusted by variable resistors VR1 and VR2. The audio signals are finally available at the negative ends of capacitors C13 and C17. The circuit wired around relay driver transistor T3 serves as a simple source selector. This is added deliberately to help the user share the common PC sound card line-input terminal for operating some
ELECTRONICS FOR YOU ❚ OCTOBER 2001
other audio device as well. When the preamplifier is in ‘off’ state, switching relay RL1 is off and it allows connection of external signals to the sound card. When the preamplifier is turned ‘on’, the relay is energised by transistor T3 after a short delay determined by the values of resistor R21 and capacitor C23. On energisation, the relay contacts changeover the signals to internal source, i.e. the head preamplifier. After constructing the whole circuit on a veroboard, enclose it in a mini metallic cabinet with level controls and sock-
ets at suitable points. Use a regulated 1A, 12V DC power supply for powering the whole circuit including the tape deck mechanism. (A 1A, 18V AC secondary transformer with 4700µF, 40V electrolytic capacitor and 78M12 regulator is sufficient.) You can use any kind of tape deck mechanism with this circuit. Use of goodquality playback head and well-screened wires are recommended.
TELEPHONE CALL ME TER USING CALCULATOR AND COB
I
n this circuit, a simple calculator, in conjunction with a COB (chip-onboard) from an analogue quartz clock, is used to make a telephone call meter. The calculator enables conversion of STD/ ISD calls to local call equivalents and always displays current local call-meter reading. The circuit is simple and presents an elegant look, with feather-touch operation. It consumes very low current and is fully battery operated. The batteries used last more than a year. Another advantage of using this circuit is that it is compatible with any type of pulse rate format, i.e. pulse rate in whole number, or whole number with decimal part. Recently, the telephone department announced changes in pulse rate format, which included pulse rate in whole number plus decimal part. In such a case, this circuit proves very handy. To convert STD/ISD calls to local calls, this circuit needs accurate 1Hz clock pulses, generated by clock COB. This COB is found inside analogue quartz wall clocks or time-piece mechanisms. It consists of IC, chip capacitors, and crystal that one can retrieve from scrap quartz clock mechanisms. Normally, the COB inside clock mechanism will be in good condition. However, before using the COB, please check its serviceability by applying 1.5V DC across terminals C and D, as shown in the figure. Then check DC voltage across terminals A and B; these terminals in a clock are connected to a coil. If the COB is in good condition, the multimeter needle would deflect forward and backward once every second. In fact, 0.5Hz clock is available at terminals A and B, with a phase difference of 90o. The advantage of using this COB is that it works on a 1.5V DC source.
The clock pulses available from terminal A and B are combined using a bridge, comprising diodes D1 to D4, to
always be included before counting the calls. For making call in pulse rate 4, slide
obtain 1Hz clock pulses. These clock pulses are applied to the base of transistor T1. The collector and emitter of transistor T1 are connected across calculator’s ‘=’ terminals. The number of pulses forming an equivalent call may be determined from the latest telephone directory. However, the pulse rate (PR) found in the directory cannot be used directly in this circuit. For compatibility with this circuit, the pulse rate applicable for a particular place/distance, based on time of the day/holidays, is converted to pulse rate equivalent (PRE) using the formula PRE = 1/PR. You may prepare a look-up table for various pulse rates and their equivalents (see Table). Suppose you are going to make an STD call in pulse rate 4. Note down from the table the pulse rate equivalent for pulse rate 4, which is 0.25. Please note that on maturity of a call in the telephone exchange, the exchange call meter immediately advances to one call and it will be further incremented according to pulse rate. So one call should
switch S1 to ‘off’ (pulse set position) and press calculator buttons in the following order: 1, ‘+’, 0.25, ‘=’. Here, 1 is initial count, and 0.25 is PRE. Now calculator displays 1.25. This call meter is now ready to count. Now make the call, and as soon as the call matures, immediately slide switch S1 to ‘on’ (start/standby position). The COB starts generating clock pulses of 1 Hz. Transistor T1 conducts once every second, and thus ‘=’ button in calculator is activated electronically once every second. The calculator display starts from 1.25, advancing every second as follows: 1.25, 1.5, 1.75, 2.00, 2.25, 2.50, and so on. After finishing the call, immediately slide switch S1 to ‘off’ position (pulse set position) and note down the local call meter reading from the calculator display. If decimal value is more than or equal to 0.9, add another call to the whole number value. If decimal value is less than 0.9, neglect decimal value and note down only whole numbers. To store this local call meter reading into calculator memory, press ‘M+’ button. Now local call meter reading is stored in memory and is added to the previous local call meter reading. For continuous display of current local call meter reading, press ‘MRC’ button and slide switch S1 to ‘on’ (start/standby position). The current local call meter reading will blink
LOOKUP TABLE Pulse rate (PR) 2 Pulse rate eqlt. (PRE)
2.5
3
4
6
8
12
16
24
32
36
48
0.500 0.400 0.333 0.250 0.166 0.125 0.083 0.062 0.041 0.031 0.027 0.020
Note: Here PRE is shown up to three decimal places. In practice, one may use up to five
or six decimal places.
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once every second. In prototype circuit, the author used TAKSUN calculator. The display height was 1 cm. In this calculator, he substituted the two button-type batteries with two externally connected 1.5V R6 type batteries to run the calculator for more than an year. The power ‘off’ button terminals were made dummy by affixing cellotape on contacts to avoid erasing of memory, should someone accidentally press the power ‘off’ button. This calculator has auto ‘off’ facility. Therefore, some button needs to be pressed frequently to keep the calculator ‘on’. So, in the idle condition, the ‘=’ but-
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ELECTRONICS PROJECTS Vol. 21
ton is activated electronically once every second by transistor T1, to keep the calculator continuously ‘on’. Useful hints. Solder the ‘=’ button terminals by drilling small holes in its vicinity on PCB pattern using thin copper wire and solder it neatly, such that the ‘=’ button could get activated electronically as well as manually. Take the copper wire through a hole to the backside of the PCB, from where it is taken out of the calculator as terminals G and H. At calculator’s battery terminals, solder two wires to ‘+’ and ‘–’ terminals. These wires are also taken out from cal-
culator as terminals E and F. Affix COB on a general-purpose PCB and solder the remaining components neatly. For giving the unit an elegant look, purchase a jewellery plastic box with flip-type cover (size 15cm x 15cm). Now fix the board, calculator, and batteries, along with holder inside the jewellery box. Then mount the box on the wall and paste the look-up table inside the box cover in such a way that on opening the box, it is visible on left side of the box. Caution. The negative terminals of battery A and battery B are to be kept isolated from each other for proper operation of this circuit.
Telephone Conversation Recorder
T
his circuit enables automatic switching-on of the tape recorder when the handset is lifted. The tape recorder gets switched off when the handset is replaced. The signals are suitably attenuated to a level at which they can be recorded using the 'MICIN' socket of the tape recorder. Points X and Y in the circuit are connected to the telephone lines. Resistors R1 and R2 act as a voltage divider. The voltage appearing across R2 is fed to the 'MIC-IN' socket of the tape recorder. The values of R1 and R2 may be changed depending on the input impedance of the tape recorder's 'MIC-IN' terminals. Capacitor C1 is used for blocking the flow of DC. The second part of the circuit controls relay RL1, which is used to switch on/off the tape recorder. A voltage of 48 volts appears across the telephone lines in on-hook condition. This voltage drops to about 9 volts when the handset is lifted. Diodes D1 through D4 constitute a bridge rectifier/polarity guard. This
ELECTRONICS PROJECTS Vol. 21
ensures that transistor T1 gets voltage of proper polarity, irrespective of the polarity of the telephone lines. During on-hook condition, the output from the bridge (48V DC) passes through 12V zener D5 and is applied to the base of transistor T1 via the voltage divider comprising resistors R3 and R4. This switches on transistor T1 and its collector is pulled low. This, in turn, causes transistor T2 to cut off and relay RL1 is not energised. When the telephone handset is lifted, the voltage across points X and Y falls below 12 volts and so zener diode D5 does not conduct. As a result, base of transis-
tor T1 is pulled to ground potential via resistor R4 and thus is cut off. Thus, base of transistor T2 gets forward biased via resistor R5, which results in the energisation of relay RL1. The tape recorder is switched 'on' and recording begins. The tape recorder should be kept loaded with a cassette and the record button of the tape recorder should remain pressed to enable it to record the conversation as soon as the handset is lifted. Capacitor C2 ensures that the relay is not switched on-and-off repeatedly when a number is being dialled in pulse dialling mode.
TELEPHONE LINE BASED AUDIO MUTING AND LIGHT-ON CIRCUIT
V
ery often when enjoying music or watching TV at high audio level, we may not be able to hear a telephone ring and thus miss an important incoming phone call. To overcome this situation, the circuit presented here can be used. The circuit would automatically light a bulb on arrival of a telephone ring and simultaneously mute the music system/TV audio for the duration the telephone handset is off-hook. Lighting of the bulb would not only indicate an incoming call but also help in locating the telephone during darkness. On arrival of a ring, or when the handset is offhook, the inbuilt transistor of IC1 (opto-coupler) conducts and capacitor C1 gets charged and, in turn, transistor T1 gets forward biased. As a result, transistor T1 conducts, causing energisation of relays RL1, RL2, and RL3. Diode D1 connected in antiparallel to inbuilt diode of IC1, in shunt with resistor R1, provides an easy path for AC current and helps in limiting the voltage across inbuilt diode to a safe value during the ringing. (The RMS value of ring voltage lies between 70 and 90 volts RMS.) Capacitor C1 maintains necessary voltage for continuously forward biasing transistor T1 so that the relays are not de-energised during the negative half cycles and off-period of ring signal. Once the handset is picked up, the
relays will still remain energised because of low-impedance DC path available (via cradle switch and handset) for
The timer IC2 (555) is configured in monostable mode and connected between transistor T1 and relay units provides a
the in-built diode of IC1. After completion of call when handset is placed back on its cradle, the low-impedance path through handset is no more available and thus relays RL1 through RL3 are deactivated. As shown in the figure, the energised relay RL1 contacts switch on the light, while energisation of relay RL2 causes the path of TV speaker lead to be opened. (For dual-speaker TV, replace relay RL2 with a DPDT relay of 6V, 200 ohm.)
holding time of around 0.5 minutes. Similarly, energisation of DPDT relay RL3 opens the leads going to the speakers and thus mutes both audio speakers. Use ‘N/C’ contacts of relay RL3 in series with speakers of music system and ‘N/C’ contacts of RL2 in series with TV speaker. Use ‘N/O’ contact of relay RL1 in series with a bulb to get the visual indication of an incoming call as well as light during off-hook period.
ELECTRONICS PROJECTS Vol. 21
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U
TELEPHONE RINGER USING TIMER ICs
sing modulated rectangular waves of different time periods, the circuit presented here produces ringing tones similar to those produced by a telephone. The circuit requires four astable multivibrators for its working. Therefore two 556 ICs are used here. The IC 556
contains two timers (similar to 555 ICs) in a single package. One can also assemble this circuit using four separate 555 ICs. The first multivibrator produces a rectangular waveform with 1-second ‘low’ duration and 2-second ‘high’ duration. This waveform is used to control the next multivibrator that produces
another rectangular waveform. A resistor R7 is used at the collector of transistor T2 to prevent capacitor C3 from fully discharging when transistor T2 is conducting. Preset VR1 must be set at such a value that two ringing tones are heard in the loudspeaker in one second. The remaining two multivibrators are used to produce ringing tones corresponding to the ringing pulses produced by the preceding multivibrator stages. When switch S1 is closed, transistor T1 cuts off and thus the first multivibrator starts generating pulses. If this switch is placed in the power supply path, one has to wait for a longer time for the ringing to start after the switch is closed. The circuit used also has a provision for applying a drive voltage to the circuit to start the ringing. Note that the circuit is not meant for connection to the telephone lines. Using appropriate drive circuitry at the input (across switch S1) one can use this circuit with intercoms, etc. Since ringing pulses are generated within the circuit, only a constant voltage is to be sent to the called party for ringing. EFY Lab note. To resemble the actual telephone ringing a 400 Hz tone is switched on in the following sequence: 400ms on, 200ms off, 400ms on and 2000ms off and then repeat.
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Telecom Headset
A
compact, inexpensive and low component count telecom head-set can be constructed using two readily available transistors and a few other electronic components. This circuit is very useful for hands-free operation of EPABX and pager communication. Since the circuit draws very little current, it is ideal for parallel operation with electronic telephone set. Working of the circuit is simple and straightforward. Resistor R1 and an ordinary neon glow-lamp forms a complete visual ringer circuit. This simple arrangement does not require a DC blocking capacitor because, under idle conditions, the telephone line voltage is insufficient to ionise the neon gas and thus the lamp does not light. Only when the ring signal is being received, it flashes at the ringing rate to indicate an incoming call. The bridge rectifier using diodes D1 through D4 acts as a polarity guard which protects the electronic circuit from any reversal in the telephone line polarity. Zener diode D5 at the output of this bridge rectifier is used for additional circuit protection. Section comprising transistor T1, resistors R2, R3 and zener diode D6 forms a constant voltage regulator that provides a low voltage output of about 5 volts. Dial tone and speech signals from exchange are coupled to the audio amplifier stage built around transistor T2 and related parts, i.e. resistors R7, R6 and capacitor C5. Amplified signals from col-
lector of transistor T2 are coupled to dynamic receiver RT-200 (used as earpiece) via capacitor C7. A condenser microphone, connected as shown in the circuit, is used as transmitter. Audio signals developed across the microphone are coupled to the base of transistor T1 via capacitor C3. Resistor R4 determines the DC bias required for the microphone. After amplification by transistor T1, the audio signals are coupled to the telephone lines via the
diode bridge. The whole circuit can be wired on a very small PCB and housed in a medium size headphone, as shown in the illustration. For better results at low line currents, value of resistor R2 may be reduced after testing.
ELECTRONICS PROJECTS Vol. 20
CIRCUIT
IDEAS
Telephone Number Display BHASKAR BANERJEE
T
he given circuit, when connected in parallel to a telephone, displays the number dialled from the telephone set using the DTMF mode. This circuit can also show the number dialled from the phone of the called party. This is particularly helpful for receiving any number over the phone lines. The DTMF signal—generated by the phone on dialling a number—is decoded by DTMF decoder CM8870P1 (IC1), which converts the received DTMF signal into its equivalent BCD number that corresponds to the dialled number. This binary number is stored sequentially in 10 latches each time a number is dialled from the phone. The first number is stored in IC5A (1/2 of CD4508) while the second number is stored in IC5B and so on. The binary output from IC1 for digit ‘0’ as decoded by IC1 is 10102 (=1010), and this cannot be displayed by the seven-segment decoder, IC10. Therefore the binary output of IC1 is passed through a logic-circuit which converts an input of ‘10102’ into ‘00002’ without affecting the inputs ‘1’ through ‘9’. This is accomplished by gates N13 through N15 (IC11) and N1 (IC12). The storing of numbers in respective latches is done by IC2 (4017). The data valid output from pin 15 of IC1 is used to clock IC2. The ten outputs of IC2 are sequentially connected to the store and clear inputs of all the latches, except the last one, where the clear input is tied to ground. When an output pin of IC2 is high, the corresponding latch is cleared of previous data and kept ready for storing new data. Then, on clocking IC2, the same pin becomes low and the data present at the inputs of that latch at that instant gets stored and the next latch is cleared and kept ready. The similar input and output pins of all latches are connected together to ELECTRONICS FOR YOU n MAY '99
AINA
R. R
CIRCUIT form two separate input and output buses. There is only one 7-segment decoder/ driver IC10 for all the ten displays. This not only reduces size and cost but reduces power requirement too. The output from a latch is available only when its disable pins (3 and 15) are brought low. This is done by IC3, IC12 and IC13. IC3 is clocked by an astable multivibrator IC4 (555). IC3 also drives the displays by switching corresponding transistors. When a latch is enabled, its corresponding display is turned on and the content of that latch, after de-
coding by IC10, gets displayed in the corresponding display. For instance, contents of IC5A are displayed on display ‘DIS1,’ that of IC5B on ‘DIS2’ and so on. The system should be connected to the telephone lines via a DPDT switch (not shown) for manual switching, otherwise any circuit capable of sensing handset’s off-hook condition and thereby switching relays, etc. can be used for automatic switching. The power-supply switch can also be replaced then. Such circuits, under different captions, can be found in EFY’s back issues. Though this circuit is capable of showing a maxi-
ELECTRONICS FOR YOU n MAY '99
IDEAS
mum of ten digits, one can reduce the display digits as required. For doing this, connect the reset pin of IC2, say, for a 7-digit display, with S6 output at pin 5. The present circuit can be built on a veroboard and housed in a suitable box. The displays are common-cathode type. To make the system compact, small, 7segment displays can be used but with some extra cost. Also, different colour displays can be used for the first three or four digits to separate the exchange code/STD code, etc. The circuit can be suitably adopted for calling-line display.
Teleremote Control
H
ere is a teleremote circuit which enables switching ‘on’ and ‘off’ of appliances through telephone lines. It can be used to switch appliances from any distance, overcoming the limited range of infrared and radio remote controls. The circuit can be used to switch up to nine appliances (corresponding to the digits 1 through 9 of the telephone key-pad). The DTMF signals on telephone instrument are used as control signals. The digit ‘0’ in DTMF mode is used to toggle between the appliance mode and normal telephone operation mode. Thus the telephone can be used to switch on or switch off the appliances also while being used for normal conversation. The circuit uses IC KT3170 (DTMF-to-BCD converter), 74154 (4-to-16-line demult-iplexer), and five CD4013 (D flip-flop) ICs. The working of the circuit is as follows. Once a call is established (after hearing ring-back tone), dial ‘0’ in DTMF mode. IC1 decodes this as ‘1010,’ which is further demultiplexed by IC2 as output O10 (at pin 11) of IC2 (74154). The active low output of IC2, after inversion by an inverter gate of IC3 (CD4049), becomes logic 1. This is used to toggle flip-flop-1 (F/F-1) and relay RL1 is energised. Relay RL1 has two changeover contacts, RL1(a) and RL1(b). The energised RL1(a) contacts provide a 220-ohm loop across the telephone line while RL1(b) contacts inject a 10kHz tone on the line, which indicates to the caller that appliance mode has been selected. The 220-ohm loop on telephone line disconnects the ringer from the telephone line in the exchange. The line is now connected for appliance mode of operation. If digit ‘0’ is not dialed (in ELECTRONICS PROJECTS Vol. 20
DTMF) after establishing the call, the ring continues and the telephone can be used for normal conversation. After selection of the appliance mode of operation, if digit ‘1’ is dialed, it is decoded by IC1 and its output is ‘0001’. This BCD code is then demultiplexed by 4-to-16-line demultiplexer IC2 whose corresponding output, after inversion by a CD4049 inverter gate, goes to logic 1 state.
This pulse toggles the corresponding flip-flop to alternate state. The flip-flop output is used to drive a relay (RL2) which can switch on or switch off the appliance connected through its contacts. By dialing other digits in a similar way, other appliances can also be switched ‘on’ or ‘off .’ Once the switching operation is over, the 220-ohm loop resistance and 10kHz
tone needs to be removed from the telephone line. To achieve this, digit ‘0’ (in DTMF mode) is dialed again to toggle flip-flop-1 to de-energise relay RL1, which terminates the loop on line and the 10kHz tone is also disconnected. The telephone line is thus again set free to receive normal calls. This circuit is to be connected in parallel to the telephone instrument.
ELECTRONICS PROJECTS Vol. 20
CIRCUIT
IDEAS
Time Switch his circuit is especially designed for those who often need to wake up early in the morning. Ordinary alarms in electronic watches are not loud enough and very often they fail to wake up. The switch circuit described here will come handy; it can be used to switch on a TV, radio or tape recorder etc, which will not allow even the laziest amongst us to ignore their sound for too long. Besides, this time switch can also be used to
T
out having to flip the mode switch (i.e. mode switch can be omitted). Please refer to the back panel diagram of a typical analogue clock and the audio jack, to see how the existing buzzer of the clock is required to be wired to the audio output from the clock. This will ensure that when plug is inserted in the audio jack, the clock’s buzzer will remain off and not consume any power
the position of mode switch. At the time of alarm, when point A connected to collector of transistor T1 passes through logic 0 state, the output logic state of both the gates will toggle. Assuming that mode switch is flipped to ‘Mode Off’ position at poweron-reset (when point D is at logic 1), initially diode D1 would be in blocking state and transistor T2 would be forward biased via resistor R5 and diodes D2 and D3. As a result, the relay is in energised state, which makes output power available at output socket1 and cuts it off from socket-2. At alarm time, the audio signal toggles logic output states of both gates N1 and N2. As a result, point D goes to logic 0 state. Diode D1 conducts, taking the voltage at junction of diodes D1 and D2 to near about 1 volt. Diode D3 ensures that its series combination with
switch on/off any other electric or electronic gadget at any time. What you need is a simple analogue electronic clock with alarm facility and a small circuit to implement the time switch. This time switch has two modes. One is ‘time-on’ mode and the other is ‘time-off ’ mode. In time-on mode, you set up the alarm in your clock as per normal procedure and at the set time this switch turns on the gadget connected at the output socket-1. In timeoff mode, it turns your gadget off at the set time. The optional output socket-2 is wired in such a way that when you use this socket, the mode changes with-
unnecessarily. The audio alarm output from the clock is coupled to the AF detector built around low-power switching transistor T1. During alarm, the collector of transistor T1 will fluctuate around ground level and Vcc. During absence of audio alarm input, the collector of transistor T1 is held at Vcc potential. The next stage consists of an S-R latch built around NAND gates N1 and N2. Capacitor C2 and resistor R4 are used for power-on-reset. On switching the power supply, gate N2 output will acquire logic 1 and that of gate N1 logic 0. This is the initial state, irrespective of
diode D2 puts them in blocking mode. Capacitor C3 meanwhile discharges via resistor R6 and the voltage at base of transistor T2 approaches towards ground level, cutting off transistor T2 and de-energising relay RL1. Now the power at output socket-1 would be cut off while it becomes available in socket-2. If the above operation is repeated with switch S1 in ‘Mode On,’ the power would initially not be available in socket-1 (but available in socket-2). But after the alarm, the power would become available in socket-1 and not in socket-2.
ATA Y. K
RIA
AVNISH PUNDIR
ELECTRONICS FOR YOU n MARCH '99
D
Tiny Dew Sensor
ew (condensed moisture) adversely affects the normal performance of sensitive electronic devices. A low-cost circuit described here can be used to switch off any gadget automatically in case of excessive humidity. At the heart of the circuit is an inexpensive (resistor type) dew sensor element. Although dew sensor elements are widely used in video cassette players and recorders, these may not be easily available in local market. However, the
ELECTRONICS PROJECTS Vol. 20
same can be procured from authorised service centres of reputed companies. The author used the dew sensor for FUNAI VCP model No. V.I.P. 3000A (Part No: 6808-08-04, reference no. 336) in his prototype. In practice, it is observed that all dew sensors available for video application possess the same electrical characteristics irrespective of their physical shape/size, and hence are interchangeable and can be used in this project. The circuit is basically a switching type circuit made with the help of a
popular dual op-amp IC LM358N which is configured here as a comparator. (Note that only one half of the IC is used here.) Under normal conditions, resistance of the dew sensor is low (1 kilo-ohm or so) and thus the voltage at its non-inverting terminal (pin 3) is low compared to that at its inverting input (pin 2) terminal. The corresponding output of the comparator (at pin 1) is accordingly low and thus nothing happens in the circuit. When humidity exceeds 80 per cent, the sensor resistance increases rapidly. As a result, the non-inverting pin becomes more positive than the inverting pin. This pushes up the output of IC1 to a high level. As a consequence, the LED inside the opto-coupler is energised. At the same time LED1 provides a visual indication. The opto-coupler can be suitably interfaced to any electronic device for switching purpose. Circuit comprising diode D1, resistors R8 and R6 and capacitor C1 forms a lowvoltage, low-current power supply unit. This simple arrangement obviates the requirement for a bulky and expensive step-down transformer.
TOUCH-SENSITIVE MUSICAL BELL WITH TIMER
T
his circuit is built around CMOS IC CD4011 and popular melody generator IC UM66. When touch plates are bridged by hand for a moment. the circuit starts to generate music. After a few seconds the music automatically stops. Maximum supply voltage for this circuit is +5 volts. The IC UM66 can not operate beyond 3.3V voltage. IC 7805 regulator based power supply can be used to power this circuit. Time delay can be changed by chang-
ing values of capacitor C1 and resistor R2. Three silicon diodes connected in series between pin 2 of UM66 IC and positive
5-volt rail keep voltage applied to pin2 of UM66 below 3.2 volts because of the drop of approximately 1.8 volts across them.
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CIRCUIT IDEAS
AUTOMATED TRAFFIC SIGNAL CONTROLLER VIKRAM BANERJEE MRINAL KANTI MANDAL DR ANIRUDHA GHOSAL
RUP
T
NA
of 8 seconds each. For the left- and rightturning traffic and pedestrians crossing from north to south, south to north, east to west, and west to east, only green and red signals are used. Table I shows the simultaneous states of the signals for all the traffic. Each row represents the status of a signal for 8
sity is high. This controller allows the pedestrians to safely cross the road during certain periods. 3. The controller uses digital logic, which can be easily implemented by using logic gates. 4. The controller is a generalised one and can be used for different roads with
his automated traffic signal controller can be made by suitably programming a GAL device. (For GAL programming you may refer to the con-
ANJA
Fig. 1: Flow of traffic in all possible directions
TABLE I Simultaneous States of Signals for All the Traffic X
Y
Z
B-C/B-G Lt/Rt
B-E St
D-E/D-A Lt/Rt
D-G St
F-G/F-C Lt/Rt
F-A St
H-A/H-E Lt/Rt
HC St
WALK (N-S)/(S-N)
WALK (E-W)/(W-E)
0 0 0 0 1 1 1 1
0 0 1 1 0 0 1 1
0 1 0 1 0 1 0 1
R R R G R R R R
R G G Y R R R R
R R R R R R R G
R R R R R G G Y
G R R R R R R R
G G Y R R R R R
R R R R G R R R
R R R R G G Y R
R G G R R R R R
R R R R R G G R
struction project published on page 52 in EFY’s September issue.) Its main features are: 1. The controller assumes equal traffic density on all the roads. 2. In most automated traffic signals the free left-turn condition is provided throughout the entire signal period, which poses difficulties to the pedestrians in crossing the road, especially when the traffic den-
slight modification. 5. The control can also be exercised manually when desired. The time period for which green, yellow, and red traffic signals remain ‘on’ (and then repeat) for the straight moving traffic is divided into eight units of 8 seconds (or multiples thereof) each. Fig. 1 shows the flow of traffic in all permissible directions during the eight time units
seconds. As can be observed from the table, the ratio of green, yellow, and red signals is 16:8:40 (=2:1:5) for the straight moving traffic. For the turning traffic the ratio of green and red signals is 8:56 (=1:7), while for pedestrians crossing the road the ratio of green and red signals is 16:48 (=2:6). In Table II (as well as Table I) X, Y, and Z are used as binary variables to NOVEMBER 2002
ELECTRONICS FOR YOU
CIRCUIT IDEAS TABLE II Boolean Functions for All the Signal Conditions Signal
Reference
Boolean functions
Green B-C(Lt)/B-G (Rt) X’YZ Green B-E (St) XYZ’+X’Y’Z Red B-E (St) X+Y’Y’Z’ Yellow B-E (St) X’YZ Green D-E (Lt)/D-A (Rt) XYZ Green D-G (St) XYZ’+XY’Z Red D-G (St) X’+XY’Z’ Yellow D-G (St) XYZ Green F-G(Lt)/F-C (Rt) X’Y’Z’ Green F-A (St) X’Y’ Red F-A (St) X+X’YZ Yellow F-A (St) X’YZ’ Green H-A (Lt)/H-E (Rt) XY’Z’ Green H-C (St) XY’ Red H-C (St) X’+XYZ Yellow H-C (St) XYZ’ Green Walk (N-S/S-N) X’YZ’+X’Y’Z Green Walk (E-W/W-E) XYZ’+XY’Z Note. X’, Y’, and Z’ denote complements of variables X, Y, and Z, respectively.
Fig. 2: The circuit diagram for traffic light signalling ELECTRONICS FOR YOU
NOVEMBER 2002
depict the eight states of 8 seconds each. Letters A through H indicate the left and right halves of the roads in four directions as shown in Fig. 1. Two letters with a dash in between indicate the direction of permissible movement from a road. Straight direction is indicated by St, while left and right turns are indicated by Lt and Rt, respectively. The Boolean functions for all the signal conditions are shown in Table II. The left- and the right-turn signals for the traffic have the same state, i.e. both are red or green for the same duration, so their Boolean functions are identical and they should be connected to the same con-
trol output. The circuit diagram for realising these Boolean functions is shown in Fig. 2. Timer 555 (IC1) is wired as an astable multivibrator to generate clock signal for the 4-bit counter 74160 (IC2). The time duration of IC1 can be adjusted by varying the value of resistor R1, resistor R2, or capacitor C2 of the clock circuit. The ‘on’ time duration T is given by the following relationship: T = 0.695C2(R1+R2) IC2 is wired as a 3-bit binary counter by connecting its Q3 output to reset pin 1 via inverter N1. Binary outputs Q2, Q1, and Q0 form variables X, Y, and Z, respectively. These outputs, along with their complimentary outputs X’, Y’, and Z’, respectively, are used as inputs to the rest of the logic circuit to realise various outputs satisfying Table I. You can simulate various traffic lights using green, yellow, and red LEDs and feed the outputs of the circuit to respective LEDs via current-limiting resistors of 470 ohms each to check the working of the circuit. Here, for turning traffic and pedestrians crossing the road, only green signal is made available. It means that for the remaining period these signals have to be treated as ‘red’. In practice, the outputs of Fig. 2 should be connected to solidstate relays to operate high-power bulbs. Further, if a particular signal condition (such as turning signal) is not applicable to a given road, the output of that signal condition should be
CIRCUIT IDEAS connected to green signal of SIG-B SIG-D SIF-F SIG-H WALK(N-S) WALK(E-W) the next GGRY GGRY GGRY GGRY GR GR state (refer 0010 0100 1100 0010 01 01 Table I). 0100 0100 0100 0010 10 01 T h e 0100 0100 0001 0010 10 01 traffic sig1001 0100 0010 0010 01 01 nals can 0010 0100 0010 1100 01 01 also be 0010 1000 0010 0100 01 10 controlled 0010 1000 0010 0001 01 10 manually, 0010 0011 0010 0010 01 01 if desired. Note. The first column under G (green) in each group of four signals indicates the Any signal turn signal, while the next three columns under GRY indicate signal for the straight state can be traffic. established Table III Execution Results of Software Program
by entering the binary value corresponding to that particular state into the parallel input pins of the 3-bit counter. Similarly, the signal can be reset at any time by providing logic 0 at the reset pin (pin 1) of the counter using an external switch. A software program to verify the functioning of the circuit using a PC is given below. (Source code and executable file will be provided in the next month’s EFY-CD.) When executing the program, keep pressing Enter key to get the next row of results. The test results on execution of the program is shown in Table III. This circuit costs around Rs 125.
TRAFFIC.C #include #include #define TRUE 1 #define False 0 int not(int x); int or2(int x,int y); int or3(int x,int y,int z); int and2(int x,int y); int and3(int x,int y,int z); int main(void) { int a,b,c; int seq,green_bl,green_bs,red_bs,yellow_bs; int green_dl,green_ds,red_ds,yellow_ds; int green_fl,green_fs,red_fs,yellow_fs; int green_hl,green_hs,red_hs,yellow_hs; int walk_ns,stop_ns; int walk_ew,stop_ew; clrscr(); printf(“ SIG-B SIG-D SIF-F SIG-H WALK(N-S) WALK(E-W)\n”); printf(“G G R Y G G R Y G G R Y G G R Y GR G R\n”); for(seq=0;seq<8;seq++) {
c=(seq&1);b=(seq&2)>>1;a=(seq&4)>>2; green_bl=and3(not(a),b,c); green_bs=or2(and3(not(a),b,not(c)),and3(not(a),not(b),c)); red_bs=or2(a,and3(not(a),not(b),not(c))); yellow_bs=and3(not(a),b,c); green_dl=and3(a,b,c); green_ds=or2(and3(a,b,not(c)),and3(a,not(b),c)); red_ds=or2(not(a),and3(a,not(b),not(c))); yellow_ds=and3(a,b,c); green_fl=and3(not(a),not(b),not(c)); green_fs=and2(not(a),not(b)); red_fs=or2(a,and3(not(a),b,c)); yellow_fs=and3(not(a),b,not(c)); green_hl=and3(a,not(b),not(c)); green_hs=and2(a,not(b)); red_hs=or2(not(a),and3(a,b,c)); yellow_hs=and3(a,b,not(c)); walk_ns=green_bs; stop_ns=or3(and3(not(a),not(b),not(c)),and3(not(a),b,c),a); walk_ew=green_ds; stop_ew=or3(not(a),and3(a,b,c),and3(a,not(b),not(c))); printf(“%d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d\n”, green_bl,green_bs,red_bs,yellow_bs, green_dl,green_ds,red_ds,yellow_ds, green_fl,green_fs,red_fs,yellow_fs, green_hl,green_hs,red_hs,yellow_hs,
walk_ns,stop_ns, walk_ew,stop_ew); getch(); } return; } int and2(int x,int y) { return(x && y); } int and3(int x,int y,int z) { return(x && y && z); } int or2(int x,int y) { return(x || y); } int or3(int x,int y,int z) { return(x || y || z); } int not(int x) { return(!x); }
NOVEMBER 2002
ELECTRONICS FOR YOU
Ultra Low Drop Linear Regulator
T
he circuit is a MOSFET based linear voltage regulator with a voltage drop of as low as 60 mV at 1 ampere. Drop of a fewer millivolts is possible with better MOSFETs having lower RDS(on) resistance. The circuit in Fig. 1 uses 15V-0-15V secondary output from a step-down transformer and employs an n-channel MOSFET IRF540 to get the regulated 12V output from DC input, which could be as low as 12.06V. The gate drive voltage required for the MOSFET is generated using a voltage doubler circuit consisting of diodes D1 and D2 and capacitors C1 and C4. To turn the MOSFET fully on, the gate terminal should be around 10V above the source terminal which is connected to the output here. The voltage doubler feeds this voltage to the gate through resistor R1. Adjustable shunt regulator TL431 (IC2) is used here as an error amplifier, and it dynamically adjusts the gate voltage to maintain the regulation at the output. With adequate heatsink for the MOSFET, the circuit can provide up to 3A output at slightly elevated minimum voltage
Fig. 1
ELECTRONICS PROJECTS Vol. 20
drop. Trimpot VR1 in the circuit is used for fine adjustment of the output voltage. Combination of capacitor C5 and resistor R2 provides error-amplifier compensation. The circuit is provided with a short-circuit crow-bar protection to guard the components against overstress during accidental short at the output. This crow-bar protection will work as follows: Under Fig. 2 normal working conditions, the voltage across capacitor C3 will be 6.3V and diode D5 will be in the off state since it will be reverse-biased with the output voltage of 12V. However, during output short-circuit condition, the output will momentarily drop, causing D5 to conduct and the opto-triac MOC3011 (IC1) will get
triggered, pulling down the gate voltage to ground, and thus limiting the output current. The circuit will remain latched in this state, and input voltage has to be
switched off to reset the circuit. The circuit shown in Fig. 2 follows a similar scheme. It can be utilised when the regulator has to work from a DC rail in place of 15V-0-15V AC supply. The gate voltage here is generated using an LM555 charge pump circuit as follows: When 555 output is low, capacitor C2 will get charged through diode D1 to the input voltage. In the next half cycle, when the 555 output goes high, capacitor C3 will get charged to almost double the input voltage. The rest of the circuit works in a similar fashion as the circuit of Fig. 1. The above circuits will help reduce power-loss by allowing to keep input voltage range to the regulator low during initial design or even in existing circuits. This will keep the output regulated with relatively low input voltage compared to the conventional regulators. The minimum voltage drop can be further reduced using low R DS(on) MOSFETs or by paralleling them.
CIRCUIT
IDEAS
UNDER-/OVER-VOLTAGE BEEP FOR MANUAL STABILISER
MAR IL KU SUN
K. UDHAYA KUMARAN anual stabilisers are still popular because of their simple construction, low cost, and high reliability due to the absence of any relays while covering a wide range of mains AC voltages compared to that handled by automatic voltage stabilisers. These are used mostly in homes and in business centres for loads such as lighting, TV, and fridge, and in certain areas where the mains AC voltage fluctuates between very low (during peak hours) and abnormally high (during non-peak hours). Some manual stabilisers available in the market incorporate the high-voltage
M
eration is very irritating and inconvenient for the user. This under-/over-voltage audio alarm circuit designed as an add-on circuit for the existing manual stabilisers overcomes the above problem. Whenever the stabiliser’s output voltage falls below a preset low-level voltage or rises above a preset high-level voltage, it produces different beep sounds for ‘high’ and ‘low’ voltage levels—short-duration beeps with short intervals between successive beeps for ‘high’ voltage level and slightly longerduration beeps with longer interval between successive beeps for ‘low’ voltage
auto-cut-off facility to turn off the load when the output voltage of manual stabiliser exceeds a certain preset high voltage limit. The output voltage may become high due to the rise in AC mains voltage or due to improper selection by the rotary switch on manual stabiliser. One of the major disadvantage of using a manual stabiliser in areas with a wide range of voltage fluctuations is that one has to keep a watch on the manual stabiliser’s output voltage that is displayed on a voltmeter and keep changing the same using its rotary switch. Or else, the output voltage may reach the preset autocut-off limit to switch off the load without the user’s knowledge. To turn on the load again, one has to readjust the stabiliser voltage using its rotary switch. Such op-
level. By using these two different types of beep sounds one can readily readjust the stabiliser’s AC voltage output with the help of the rotary switch. There is no need of frequently checking voltmeter reading. It is advisable to preset the high-level voltage 10V to 20V less than the required high-voltage limit for auto-cut-off operation. Similarly, for low level one may preset low-level AC voltage 20V to 30V above minimum operating voltage for a given load. The primary winding terminals of step-down transformer X1 are connected to the output terminals of the manual stabiliser. Thus, 9V DC available across capacitor C1 will vary in accordance with the voltage available at the output terminals of the manual stabiliser, which is ELECTRONICS FOR YOU ❚ MAY 2001
used to sense high or low voltage in this circuit. Transistor T1 in conjunction with zener diode ZD1 and preset VR1 is used to sense and adjust the high-voltage level for beep indication. Similarly, transistor T2 along with zener ZD2 and preset VR2 is used to sense and adjust low voltage level for beep indication. When the DC voltage across capacitor C1 rises above the preset high-level voltage or falls below the preset low-level voltage, the collector of transistor T2 becomes high due to non-conduction of transistor T2, in either case. However, if the DC voltage sampled across C1 is within the preset high- and low-level voltage, transistor T2 conducts and its collector voltage gets pulled to the ground level. These changes in the collector voltage of transistor T2 are used to start or stop oscillations in the astable multivibrator circuit that is built around transistors T3 and T4. The collector of transistor T4 is connected to the base of buzzer driver transistor T5 through resistor R8. Thus when the collector voltage of transistor T4 goes high, the buzzer sounds. Preset VR3 is used to control the volume of buzzer sound. In normal condition, the DC voltage sampled across capacitor C1 is within the permissible window voltage zone. The base of transistor T3 is pulled low due to conduction of diode D2 and transistor T2. As a result, capacitor C2 is discharged. The astable multivibrator stops oscillating and transistor T4 starts conducting because transistor T3 is in cut-off state. No beep sound is heard in the buzzer due to conduction of transistor T4 and non-conduction of transistor T5. When the DC voltage across capacitor C1 goes above or below the window voltage level, transistor T2 is cut off. Its collector voltage goes high and diode D2 stops conducting. Thus there is no discharge path for capacitor C2 through diode D2. The astable multivibrator starts
CIRCUIT
oscillating. The time period for which the beep is heard and the time interval between two successive beeps are achieved with the help of the DC supply voltage, which is low during low-level voltage sampling and high during high-level voltage
IDEAS
sampling. The time taken for charging capacitors C2 and C3 is less when the DC voltage is high and slightly greater when the DC voltage is low for astable multivibrator operation. Thus during lowlevel voltage sensing the buzzer beeps for
ELECTRONICS FOR YOU ❚ MAY 2001
longer duration with longer interval between successive beeps compared to that during high-voltage level sensing. This circuit can be added to any existing stabiliser (automatic or manual) or UPS to monitor its performance.
UPS FOR CORDLESS TELEPHONES
C
ordless telephones are very popular nowadays. But they have a major drawback, i.e. they cannot be operated during power failure. Therefore usually another ordinary telephone is connected in parallel to the
Fig. 1: Block diagram of UPS.
cordless telephone. This results in lack of secrecy. UPS is a permanent solution to this problem.
out, irrespective of the presence of the AC mains. When the AC mains is present, the same is converted into DC and fed to the inverter. A part of the mains rectified output is used to charge the battery. When the mains power fails, the DC supply to the inverter is from the battery and from this is obtained AC at the inverter output. This is shown in fig.1. The circuit wired around IC CD4047 is an astable multivibrator operating at a frequency of 50 Hz. The Q and Q outputs of this multivibrator directly drive power MOSFETS IRF540. The configuration used is push-pull type. The inverter output is filtered and the spikes are reduced using
Fig. 3: Proposed layout of front and rear panels.
battery, one may use two 6V, 4Ah batteries (SUNCA or any other suitable brand). The circuit can be easily assembled on a general-purpose PCB and placed inside a metal box. The two transformers may be mounted on the chassis of the box. Also,
Fig. 2: Circuit diagram of UPS
Since the UPS is meant only for the cordless telephone, its output power is limited to around 1.5W. This is sufficient to operate most cordless telephones. as these employ only small capacity adapters (usually 9V/12V, 500mA), to enable the operation of the circuit and to charge the battery present in the handset. The UPS presently designed is of online type. Here the inverter is ‘on’ through-
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ELECTRONICS PROJECTS Vol. 19
MOV (metal oxide varistor). The inverter transformer used is an ordinary 9V-0-9V, 1.5A mains transformer readily available in the market. Two LEDS (D6 and D7) indicate the presence of mains/battery. The mains supply (when present) is stepped down, rectified and filtered using diodes D1 through D4 and capacitor C1. A part of this supply is also used to charge the battery. In place of a single 12V, 4Ah
the two batteries can be mounted in the box using supporting clamps. The front and back panel designs are shown in the Fig. 3. The same circuit can deliver up to 100W, provided the inverter transformer and charging transformer are replaced with higher current rating transformers, so that the system can be used for some other applications as well.
circuit
ideas
Versatile CMOS/TTL Logic And Clock Probe
EFY Lab
F
or fault diagnosis of any logic circuit, you need a probe that can test the logic level or existence of clock activity. The circuit shown here can be used to test CMOS and TTL logic circuits for logic states and also for the presence of clock activity from a few hertz to more than 10 MHz, at any
point of the logic circuit. Supply for the probe circuit is taken from the circuit under test using alligator clips. In the circuit, LM319 dual-comparator is connected as a window detector. The non-inverting pin of comparator N1 is biased to nearly 2V when switch S1 is in TTL position and 80 per cent of Vcc in CMOS position. The output of N1
Test Results Test conditions
Specified level
Observed level
Red LED
Green LED
Yellow LED
Buzzer sound Off
TTL (5V) Low
<0.8V
<0.8V
Off
On
Off
High
>2V
≥2.1V
On
Off
Off
Off
Clock
TTL compatible
1 Hz to 10 MHz Off or even more
Momentarily on/off
On for 3 seconds
On for 3 seconds
Low
<2.5V
≤2.35V
Off
On
Off
Off
High
>9.5V
>9.5V
On
Off
Off
Off
Clock
CMOS compatible
1 Hz to 10 MHz Off or even more
Momentarily on/off
‘On’ for 3 seconds
‘On’ for 3 seconds
CMOS (12V)
9 2 • F e b r ua ry 2 0 0 8 • e l e c t ro n i c s f o r yo u
edi
s.c. dwiv
goes low only when logic input at the probe tip exceeds the biasing voltage and, as a result, the red LED lights up to indicate logic 1 state at the probe tip. Similarly, the inverting pin of comparator N2 is biased at nearly 0.8V (in TTL position of switch S1) and 20 per cent of Vcc (in CMOS position of switch S1). Only when the input volt-
age at probe tip is less than the biasing voltage, will its output drop low to light up the green LED to indicate logic 0 state. The probe tip is also connected to the input of CD4049 (N3) via capacitor C1 to pass AC/clock signals. It simply acts as a buffer and couples only the high-to-low going signals at the input/output of the gate to the input of next gate N4. The output of gate N4 is further coupled to gate N5, which is wired as a monostable. A positive feedback from w w w. e f y m ag . co m
circuit
ideas
the output of gate N5 to the input of gate N4 ensures that unless capacitor C4 (0.47µF) discharges sufficiently via 4.7-mega-ohm resistor, further clock pulses at the input of N4 will have no effect.
w w w. e f y m ag . co m
Gate N6 is used for driving a yellow LED (indicating oscillatory input at probe tip), which will be switched on for a brief period. The output of gate N6 is further used to inhibit/enable the oscillator formed by gates N7
and N8. It briefly activates the buzzer to beep during mono period, indicating oscillatory input at the probe tip. Thus we have audio-visual indication during clock/oscillatory input at the probe tip.
e l e c t ro n i c s f o r yo u • F e b r ua ry 2 0 0 8 • 9 3
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IDEAS
VERSATILE ZENER DIODE TESTER K. UDHAYA KUMARAN
Z
ener diodes available in the market are specified according to their breakdown voltage as well as tolerance. The tolerance may vary from 5 per cent to 20 per cent. The circuit of a versatile zener diode tester presented here enables you to verify the specified breakdown voltage and tolerance values. In addition, you can check the dynamic impedance of a zener diode. The dynamic impedance characteristics of a zener diode determine as to how well the zener diode regulates its own breakdown voltage. Thus this circuit can be used to compare the dynamic impedance characteristics of zener diodes from a lot and segregate/categorise them accordingly. For full-fledged zener diode testing you will have to refer to the manufacturer’s datasheet to check zener diode parameters such as zener voltage, power, and current (maximum/nominal) ratings. In addition, temperature coefficient and dynamic impedance have also to be checked if zener diode is to be used for critical functions such as voltage reference for TABLE I Minimum and Maximum Test Current Values Zener diode values IT(min) IT(max) 3.3V to 4.3V 10mA 15mA 4.7V to 18V 5mA 10mA 20V to 39V 2mA 4mA Note: Zener diode power ratings are 250 mW, 400 mW, and 500 mW. TABLE II Minimum and Maximum Test Current Values IT(max) Zener diode values IT(min) 3.3V to 12V 10mA 15mA 13V to 27V 5mA 10mA 30V to 43V 2mA 5mA 47V to 75V 1.5mA 3mA 82V to 120V 1mA 2mA Note: Zener diode power rating is 1 watt.
digital voltmeters, control systems, and precision power-supply circuits. However, for a common hobbyist it is not necessary to check zener diodes critically, and only checking its dynamic impedance characteristic is sufficient. Dynamic impedance implies the degree of change in a zener diode’s voltage with the change in current. Expressed in ohms, it equals the small change in zener
operation. In quick-test mode, you can perform a rough check of zener diode’s breakdown voltage up to 47 volts. In quality-test mode, you can check dynamic impedance characteristic for zener diodes from 3.3V to 120V. Commonly available step-down transformers X1 and X2 (230V AC primary to 9V AC, 750 mA sec. each) are connected back-to-back as shown in the figure. A bridge rectifier followed by filter capacitor C1 converts the output from X2 transformer to DC. Neon lamp L1 indicates the presence of higher DC voltage (220V approximately) across capacitor C1, which is used to test various zener diode values from 3.3V to 120V.
voltage divided by the corresponding change in zener current (centered around the test current figure prescribed in datasheets by manufacturers). From datasheets it is observed that test current value is high for low-voltage zener diodes and low for higher-voltage zener diodes. However, the dynamic impedance value will be low for low-voltage zener diodes and vice versa for higher-voltage zener diodes. To test 3.3V to 120V zener diodes by the practical dynamic impedance method, you need to have a variable voltage (0 to above 120V) and current (1 mA to 150 mA) supply source. Designing this type of power supply is quite complicated and is prone to damage if excess current is drawn accidentally. The zener diode tester circuit presented here has been designed considering the above factors. It is capable of testing zener diodes of breakdown voltage ratings of upto 120V and wattage ratings of 250 mW, 400 mW, 500 mW, and 1W. The circuit can be deployed in quicktest mode as also in quality-test mode of
An advantage of using this high-voltage circuit is that the current gets restricted to a low value. It delivers only 3 mA (approx.) when testing zener diodes with higher breakdown values (e.g. 120V zener diode), but while testing zener diodes of low breakdown values, such as 3.3V, it delivers a current slightly above 20 mA. Such power-supply characteristics suit our requirement, as stated earlier. Since a small current is used for testing of zener diodes, there is no danger of zener diodes getting damaged during testing using the dynamic impedance method. Before using the circuit, check DC voltage across test terminals A and B without connecting any zener diode and then flip toggle switch S2 to quick-test position. DC voltage available across terminals A and B will be around 200V DC. Now put toggle switch to quality-test position. DC voltage can now be adjusted from 6V DC to 200V DC (approx.) with the help of potentiometer VR1. After these preliminary checks, the circuit is ready for operation. To test zener diode by quick-test method, connect zener diode across termi-
MAR IL KU SUN
ELECTRONICS FOR YOU ❚ JUNE 2001
CIRCUIT
nals A and B and flip switch S1 to ‘on’ position. Note down DC voltage in digital multimeter M2, which is the rough breakdown voltage. In quick-test method you can test zener diode values up to 47 volts safely. For higher-value zener diodes you will have to increase the value of resistor R3 suitably. If zener diode presents a short, digital multimeter M2 will read ‘0’ volts. To perform quality test on the same zener diode, turn switch S1 ‘off’ and remove zener diode from across terminals A and B. Now turn switch S1 ‘on’ and adjust potentiometer VR1 to obtain DC voltage (on digital multimeter) across terminals A
IDEAS
and B equal to the one found during quick test method. Now keep potentiometer VR2 in mid position and connect zener diode across terminals A and B. (Note. Before testing zener diode, refer Table I and Table II for the minimum test current (ITmin) and maximum test current (ITmax) required for various zener diode values, depending upon their wattage rating.) Test current is adjusted using potentiometer VR2 and measured using meter M1 (A 0-25mA analogue milliampere meter or a 0-20mA digital multimeter can be used.)
ELECTRONICS FOR YOU ❚ JUNE 2001
Now adjust potentiometer VR2 and note down changes in zener voltage during ITmin and ITmax conditions. If the required current is not available, increase DC voltage by adjusting potentiometer VR1 suitably. While changing test current from ITmin to ITmax, the voltage variation across zener diode should be less than 1 volt for lower-value zener diodes and a few volts for higher-value zener diodes. A voltage variation of more than this value indicates that zener diode is not properly regulating. When comparing zener diodes of same values, the zeners showing less voltage deviation would regulate better.
WATCH-DOG FOR TELEPHONES
M
ost of the telephone security devices available in market are simple but quite expensive. These devices provide blinking or beeping type line-tap/misuse indications. Quite often they do not offer guaranteed protection against unauthorised operation. A very simple and unique circuit of a telephone watch-dog to safeguard subscriber telephone lines against any fraud is described here. This little circuit keeps continuous watch over the telephone lines and sounds
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ELECTRONICS PROJECTS Vol. 19
an alarm in case of any misuse. In addition it transmits a loud tone through the telephone lines to prevent further misuse. When switch S1 is turned on, the normal (on-hook) telephone line voltage at the output of bridge-rectifier diodes D1 to D4 is approximately 48 volts, which being well above the break-down voltage of zener diode D5, the diode conducts. As a result transistor T2 gets forward biased. This effectively grounds the base of transistor T1 which is thus
cut off and the remaining circuit does not get any power supply. In this state, only a small (negligible) current is taken by the circuit, which will not affect the telephone line condition. However, when handset of any telephone connected to the telephone lines is lifted (off-hook), line voltage suddenly drops to about 10 volts. As a result, transistor T2 is switched off and transistor T1 gets forward biased via resistor R1. Now, the astable multivibrator built around timer IC1 starts oscillating and the speaker starts sounding. Output of the astable multivibrator is also connected to the base of transistor T1 through capacitor C5. As a result, only a loud (and irritating) tone is heard in the ear-piece of the unauthorised telephone instrument. This circuit can be constructed on a veroboard using easily available low-cost components and it can be connected to any telephone line without the fear of malfunctioning. No extra power supply is required as it draws power from the telephone line for operation. EFY Note: Please disconnect the gadget when you are yourself using the telephone as it cannot distinguish between authorised and unauthorised operation.
Water Level Indicator With Alarm Vijay D. Sathe
H
ere is a simple, versatile circuit which indicates the level of water in a tank. This circuit produces alarm when water level is below the lowest level L1 and also when water just touches the highest level L12. The circuit is designed to display 12 different levels. However, these display levels can be increased or decreased depending upon the level resolution required. This can be done by increasing or decreasing the number of level detector metal strips (L1 through L12) and their associated components. In the circuit, diodes D1, D2 and D13 form half-wave rectifiers. The rectified output is filtered using capacitors C1 through C3 respectively. Initially, when water level is below strip L1, the mains supply frequency oscillations are not transferred to diode D1. Thus its output is low and LED1 does not glow. Also, since base voltage of transister T1 is low, it is in cut-off state and its collector voltage is high, which enables melody generating IC1 (UM66) and alarm is sounded. When water just touches level detector strip L1, the supply frequency oscillations are transferred to diode D1. It rectifies the supply voltage and a positive DC voltage develops across capacitor C1, which lights up LED1. At the same time base voltage for transistor T1 becomes high, which makes it forward biased and its collector voltage falls to near-ground potential. This disables IC1 (UM66) and alarm is inhibited. Depending upon quantity of water present in the tank, corresponding level indicating LEDs glow. It thus displays intermediate water levels in the tank in bar-graph style. When water in the tank just touches the highest level detector strip L12, the DC voltage is developed across capacitor C2. This enables melody generating IC1 (UM66) and alarm is again sounded. ELECTRONICS PROJECTS Vol. 20
Water-Level Controller joydeep kumar chakraborty
I
n most houses, water is first stored in an underground tank (UGT) and from there it is pumped up to the overhead tank (OHT) located on the roof. People generally switch on the pump when their taps go dry and switch off the pump when the overhead tank starts overflowing. This results in the unnecessary wastage and sometimes non-availability of water in the case of emergency. The simple circuit presented here makes this system automatic, i.e. it switches on the pump when the water level in the overhead tank goes low and switches it off as soon as the water level reaches a pre-determined level. It also prevents ‘dry run’ of the pump in case the level in the underground tank goes below the suction level. In the figure, the common probes connecting the underground tank and the overhead tank to +9V supply are marked ‘C’. The other probe in underground tank, which is slightly above the ‘dry run’ level, is marked ‘S’. The low-level and high-level probes in the overhead tank are marked ‘L’ and ‘H’, respectively. When there is enough water in the underground tank, probes C and S are connected through water. As a result, transistor T1 gets forward biased and starts conducting. This, in turn, switches transistor T2 on. Initially, when the overhead tank is empty, transistors T3 and T5 are in cut-off state and hence pnp transistors T4 and T6 get forward biased via resistors R5 and R6, respectively. As all series-connected transistors T2, T4, and T6 are forward biased, they conduct to energise relay RL1 (which is also connected in series with transistors T2, T4, and T6). Thus the supply to the pump motor gets completed via the lower set of relay contacts (assuming that switch S2 is on) and the pump starts filling the overhead tank. Once the relay has energised, transistor T6 is bypassed via the upper set of contacts of the relay. As soon as the water level touches probe L in the overhead tank, transistor T5 gets forward
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biased and starts conducting. This, in turn, reverse biases transistor T6, which then cuts off. But since transistor T6 is bypassed through the relay contacts, the pump continues to run. The level of water continues to rise. When the water level touches probe H, transistor T3 gets forward biased and starts conducting. This causes reverse biasing of transistor T4 and it gets cut off. As a result, the relay de-energises and the pump stops. Transistors T4 and T6 will
be turned on again only when the water level drops below the position of L probe. Presets VR1, VR2, and VR3 are to be adjusted in such a way that transistors T1, T3, and T5 are turned on when the water level touches probe pairs C-S, C-H, and C-L, respectively. Resistor R4 ensures that transistor T2 is ‘off’ in the absence of any base voltage. Similarly, resistors R5 and R6 ensure that transistors T4 and T6 are ‘on’ in the absence of any base voltage. Switches S1 and S2 can
be used to switch on and switch off, respectively, the pump manually. You can make and install probes on your own as per the requirement and facilities available. However, we are describing here how the probes were made for this prototype. The author used a piece of nonmetallic conduit pipe (generally used for domestic wiring) slightly longer than the depth of the overhead tank. The common wire C goes up to the end of the pipe
through the conduit. The wire for probes L and H goes along with the conduit from the outside and enters the conduit through two small holes bored into it as shown in Fig. 2. Care has to be taken to ensure that probes H and L do not touch wire C directly. Insulation of wires is to be removed from the points shown. The same arrangement can be followed for the underground tank also. To avoid any false triggering due to interference, a shielded wire may be used.
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