Fridge Door Alarm Circuit Diagram:
Components:
Working of Circuit:
R1______________10K 1/4W Resistor R2_____________Photo R2_____________Phot o resistor (any type) R3_______________2M2 1/4W Resistor R4_______________1M 1/4W Resistor
The circuit, enclosed in a small box, should be placed in the fridge near the lamp or close to the opening. With the door closed, the interior of the fridge is in dark, the photo resistor R2 presents a high resistance (>200K) thus clamping IC1 by holding C1 fully charged across R1 and D1. When a beam of C1______________10µF 25V Electrolytic Electrolytic Capacitor light enters from the opening, or the fridge lamp lights, the C2_____________100nF 63V Polyester Polyester Capacitor Capacitor photo resistor lowers its resistance (<2K) stopping C1charging current. Therefore IC1, wired as an astable multivibrator, starts D1____________1N4148 75V 150mA Diode oscillating at a very low frequency and after a period of about 20 seconds its output pin (#3) goes high, enabling IC. This chip IC1,IC2_________7555 IC1,IC2_________7555 or TS555CN CMos Timer ICs is also also wired wired as an astabl astablee multiv multivibr ibrato ator, r, drivin driving g the Piezo Piezo sounder intermittently at about 5 times per second. The alarm is BZ1___________Piezo BZ1___________Piezo sounder (incorporating (incorporating 3KHz oscillator) activated for about 17 sec. then stopped for the same time period and the cycle repeats until the fridge door closes. B1____________3V Battery (2 x 1.5V AA, AAA or smaller type Cells in series) Photo resistor: Introduction:
A refrigerator or freezer door that is left open or ajar may cause the food food contents contents to spoil. spoil. Moreov Moreover er it also reduces reduces the efficiency of the fridge by needlessly running the compressor in a futile effort to keep the contents cool. The perishable goods like fishes, vegetables etc. are more at risk if the fridge door is left open. This is where the Fridge Alarm is useful. The fridge alar alarm m oper operat ates es by dete detect ctin ing g when when any any ligh lightt ente enters rs the the compartment area. The alarm will sound if the light is present for longer than the preset period and will continue to sound until the door is closed. closed. In practice, the preset preset period is adjusted adjusted so that in normal use the alarm will not sound. It will sound when the door is left wide open for too long or if left slightly ajar. It is great for indicating when someone is standing with the door open for too long and a real asset in warning when the door looks shut but is still partially ajar.
A photo photo resistor resistor is a sensor sensor whose whose resistanc resistancee varies varies with light intens intensity ity.. Most Most decrea decrease se in resist resistanc ancee as the light light intens intensity ity increa increases ses.. In a typica typicall micro microcon contro trolle llerr applic applicati ation, on, this this resist resistanc ancee must must be conver converted ted to a voltag voltagee so that that an A2D converter converter can measure measure it. The easiest easiest way to do this is with a voltage divider circuit. Working of Photo resistor: A voltag voltagee divide dividerr is just just two resist resistors ors in series series connec connected ted between a voltage supply and ground. If R1 is connected to the voltage supply and R2 is connected to ground then the voltage at the junction between the two resistors is: V = V supply * (R2 / (R1 + R2)). If R1 is the photo resistor resistor,, the voltage voltage will will incre increase ase with with increasing light intensity. intensity. If R2 is the photo resistor, resistor, the voltage will decrease with increasing light intensity.
Sample Photo resistor Circuits
Theory:
Here, we use R2 as photo resistor and hence the voltage IC7555CN: decreases with increasing light intensity. Thus when the door of the fridge is closed, the light intensity is zero and the buzzer does not sound, but sounds only when the door is open i.e. when the voltage is minimum. To understand what happens when a semiconductor absorbs a photon material it is necessary to brief review energy band gap theory. Each electron in the orbit in an atom has a certain allowable discrete energy level. Values other than these specific levels are forbidden energy levels. The two interest bands in a semiconductor material, are the valence band and the conduction band, which are separated by the forbidden gap, as Description: shown in figure. The ICM7555 is a CMOS timer providing significantly improved performance over the standard NE/SE555 timer, while at the same time being a direct replacement for those devices in most applications. Improved parameters include low supply current, wide operating supply voltage range, low THRESHOLD, TRIGGER, and RESET currents, no crowbarring of the supply current during output transitions, higher frequency performance and no requirement to decouple CONTROL VOLTAGE for stable operation. The ICM7555 is a stable controller capable of producing accurate time delays or frequencies.
The valence band is the highest energy band that is filled by electrons. The conduction band is partially filled with electron that moved across the forbidden gap from one band to another. Features of IC 7555 CMOS Timer: Light, of energy equal to or greater than the energy gap between the valence and the conduction bands incident upon a • Exact equivalent in most applications for NE/SE555 semiconductor material, can force electrons from the valence to • Low supply current: 80mA (typ) • Extremely low trigger, threshold, and reset currents: 20pA the conduction band and thereby change the conductivity. Light dependent resistor (LDR) is made from cadmium (typ) sulphide. Cadmium Sulphide is a II-VI semiconductor. (It is so • High-speed operation: 500kHz guaranteed called because Cadmium is in group II and Sulphide is in group • Wide operating supply voltage range guaranteed VI.) LDR is commonly used in auto-focusing camera for light • Can be used with higher-impedance timing elements than the bipolar 555 for longer time constants metering. • Timing from microseconds through hours • Operates in both astable and monostable modes • Adjustable duty cycle • High output source/sink driver can drive TTL/CMOS • Typical temperature stability of 0.005%/°C at 25°C • Rail-to-rail outputs.
APPLICATIONS of IC:
• Precision timing • Pulse generation • Sequential timing • Time delay generation • Pulse width modulation • Pulse position modulation • Missing pulse detector Troubleshooting:
The commonly available photo resistor does not show much difference in its resistance in light as well as dark. Hence the resistance keeps on varying and is not constant. Due to this IC1 remains in ON state even in dark which eventually triggers IC2 to keep the buzzer ON. Thus the photo resistor used should be of high sensitivity. Applications:
Since the above circuit is a light operated device, it can be used for security of lockers. In this case the Delay time can be varied by varying capacitors C1 and/or resistance R3. Limitations:
The above circuit cannot work in high moisture areas; hence it should be switched off and taken out during defrosting.
