Laser Torch Based Voice Transmitter Amp Receiver

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MOUNT ZION COLLEGE COLLEGE OF ENGINEERING

(Affiliated to M.G University&Approved University&Approved by A.I.C.T.E) Kadammanitta, Pathanamthitta Pathanamthitta

Kerala-689649 Email: mountzion01@ sify.com Website: www.mountzioncollege.org

DEPARTMENT OF ELECTRONICS& COMMUNICATION

CERTIFICATE Certified that this is a bonafide record of the mini project wor work

done by

SAM SAM

SUNN SUNNY Y

(5660 56601) 1) of

SIXTH XTH

seme semesster ter,

Ele Electro tronic nics

&Communic &Communicatio ation n Engineeri Engineering, ng, unde underr Mahatma Mahatma Gandhi Gandhi Universit University y during during the year 2010.

Project Guide

HOD

Internal Examiner

External Examiner

www.final-yearprojects.co.cc | www.troubleshoot4free.com/fyp/ MINI PROJECT 2010

LASER TORCH BASED VOICE TRANSMITTER AND RECEIVER

ACKNOWLEDGEMENT

To bring something into existence is truly the work of ALMIGHTY. We thank GOD ALMIGHTY for making this venture a success.

We express our wholehearted thanks to the Management of the the coll colleg ege, e, Mr. Chairm rman an,, for for prov provid idin ing g us an Mr. Abraha Abraham m Kalamm Kalammann annil il, Chai opport opportuni unity ty to do our studie studiess in this this esteem esteemed ed instit instituti ution. on. We thank thank our Princi Principal pal,, Prof.P.C.Mohandas for for prov provid idin ing g the the faci facili liti ties es for for our our stud studie iess and and cons consta tant nt

encouragement in all achievements.

At the outset we wish to place on record our sincere thanks to quite a few people without whose help, this venture would not have been a success.

We would like to express profound gratitude to our Head of the department, Mr. Rangit Varghese , for his encouragem encouragement ent and for providing providing all facilities for carrying out this project. We express our highest regard and sincere thanks to our project project Co-ord Co-ordina inator tors, s, Mr. Sree who prov provid ided ed the the neces necessar sary y Sreeji ji Krishnan Krishnan, who guidance and serious advice to carry out this project. We also express my gratitude to our Project Guide, Mr. Sudheesh S.R , for her apt suggestions and support. Our sincere thanks to all the staff members of the department of Electronics and Communication who guided as throughout the entire course

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PREFACE

Because Because of the differences differences in our college college level studies and industry level requirements, we are allotted a project to get knowledge about the on goings goings at industries. I did the mini project that covered covered up a practical practical knowledge knowledge of what I have studied so far in books. I did experienced an exposure to various electronics devices and equipments which I would not have able to get easily anywhere else. I learnt a lot about processes of communication like sampling, quantization, detection, error correction, correction, broadcasting broadcasting and reception of signals. signals. All the topics which were dealt with in the project duration are mentioned in an easy manner here in the report which I am submitting to our college for reference purpose I am highly thankful to the college faculty and the management for the insertion of such a training period in our curriculum.

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CONTENTS

1. INTRODUCTION 2. BLOCK DIAGRAM

3. BLOCK BLOCK DIAGRA DIAGRAM M EXPL EXPLANA ANATIO TION N 3.1.

CONDENSERMICROPHONE

3.2.

TRANSMITTING SE SECTION

3.3.

LASER TORCH

3.4.

RECEIVING SE SECTION

3.5.

LOUD SPEAKER

1. CIRC CIRCUI UIT T DIAG DIAGRA RAM M 4.1.

TRANSMITTER

4.2.

RECEIVER

2. COMPONENTSTUDY 5.1.

OPERATIONAL AMPLIFIER 5.1.1. IC 741 5.1.1.1.

NEGATIVE FEEDBACK CONTROL

5.2.

VR (potentiometer/resistan (potentiometer/resistance ce variac/trimmer) variac/trimmer)

5.3.

CAPACITOR

5.4.

DIGITAL MULTIMETER (DMM)

5.5.

BATTERY (9 VOLT)

5.6.

LASER TORCH

5.7.

MICROPHONE 5.7.1. CONDENSER MICROPHONE

5.8.

INTEGRATED CIRCUIT (IC)

5.9.

PHOTODIODES 5.9.1. PRINCIPLE OF OPERATION 5.9.2. APPLICATIONS

5.10.

PHOTOTRANSISTORS

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3. CIRCUIT DESCRIPTION 6.1.

TRANSMITTER

6.2.

RECEIVER 6.2.1. 6.2.1. CONDEN CONDENSER SER MICRO MICROPH PHONE ONE

1. WORKING 2. PCB DESIGN DESIGN AND FABRIC FABRICATI ATION ON 8.1.

PCB DESIGN

8.2.

PCB FABRICATION

3. COMP COMPON ONEN ENTS TS LAYO LAYOUT UT 9.1.

TRANSMITTER

9.2.

RECEIVER

4. PRINTE PRINTED D CIRCUI CIRCUIT T BOAR BOARD D (PCB) (PCB) LAYO LAYOUT UT 9.1.

TRANSMITTER

9.2.

RECEIVER

5. LIST OF TOOLS TOOLS AND INSTRUMEN INSTRUMENTS TS REQUIR REQUIRED ED 6. COMP COMPON ONEN ENTS TS REQU REQUIR IRED ED 12.1.

TRANSMITTER

12.2.

RECEIVER

7. CONSTR CONSTRUCT UCTION ION AND TESTIN TESTING G 13.1.

CONSTRUCTION

13.2.

TESTING

8. SETTIN SETTING G UP LINK LINK AND AND PRECA PRECAUTI UTIONS ONS 14.1 SETTING UP A 14.2.

PRECAUTIONS 14.2.1. LASER SAFETY

9. ADVAN DVANTA TAG GES 10.DISADVANTAGES 11.PROBLEM FACED 12.APPLICATIONS 13.CONCLUSION 3 DEPT.OF E & C

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REFERENCES APPENDIX

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LIST OF FIGURES 1. TRANSM TRANSMITT ITTER… ER………… ……………… ………………… ………………… ……………… ………..… ..… xii 2. RECEIV RECEIVER… ER…………… ………………… ……………… ………………… ………………… …………… …… xiii xiii 3. SYMBOL SYMBOL OF OP AMP……………………… AMP………………………………….. …………...…….xi .…….xiv v 4. SYMBO SYMBOL L OF OF IC 741……… 741………………… ………………… ……………… ………….. …...…… .…… xv 5. NEGATIVE NEGATIVE FEED FEED BACK BACK CONTRO CONTROL L CIRCUIT………… CIRCUIT………….….xv .….xvii 6. CERAM CERAMIC IC CAPACI CAPACITOR TOR……… ……………… ………………… ………………… ……….… .… xvii xvii 7. 9V BATTERY……… BATTERY………………………… ………………………………. ……………...……… ..……….xvii .xviiii 8. LASER LASER TORCH… TORCH…………… ………………… ……………… ………………… ………………… ………... ...xix xix 9. REVERSE REVERSE BIASED CIRCUIT……………… CIRCUIT………………………….. …………...…….xx .…….xxii 10.PHOTODIODE SYMBOL……………………………………..xxii 11.PHOTODIODE CIRCUIT………………………………… …xxii 12. PHOTOTRANSISTOR SYMBOL……………………….... …xxv 13.PHOTOTRANSISTOR USED AS A PHOTODIODE………..xxvi 14.COMPONENTS LAYOUT…………………………………..xxxiv 15. PCB LAYOUT………………………………………………xxxiv

LIST OF TABLES

1. COMPONENTS REQUIRED FOR TRANSMITTER…xxxvii 2. COMPONENTS REQUIRED FOR RECEIVER……….xxxviii

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1. INTRODUCTION

Laser as a communication medium medium can provide a good substitute for the pr e s e nt da y c om mu ni ca t io n s ys t e ms a s th e pr ob l e m of in te r f e r en c e f a c e d in case of electromagnetic waves is not there and high deal of secrecy is a va va il il ab ab le le .

L as as er er

c om om mu mu ni ni ca ca ti ti on on s

o ff ff er er s

a

v ia ia bl bl e

a lt lt er er na na ti ti ve ve

to

RF

communications for inter satellite links and other applications where high pe r f or m an ce li nk s ar e a ne ce s s it y. Hi gh d at a ra te , s ma l l an te nn a s i ze , n ar ro w be a m d iv er ge nc e , an d a n ar r o w f ie l d of vi ew a r e c ha ra c t e r is t i c s of la s e r commun communica icatio tions ns that that offer offer a numb number er of poten potentia tiall advant advantage agess for system system desig design. n. The The presen presentt paper paper involv involves es the study study of wirele wireless, ss, open open chann channel el communication system using laser a carrier for voice signals. Using this circuit we can communicate with your own neighbours wirelessly. Instead of RF signals, light from a laser torch is used as the carrier in the circuit. The laser laser torch torch can transmi transmitt light light up to a distan distance ce of about about 500 500 meters meters.. The The ph o to tr a ns i s t or o f t he r ec ei ve r m us t be a c cu r a te l y or i e nt e d to wa r d s t he l as er be a m f r om t he t or c h. I f t he re i s a ny o bs t ru c t io n in t he p at h of la s e r be am , no sounds will be heard from the receiver.

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2. BLOCK DIAGRAM

L A S E R T O R C H

R S E E C C I T E V I O I N N G

T R S A E N C S T M I I O T N T I N G

L O U D S P E A K E R

M C O I C N D E N S E R

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3. BLOCK DIAGRAM EXPLANATION

3.1 CONDENSER MICROPHONE

It is also called a capacitor or electrostatic microphone. Condenser means capacitor, which stores energy in the form of an electric field. Condenser microphones require power from a battery or external source. Condenser also tends to be more sensitive and responsive than dynamic, making them well suited to capturing subtle nuances in a sound. The diaphragm vibrates when struck by sound waves, changing the distance between the two plates and therefore changing the capacitance. Specifically Specifically when the plates are closer together capacitance increases and a charge current occurs and this current will be used to trigger the transmitting section.

3.2 TRANSMITTING SECTION

The transmitt transmitter er section section comprises comprises condenser condenser microphon microphone, e, transistor transistor amplifier BC548 followed by an op-amp stage built around IC1. The gain of the op-amp can be controlle controlled d with the help of 1-mega ohm pot meter meter VR1. The AF output from IC1 is coupled to the base of transistor Bd139, which in turn, turn, modul modulate atess the the laser laser beam. beam. The The transm transmitt itter er uses uses 9V power power suppl supply. y. however, the 3-volt laser torch ( after the 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.

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3.3 LASER TORCH

Here we use the light rays coming from laser torch as the medium for transmission. Lase Laserr had had pote potent ntia iall for for the the tran transf sfer er of data data at extr extrem emel ely y high high rate rates, s, spec specif ific ic advanc advanceme ements nts were were needed needed in compon component ent perfor performan mance ce and system systemss engine engineeri ering, ng, particularly for f or space-qualified hardware. Free space laser las er communications systems are wireless connections through the atmosphere. They work similar to fibre optic cable systems except the beam is transmitted through open space. The laser systems operate in the the near near infr infrare ared d regi region on of the the spect spectru rum. m. The The laser laser light light acro across ss the the link link is at a wave wavele leng ngth th of betw betwee een n 780 780 - 920 920 nm. nm. Two Two para parall llel el beam beamss are are used, sed, one one for for transmission and one for reception.

3.4 RECEIVING SECTION

The receiver circuit uses an NPN phototransistor as the light sensor that is followed by a two stage stage transi transistor stor preamp preamplif lifier ier and LM386LM386-bas based ed audio audio power power amplif amplifier. ier. The receiver doesn't 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.

3.5 LOUD SPEAKER

A loudspeaker (or "speak "speaker") er") is an electro electro acoustic acoustic transd transduce ucerr that that conver converts ts an electrical signal into sound. The speaker moves in accordance with the variations of an electrical electrical signal and causes sound sound waves to propagate propagate through through a medium medium such as air or water.

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4. CIRCUIT DIAGRAM

4.1 TRANSMITTER

Fig 4.1. Transmitter Transmitter

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4.2 RECEIVER

Fig 4.2. Receiver

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5. COMPONENT STUDY

5.1 OPERATIONAL AMPLIFIER An op amp is a high-gain, direct-coupled differential linear amplifier whose response characteristics are externally controlled by negative feedback from the output to the input. input. OP amps, widely used in computers, computers, can perform perform mathematical operations operations such as summing, integration, and differentiation. OP amps are also used as video and audio amplifiers, oscillators, etc. in the communication electronics. Because of their versatility op amps are widely used in all branches of electronics both in digital and linear circuits. OP amps amps lend lend them themsel selve vess read readil ily y to IC manu manufa factu cturi ring ng techn techniq ique ues. s. Impr Improv oved ed IC manufacturing techniques, the op amp's adaptability, and extensive use in the design of new new equi equipm pmen entt have have brou brough ghtt the the pric pricee of IC ops ops amps amps from from very very high high to very very reasonable levels. These facts ensure a very substantial role for the IC op amp in electronics. Fig shows the symbol for an op amp. Note that the operational amplifier has two inputs marked (-) and (+). The minus input is the inverting input. A signal applied to the minus terminal will be shifted in phase 180° at the output. The plus input is the non-inverting input. input. A signal applied applied to the plus terminal will appear in the same phase at the output as at the input. Because of the complexity of the internal circuitry of an op amp, the op amp symbol is used exclusively in circuit diagrams.

Fig 5.1 symbol of op-amp

5.1.1 IC-741

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An opera operati tion onal al ampl amplif ifie ierr ofte often n refer referred red to as op Amp, Amp, is a very very high high gain gain high high performance amplifier designed to amplify ac and dc s ignal voltages. Modern integrated circuit technology and large-scale production techniques have brought down the prices of such amplifiers within reach of all amateurs, experimenters and hobbyists. The Op Amp is now used as a basic gain element, like an elegant transistor, in electronic circuits. The availability of two input terminals simplifies feedback circuitry and makes the operational amplifier a highly versatile device. If a feedback is applied from the output to the inverting input terminal, the result is a negative feedback, which gives a stable amplif amplifier ier with with precis precisely ely contro controlle lled d gain gain charac characteri teristi stics. cs. On the other other hand, hand, if the feedback is applied to the non-inverting input, the result is positive feedback, which gives oscillators and multivibrator. Special effects are obtained by combination of both types of feedback.

Fig 5.1.1 symbol of IC741

5.1.1.1 NEGATIVE FEEDBACK CONTROL

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Fig 5.1.1.1. Negative feedback control circuit

The above figure shows the basic circuit, including the negative feedback loop of an op amp. The output is fed back to the inverting input terminal in order to provide negative feedback for the amplifier. The input signal is applied to the inverting input. As a result, the output will be inverted. It is possible to operate the op amp as a non-inverting amplifier by applying the signal to the plus input. In this circuit the feedback network is still connected to the inverting input.

5.2 VR (potentiometer/resistance variac/trimmer):

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fig 6.2 symbol

The potentiometer is a resistor of variable resistance. It has three terminals; a fixed resistance is found between two of the terminals and the third terminal slides along the fixed resistor. Often, it is used to control the volume in an audio amplifier.

5.3 CAPACITOR: The capacitor plays a crucial role in electronics -- it stores electrons for when they're needed most. Capacitors consist of two conducting plates placed near each other. Inside the capacitor, the terminals connect to two metal plates separated by a dielectric. The dielectric can be air, paper, plastic or anything else that does not conduct electricity and keeps the plates from touching each other..

fig 5.3. Ceramic capacitor

They can store electric charge for later discharge. Direct current through a capacitor will charge the capacitor for a short time, and then stop flowing. Alternating current, because of the changing electric fields it generates, can “flow” across a capacitor.

5.4 DIGITAL MULTIMETER (DMM)

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The DMM is an instrument instrument that is able to measure voltage, current, current, and resistance in a circuit, or across circuit components and displays its measurements on a digital display.

5.5 BATTERY (9 VOLT) If you look at any battery, you'll notice that it has two terminals. One terminal is marked (+), or positive, while the other is marked (-), or negative. In an normal flashlight batteries, the ends of the battery are the terminals. In a large car battery, there are two heavy lead posts that act as the terminals. Electrons collect on the negative terminal of the battery. If you connect a wire between the negative and positive terminals, the electrons will flow from the negative to the positive terminal as fast as they can (and wear out the battery very quickly -- this also tends to be dangerous, especially with large batteries, so it is not something you want to be doing). Normally, you connect connect some type of load to the the batt batter ery y usin using g the the wire.

Fig 5.5: 9V Battery

Inside Inside the battery battery itself, a chemic chemical al reactio reaction n produc produces es the electro electrons. ns. The speed speed of electron production by this chemical reaction (the battery's internal resistance) controls how many electrons can flow between the terminals. Electrons flow from the battery into a wire, and must travel from the negative negative to the positive terminal terminal for the chemical reaction to take place. That is why a battery can sit on a shelf for a year and still have plenty of power unless electrons are flowing from the negative to the positive terminal, the chemical reaction does not take place. Once you connect a wire, the reaction starts.

5.6 LASER TORCH 5 DEPT.OF E & C

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For this project we have removed the laser assembly from a small laser pointer. The power supply circuit is the green board attached to the brass laser head. We carry similar laser pointers in our catalog that are easily disassembled for this project. The power supply circuit came conveniently marked with a plus and a minus next to two holes in the board. We solder the black negative lead from the battery clip to the hole marked minus. We solder one of the coil leads to the hole marked plus. We solder the red positive lead of the battery clip to the other lead from the coil.

Fig 3.7. Laser torch

5.7 MICROPHONE Sound is an amazing thing. All of the different sounds that we hear are caused by minute minute pressure differences differences in the air around us. What's amazing about it is that the air transm transmits its those those pressur pressuree change changess so well, well, and so accurate accurately, ly, over over relativ relatively ely long long distances. It was a metal diaphragm attached to a needle, and this needle scratched a pattern onto a piece of metal foil. The pressure differences in the air that occurred when you spoke toward the diaphragm moved the diaphragm, which moved the needle, which was recorded on the foil. When you later ran the needle back over the foil, the vibrations scratched on the foil would then move the diaphragm and recreate the sound. The fact that this purely mechanical system works shows how much energy the vibrations in the air can have! All modern microphones are trying to accomplish the same thing as the original, original, but do it electronically electronically rather than mechanically. mechanically. A microphone microphone wants to take varying pressure waves in the air and convert them into varying electrical signals. There 7 DEPT.OF E & C

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are five different technologies commonly used to accomplish this conversion. We use condenser mic in our project.

5.7.1 5.7.1 CONDEN CONDENSER SER MICROP MICROPHON HONES ES - A cond conden ense serr micr microp opho hone ne is essentially a capacitor , with one plate of the capacitor moving in response to sound waves.

5.8 INTEGRATED CIRCUIT (IC) An integrated circuit is a pre-made circuit shrunk down to small size and put on a chip. IC’s save circuit makers time by serving common purposes like amplifying a signal which would otherwise have to be done by a new circuit built from scratch every time.

5.9 PHOTODIODES If a conventional silicon diode is connected in the reverse-biased circuit , negligible current will flow through the diode and zero voltage will develop across R1. If the diode casing is now carefully removed so that the diode's semiconductor junction is revealed, and the junction is them exposed to visible light in the same circuit, the diode current will rise, possibly to as

Fig. 5.9.1 Reverse-biased diode circuit

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high as 1 mA, producing a significant output across R1. Further investigation will show that the diode current (and thus the output voltage) is directly proportional to light intensity, and that the diode is therefore photosensitive. In practice, all silicon junctions are photosensitive, and a photodiode can be regarded as a conventional diode housed in a case that lets external light reach its photosensitive semiconductor junction. Fig.5.9.2 shows the standard photodiode symbol. In use, the photodiode is reverse biased and the output voltage is taken from across a series-connected load resistor. This resistor may be connected between the diode and ground, as in fig. 1, or between the diode and the positive supply line, as in fig.5.9.3

Fig 5.9.2 Photodiode symbol

The human eye is sensitive to a range of light radiation, radiation, as shown in fig. 5.9.4. It has a peak spectral response to the color green, which has a wave length of about 550 nm, but has a relatively low sensitivity to the color violet (400 nm) at one end of the spectrum and and to dark dark red red (700 (700 nm) nm) at the the othe other. r. Phot Photod odio iode dess also also have have spect spectra rall respo response nse characteristics, and these are determined by the chemistry used in the semiconductor junction material. Fig.5.9.4 shows typical response curves of a general-purpose photodiode, and infrared (IR) photodiode. photodiode. Photodiodes have a far lower light-sensitivity than cadmium-sulphide LDRs, but give a far quicker response to changes in light level. Generally, LDRs are ideal for use in slowacting direct-coupled light-level sensing applications, while photodiodes are ideal for use use

in

fast fast-a -act ctin ing g

AC-c AC-cou oupl pled ed

sign signal alin ing g

appl applic icat atio ions ns..

Typi Typica call

phot photod odio iode de

applications include IR remote-control circuits.

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Fig 5.9.3 Photodiode circuit

A photodiode is a semiconductor diode that functions as a photo detector. Photodiodes are packaged with either a window or optical fibre connection, in order to let in the light to the sensitive part of the device. They may also be used without a window to detect vacuum UV or X-rays. A phototransistor is in essence nothing more than a bipolar transistor that is encased in a transparent transparent case so that light can reach the base-collecto base-collectorr junction. junction. The phototran phototransistor sistor works like a photodiode, but with a much higher sensitivity for light, because the electrons that are generated by photons in base-collector junction are injected into the base, this current is then amplified by the transistor operation. A phototransistor has a slower response time than a photodiode however.

5.9.1 PRINCIPLE OF OPERATION A photodiode is a p-n junction or p-i-n structure. When light with sufficient photon energy strikes a semiconductor, photons can be absorbed, resulting in generation of a mobile electron and electron hole. If the absorption occurs in the junction's depletion

region, these carriers are swept from the junction by the built-in field of the depletion region, producing a photocurrent. Photodiodes can be used in either zero bias or reverse bias. In zero bias, light falling on the diode causes a voltage to develop across the device, leading to a current in the 5 DEPT.OF E & C

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forward bias direction. This is called the photovoltaic effect, and is the basis for solar cells — in fact; a solar cell is just a large number of big, cheap photodiodes. Diodes usually have extremely high resistance when reverse biased. This resistance is reduced when light of an appropriate frequency shines on the junction. Hence, a reverse biased diode can be used as a detector by monitoring the current running through it. Circu Circuit itss based based on this this effec effectt are more more sensit sensitiv ivee to ligh lightt than than ones ones based based on the the photovoltaic effect. Avalanche photodiodes have a similar structure; however they are operated with much higher higher reverse reverse bias. bias. This This allows allows each each photophoto-gen generat erated ed carrier carrier to be multip multiplied lied by avalanche breakdown, resulting in internal gain within the photodiode, which increases the effective responsivity of the device. Because of their greater band gap, silicon-based photodiodes generate less noise than germ german aniu ium-b m-base ased d phot photod odio iode des, s, but but germ german aniu ium m phot photod odio iode dess must must be used used for for wavelengths longer than approximately 1 µm.

5.9.2 APPLICATIONS P-N photodiodes are used in similar applications to other photodetectors, such as photoconductors, charge-coupled devices, and photomultiplier photomultiplier tubes. Photodiodes are used in consumer electronics devices such as compact disc players smoke detectors, and the receivers for remote controls in VCRs and televisions. In other consumer items such as camera light meters, clock radios (the ones that dim the display when its dark) and street lights, photoconductors are often used rather than photodiodes, although in principle principle either could be used. Photodiodes are often used for accurate measurement of light intensity in science and industry. They generally have a better, more linear response than photoconductors.

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5.10 PHOTOTRANSISTORS The standard symbol of a phototransistor, which can be regarded as a conventional transistor housed in a case that enables its semiconductor junctions to be exposed to external light. The device is normally used with its base open circuit, in either of the configurations shown in fig. 5.10.2, and functions as follows.

Fig. 5.10.1Phototransistor symbol .

.In practice, the collector and emitter current of the transistor are virtually identical and, since since the base is open open circui circuit, t, the device device is not subjected subjected to signif significa icant nt negati negative ve feedback. Consequently, the alternative fig. 5.10.2(b) circuit, in which R1 is connected to Q1 emitter, gives a virtually identical performance to that of fig. The sensitivity of a phototransistor is typically one hundred times greater than that of a photodiode, but is useful maximum operating frequency (a few hundred kilohertz) is proportionally lower 1 DEPT.OF E & C

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than that of a photodiode by using only its base and collector terminals and ignoring the emitter, as shown in fig. Phototransistors are solid-state light detectors with internal gain that are used to provide analog or digital signals. They detect visible, ultraviolet and near-infrared light from a variety of sources and are more sensitive than photodiodes, semiconductor devices that require a pre-amplifier. Phototransistors feed a photocurrent output into the base of a small signal transistor. For each illumination level, the area of the exposed collector base junction and the DC current gain of the transistor define the output.

Fig. 5.10.3 Phototransistor used as a photodiode

The base current from the incident photons is amplified by the gain of the transistor, resulting in current gains that range from hundreds to several thousands. Response time is a function function of the capacitance capacitance of the collector-base collector-base junction junction and the value of the load resistance. Photodarlingtons, a common type of phototransistor, have two stages of gain and can provide net gains greater than 100,000. Because of their ease of use, low cost and compatibility with transistor-transistor logic (TTL), phototransistors are often used in applications where more than several hundred nanowatts (nW) of optical power are available. Selecting phototransistors requires an analysis of performance specifications. Collector current is the total amount of current that flows into the collector terminal. Collector dark current is the amount of collector current for which there is no optical input. input. Typical Typically, ly, both both collect collector or curren currentt and collec collector tor dark dark curren currentt are measure measured d in milliamps (mA). Peak wavelength, the wavelength at which phototransistors are most 2 DEPT.OF E & C

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responsive, is measured in nanometers (nm). Rise time, the time that elapses when a pulse waveform increases from 10% to 90% of its maximum value, is expressed in nano nanosec secon onds ds (ns). (ns). Coll Collect ector or-em -emit itte terr brea breakd kdow own n volt voltag agee is the the volta voltage ge at which which phototransistors conduct a specified (nondestructive) current when biased in the normal direction without optical or electrical inputs to the base. Power dissipation, a measure of total power consumption, is measured in milliwatts (mW).

6. CIRCUIT DESCRIPTION There are two sections: the transmitter board and the receiver board, both powered by a separate 9V battery or a fixed voltage power supply, depending on your needs. The transmitter transmitter board has an electret microphone microphone module at one end, and the laser diode at the other end. The electronics modulates the intensity of the laser beam according to the output of the microphone. The laser diode has an inbuilt collimating lens, and is simply a module that connects to the transmitter board. The receiver uses a photodiode as the receiving element, and the onboard amplifier powers a small 4-36 ohm speaker. This board is therefore a high gain amplifier with a basic audio output stage. But what about results - are they better? Sure. Because this design uses a higher power (and visible) laser beam, the range is improved, and alignment is easier and not all that critical, especially over a few hundred meters. The quality of sound transmitted by the link is quite surprising. Clearly, this project is ideal for setting up a speech channel between two areas, say adjacent houses or offices on opposite sides of the street. Or you could use it as a link between the work shop and the house. For duplex (two way) communication, you'll obviously need two laser 'channels’. An important feature of transmission by laser beam is privacy. Because a laser beam is intentionally narrow, it's virtually impossible for someone to tap into the link without you knowing. If someone intercepts the beam, the link is broken, signaling the interception. Fibreoptic cables also have high security, as it's very difficult to splice into the cable without breaking the link. However it's theoretically possible; so for the highest security, you probably can't beat a line-of-sight laser beam.

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Where the transmission distance is no more than meter of so, a LED (or two for increased power) can be substituted for the laser diode. For instance, where the link is being used for educational purposes, such as demonstrating fibre-optic coupling, or the concept of communication over a light beam. Obviously the security of the transmission is much lower as LEDs transmit light in all directions. While this laser link can be

adapted for use as a perimeter protector. Now to a description of how it all works, it's really very simple. We'll start with the transmitter.

6.1TRANSMITTER A laser diode needs a certain value of current, called the threshold threshold current, before it emits laser light. A further increase in this current produces a greater light output. The relationship between output power and current in a laser diode is very linear, once the current is above the threshold, giving a low distortion when the beam is amplitude modulated. For example, the 65Onm 5mW laser diode used in this project has a typical threshold current of 3OmA and produces its full output when the current is raised by approximately 1OmA above the threshold to 4OmA. Further increasing the current will greatly reduce the life of the laser diode, and exceeding the absolute maximum of 8OmA will destroy it instantly. Laser diodes are very very fragile fragile and will will not survive survive electr electrosta ostatic tic dischar discharges ges and moment momentary ary surges surges!! However, if used within specifications, the typical life of one of these lasers is around 20,00 20,000 0 hours. hours. In the transm transmitt itter er circui circuitt (Fig.1 (Fig.1)) the laser laser diode diode is suppli supplied ed via an adjustable constant-current source. Note that the metal housing for the laser diode and the lens also acts as a heat sink. The laser diode should not be powered without the metal housing in place. The increasing the voltage at VR1 reduces the laser current. The setting of VR1 determines the quiescent brightness of the laser beam, and therefore the overall sensitivity of the system. The electric microphone is powered through R1 and is coupled to the non inverting input of 1C1 a via capacitor. This input is held at a fixed DC voltage to give a DC output to bias.

6.2 RECEIVER

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The transmitted signal is picked up by the photo detector diode in the receiver (shown in Fig.2). The output voltage of this diode is amplified by the common emitter amplifier around T4. This amplifier has a gain of 20 or so, and connects via VR2 to IC2, an LM386 basic power amplifier IC with a gain internally set to 20.This IC can drive a speaker with a resistance as low as four ohms, and 35OmW when the circuit is powered

from from a 9V supp supply ly.. Incr Increas easin ing g the the supp supply ly volt voltag agee will will incr increas easee the the outp output ut powe power r margin marginally ally.. Incide Incidenta ntally lly,, the photod photodiod iodee used used for this this project project has a specia speciall clear clear package, so it responds to visible light, and not just infrared.

6.3 MICROPHONE Sound is an amazing thing. All of the different sounds that we hear are caused by minute pressure differences in the air around us. What's amazing amazing about it is that the air transmits those pressure changes changes so well, and so accurately, over relatively long distances. It was a metal diaphragm attached to a needle, and this needle scratched a pattern onto a piece of metal foil. The pressure differences in the air that occurred when you spoke toward the diaphragm moved the diaphragm, which moved the needle, which was recorded on the foil. When you later ran the needle needle back over the foil, the vibrations vibrations scratched scratched on the foil would then move the diaphragm and recreate the sound. The fact that this purely mechanical system work workss show showss how how much much ener energy gy the the vibr vibrat atio ions ns in the the air air can have have!! All All mode modern rn micro microph phon ones es are tryi trying ng to acco accomp mpli lish sh the the same same thin thing g as the the orig origin inal al,, but but do it electronically rather than mechanically. A microphone wants to take varying pressure waves in the air and convert them into varying electrical signals. There are five different technologies commonly used to accomplish this conversion: 6.3.1 6.3.1 Condense Condenserr microphone microphoness - A condenser microphone is essentially a capacitor ,

with one plate of the capacitor moving in response to sound waves. The movement changes the capacitance of the capacitor, and these changes are amplified to create a measurable signal. Condenser microphones usually need a small battery to provide a voltage across the capacitor.

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7. WORKING In all of the laser communicators on this page, the laser light is amplitude modulated. This simply means that the amount of light the laser emits varies over time.

To understand what is going on, it helps to consider how a loudspeaker makes sound. A loudspeaker is a paper cone attached to a coil of wire that sits in a magnetic field from a strong permanent magnet. When an electric current flows in the loudspeaker coil, the coil becomes an electromagnet, and it moves toward or away from the permanent magnet. As it moves, the paper cone pushes on the air around it, compressing the air in front of it, and expanding the air behind it. Waves of compressed and expanded air travel to your ear, and cause your eardrum to move in time to the movements of the paper cone. The laser communicator adds two components c omponents to the loudspeaker concept. We take the electrical signal that goes to the loudspeaker, and connect it instead to the laser, so the laser gets brighter and dimmer as the electric current varies. The second component is the receiver, which converts the light back into an electric current. This current current varies in time with the first current, because the amount of light that it receives receives is varying in time. This second electric current is used to move the paper cone of a loudspeaker, just as before. However, now the loudspeaker can be quite a distance away from the original electric current, without any wires connecting the two.

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8. PCB DESIGN AND FABRICATION 8.1 PCB DESIGN Designing of PCB is a major step in the production production of PCB is a major. It forms a distinct factor in electronic performance and reliability. The productivity of a PCB, its assembly and service ability also depends on the design. The designing of a PCB consists of designing of the layout followed by the preparation of the artwork. The layout should include all the relevant aspects in details of the PCB design while the art work preparation brings it to the form required for the production process. The layout can be designed with the help of any one of the standard standard layout layout edition edition softwa softwares res such such as Eagle, Eagle, Orcad or Edwin Edwin XP. Hence a concept, clearly defining all the details of the circuits and partly of the equipm equipment ent,, is a prereq prerequis uisite ite and the actual actual layout layout can start. start.

Depend Depending ing on the

accuracy accuracy required, required, the artwork might might be produced produced a 1:1 or 2:1 even 4:1 scale. It is best prepared on a 1:1 scale.

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8.2 PCB FABRICATION

PCB fabrication involves the following steps.

1)

Firs Firstt the layo layout ut of the the PCB is gene genera rate ted d using using the the softw softwar aree ORCA ORCAD. D. First step involves drawing the circuit CIS which is a section of ORCAD. Then the layout layout is obtained using layout layout plus. This layout is printed on a paper.

2)

This This prin printed ted layout layout is tran transfer sferred red to a Mylar Mylar sheet sheet and and touc touched hed with with black black ink.

3)

The The solde solderr side side of the the Myler Myler shee sheett is place placed d on the the shini shining ng side side of the the copper copper board and is placed placed in a frame. It is than exposed exposed to sunlight, sunlight, with the Mylar sheet facing the sunlight.

4)

The The expos exposed ed cop coppe perr boar board d is put put in hydr hydrog ogen en per perox oxid idee solut solutio ion. n. It is then put in hot water; shook till unexposed unexposed region becomes transparent. transparent.

5)

This This is put put in in cold cold wat water er and and then then the the rough rough sid sidee is stru struck ck in in to the the skil skilll screen. This is then pressed and dried well.

6)

The The plast plastic ic shee sheett of the the five five - star star is remo remove ved d leavi leaving ng the the patter pattern n on the the screen.

7)

A copp copper er cla clad d shee sheett is cut cut to to the the size size and and cle clean aned ed.. This This is is then then pla place ced d under the screen.

8)

Acid Acid resis resistt ink is is spread spread on on the the scree screen, n, So So that that the the patte pattern rn of the the trac tracks ks and pad is obtained obtained on the copper clad sheet. It is dried.

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9)


The The dried dried sheet sheet is is then then etche etched d using using ferri ferricc chlor chlorid idee solut solutio ion n till till all the the unwanted copper is etched away.

10) 10)

The The unwa unwant nted ed resist resist ink ink is remove removed d usin using g sodi sodium um hydro hydroxi xide de soluti solution on,, holes are then drilled.

11)

The compon component entss are are sold soldered ered neatly neatly on the board board withou withoutt dry dry solder soldering ing

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9. COMPONENTS LAYOUT

9.1 TRANSMITTER

9.2 RECEIVER

Fig.14

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10. PRINTED CIRCUIT BOARD (PCB) LAYOUT 10.1 TRANSMITTER

10.2 RECEIVER

Fig 15

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11. LIST OF TOOLS AND INTRUMENTS REQUIRED Following tools and instruments are used for preparing the project

1

Soldering iron

2

Desoldering pump

3

Drill Machine

4

Multimeter

5

Filer

6

Tweezers

7

Screw driver

8

Dual power supply

9

Flux

10 Deso Desold lder erin ing g wick wick 11 Petrol 12 Brush 13 Sold Solder erin ing g Wire Wire

12. COMPONENTS REQUIRED

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12.1 TRANSMITTER:

Sl.No.

NAME OF THE COMPONENT

QUANTITY

1.

Resistance (8.2 K)

2

2

Resistance (1.8 M)

1

3.

Resistance (10 K)

1

4.

Resistance (15 K)

2

5.

Resistance (82 ohm)

1

6.

Variable Resistance (1 M)

1

7.

Capacitor (1 mf)

1

8.

Capacitor (0.1 mf)

1

9.

Capacitor (470 mf)

1

10.

Capacitor (1000 mf) Semiconductor T1 BC548 Semiconductor T2 BD139 Condenser MIC IC UA741 P CB

1 1 1 1 1 1

11. 12. 13. 14. 15.

Table 1

12.2 RECEIVER:

Sl.No.

NAME OF THE COMPONENT

QUANTITY

1.

Resistor (6.8 K)

1

2

Resistor (4.7M)

1

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3.

Resistor (470 K)

1

4.

Resistor (2.2 K)

2

5.

Resistor (1 K)

1

6.

Resistor (10 K)

1

7.

Variable resistor (50 K)

1

8.

Capacitor (0.01 mf)

1

9.

Capacitor(47 pf)

1

10.

Capacitor (0.1 mf)

2

11.

Capacitor (1 mf) Capacitor (100 mf)

1 2

Capacitor(10mf) Capacitor(470 mf) Semiconductor 2N5777 Semiconductor BC549 LM 386 P.C.B 8 ohm Speaker

1 1 1 2 1

12. 13. 14. 15. 16. 17. 18. 19.

1 1 Table 2

13. CONSTRUCTION AND TESTING 13.1CONSTRUCTION As the photos show, both the transmitter and the receiver are built on silk- screened PCBS. As usual fit the resistors, pots and capacitors first, taking care with the polarity of the electrolytic. IC sockets are not essential, although servicing is obviously made easier if they are used. In which case, fit these next, followed by the transistors and photo transistors The photo diode/ transistors, is mounted directly on the receiver PCB. When first mounted, the active side of the diode (Black Square inside the package) will face towards the centre of the board. You then bend the diode over by almost 180' so the active surface now faces outwards. The polarized microphone microphone element solders solders directly directly to the transmitter PCB. The negative lead is marked with a minus sign and is the lead that connects to the metal case. The laser diode is also polarized, and has three leads. Of 2 DEPT.OF E & C

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these, only two are used, shown on the circuit. Take care when soldering the laser in place, as too much heat can destroy it. The diode can be mounted on the board, or connected with leads to it. Connect a clip lead to the inside of the laser pointer where the battery touched. Usually there is a small spring to which you can attach the clip lead. The other end of the battery usually connects to the case of the laser. Since there are many many differe different nt styles styles of laser pointe pointer, r, you may have have to experi experimen mentt with with clip clip lead placement to get the laser to work with the new external battery pack. You may also have to hold down the laser's push button switch by wrapping a rubber band or some wire around it. Finally, connect the speaker and 9V battery clips, then check over the boards for any soldering errors or incorrectly installed components

13.2 TESTING First of all, it's most important that you don't look directly into the laser beam. If you do, it could cause permanent eye damage. Also, you are responsible for the safety of others near the laser, which means you must stop others from also looking into the beam, and take all necessary safety steps. This is covered by legislation. Both the receiver and the transmitter can be powered by separate 9V batteries or suitable DC supplies. Before applying power to the transmitter PCB, set VRI to its halfway position, to make sure the laser current is not excessive. To be totally sure, you could set VRI fully anticlockwise, as this setting will reduce the laser current to zero. Then apply power to the board. If the laser doesn't produce light, slowly adjust VRI clockwise. The laser diode should emit a beam with an intensity adjustable with VRI. At this stage, keep the beam intensity low, but high enough to clearly see. If you are not getting an output, check the circuit. You won't see the laser beam intensity change with the modulating signal. To check that the system is working, place the two PCBs on the workbench, spaced a meter or go apart. You might need to put a sheet of paper about 2Omm in front of the photodiode to reduce the intensity of light from the laser beam. Set the volume control 3 DEPT.OF E & C

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of the speaker to about halfway. If the volume control setting is too high you'll get acoustic feedback. Move the laser diode assembly so the beam points at the receiver's photodiode. It's useful to adjust the beam so it's out of focus at the photodiode, to make alignment even easier. You should now be able to hear the speaker reproducing any audio signal picked up by the microphone. .

14. SETTING UP LINK AND PRECAUTIONS 14.1SETTING UP A LINK Once you've tested the link, you'll probably be keen to put it to use. For a short link of say 100 meters, all you need do is position the receiver so the laser beam falls on the photodiode. Once the link is established, adjust VRI higher the laser current, the shorter will be its life. If you have an ammeter, connect it to measure the current taken by the transmitter board. Most of the current is taken by the laser, so adjust VRI to give a total current consumption of no more than 45Ma. Also, focus the laser so all of the beam is striking striking the photodiod photodiode. e. At close range, there's probably probably no need to focus the beam. In fact, because of the high output power (5mW) of the laser diode, excellent results will be obtained over reasonably short distances (20 meters or so) with rough focusing and quiescent quiescent current adjustments. adjustments. But the longer longer the distance between the transmitter transmitter and the receiver, the more critical the adjustments. For example, for distances over 20 meters, you might have to put a piece of tube over the front of the photodiode to limit the ambient light falling on it. This diode is responsive to visible light, so a high ambient light could cause it to saturate. For very long distances, say half a kilometer, you'll probably need a parabolic reflector for the laser beam, to focus it directly onto the photodiode. For short ranges (a meter or so), or for educational or testing purposes, you can use a conventional red LED. Adjust the quiescent current with VR1. The light output of a 4 DEPT.OF E & C

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LED is not focused, and simply spreads everywhere, so a reflector might help the sensitivity. Warnings The laser diode in this project is a class 3B laser and you should attach a warning label to the transmitter.. transmitter.. Remember that, as for any hazardous hazardous device, the owner of a laser is responsible for its proper use.

14.2 PRECAUTIONS 14.2.1. LASER SAFETY: Safety instructions for lasers: Laser beams may damage the eyes severely or may cause blindness if they radiate into the eyes directly or indirectly. Therefore the laser electronics must be installed in such a manner that radiation into the eyes will be impossible neither directly nor indirectly via marrow’s in the room. When using lasers with an output power higher than 1 mW, you should check about the legal regulations for prevention of accidents and be very careful. Normal laser pointers sold in shops have typically output power of 1..5 mW (power depends on laser pointer model and what country regulations say on maximum power). This power level is normally not very hazardous, but can cause permanent dotages your eye if you stare at the beam. We should be very careful with higher power lasers and lasers on that power range that emit invisible invisible radiation, radiation, because they can cause immediate immediate eye damage (and very high power lasers can cause skin burns or fire).With fire ).With any high power laser make sure that you have safe operating environment, necessary regulations/permissions and somebody that takes care that these legal regulations are observed. Lasers use coherent light which has very different different properties properties to a standard standard lighting effect. This is what makes lasers one of the most beautiful forms of light, but also one of the most dangerous light sources if not used with proper cautions 2. In the the tran transm smit itter ter sche schema mati tic, c, no ball ballast ast resis resisto torr is show shown n beca becaus usee most most small small LASER power supplies already have one built in. Yours may differ, and a resistor may be needed. 2 DEPT.OF E & C

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3. The receiver should be kept away from bright lights. You may put a piece of wax paper in front of photo transistor to keep the LASER from swamping it.

15. ADVANTAGES

1. Less costl stly 2. Circui Circuitt can can be be easil easily y cons constru tructed cted 3. Hig High data data rate rate 4. No comm commun unicat ication ion license licensess requir required. ed. 5. The laser laser transmissi transmission on is very secure because because it has has a narrow narrow beam. 6. There There are are no no recur recurrin ring g line line costs. costs. 7. Compatibil Compatibility ity with copper copper or fiber fiber interfaces interfaces and no bridge bridge or or router router requirements. 8. Lasers can can also transmit transmit through through glass, glass, however however the physica physicall properties properties of the the glass have to be considered. 9. Narr Narrow ow beam beam dive diverg rgen ence ce 1 0. Laser

transmitter transmitter and receiver receiver units units ensure ensure easy, straightforwa straightforward rd

system systemss alignm alignment ent and long-t long-term erm stable stable,, service service free free opera operatio tion, n, especially especially in inaccessib inaccessible le environm environments, ents, optical optical wireless wireless systems systems offer offer ideal ideal,, econo economic mical al altern alternati ative ve to expens expensive ive leased leased lines lines for bu i ld in gs .

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16. DISADVANTAGES

1. To avoid avoid 50Hz 50Hz hum noise noise in the speake speaker, r, keep the photot phototran ransist sistor or away from 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.

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17. PROBLEMS FACED Although this project was successfully completed, however a few hurdles that came during the construction of the circuit were the breaking of the thin electrical wires after it had been soldered and the breaking of the photodiode receiver’s leg leading to an error in reception of data. Moreover the connections with the OP-AMP chip have to be dealt with very carefully because one wrong connection may damage the whole chip. If the supply to laser is greater than it will not glow. All these things are to be taken care of, for the efficient working of the project.

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18. APPLICATIONS

1. Using this circuit circuit we can can communi communicate cate with our neighb neighbors ors wirelessly wirelessly 2. It can can be be used used in inac inaccess cessibl iblee areas. areas. 3. In future future it can be commissio commissioned ned in satellites satellites for commun communicatio ication. n. 4. It can can be be used used in conf conferen erence ce halls halls..

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19. CONCLUSION

After the successful working working of the project, project, it can be concluded that this project is suitable for easily communication. There can be further up gradations in the project which could lead to a much better system for communication. Some of the possible ways are as follows:Instead of the short range laser, high range lasers can be used which range a few hundred meters. Provisions have to be made for cases when there is no heavy traffic.

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REFERENCES

1. [Online] // wikipedia. - www.wikipedia.com. 2. [Online] // circuitstoday. - www.circuitstoday.com. 3. [Online] // electronics schematics. - www.electroschematic.com. 4. [Online] // electronics for you. - www.efy.com. 5. D.ROY CHOUDHARY SHALIN B. JAIN LINEAR INTEGRATED CIRCUITS [Book]. - DELHI : NEW AGE INTERNATIONL PUBLISHERS, THIRD EDITION 2009. 6. ELECTRONICS FOR YOU MAGAZINE [Book]. 7. GUPTA J.B. ELECTRONICS DEVICE & CIRCUITS [Book]. - INDIA : S.K. KATARIA & SONS, FIRST EDITION DEC 2000. - Vol. 1. 8. KUMAR N. SURESH ELECTRONICS DEVICE & CIRCUITS [Book]. - 2008. 9. MEHTA V.K. PRINCIPLES OF ELECTRONICS [Book]. 10. NAVAS K.A. ELECTRONICS LAB MANUAL [Book]. - [s.l.] : Rajath publishers, 2008. - Vol. 1&2. 11. RAI A. VALLAVE ELECTRONICS DEVICE & CIRCUITS [Book]. - 2007.

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APPENDIX

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General PurposeSingle Opeartional Amplifier

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Laser Torch Based Voice Transmitter Amp Receiver

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