Laser Torch Based Voice Transmitter Amp Receiver

MOUNT ZION COLLEGE OF ENGINEERING(Affiliated to M.G University&Approved by A.I.C.T.E)

Kadammanitta, 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

work done by SAM SUNNY (56601) of SIXTH semester, Electronics

&Communication Engineering, under Mahatma Gandhi University during the

year 2010.

Project Guide HOD

Internal Examiner External Examiner

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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 college, Mr. Abraham Kalammannil, Chairman, for providing us an

opportunity to do our studies in this esteemed institution. We thank our Principal,

Prof.P.C.Mohandas for providing the facilities for our studies and constant

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 encouragement and for providing all

facilities for carrying out this project. We express our highest regard and sincere thanks

to our project Co-ordinators, Mr. Sreeji Krishnan, who provided the necessary

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

1DEPT.OF E & C MZC, KADAMMANITTA



PREFACE

Because of the differences in our college level studies and

industry level requirements, we are allotted a project to get knowledge about the on

goings at industries. I did the mini project that covered up a practical 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, broadcasting and reception of 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.




CONTENTS

1. INTRODUCTION

2. BLOCK DIAGRAM

3. BLOCK DIAGRAM EXPLANATION

3.1. CONDENSERMICROPHONE

3.2. TRANSMITTING SECTION

3.3. LASER TORCH

3.4. RECEIVING SECTION

3.5. LOUD SPEAKER

1. CIRCUIT DIAGRAM

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/resistance 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




3. CIRCUIT DESCRIPTION

6.1. TRANSMITTER

6.2. RECEIVER

6.2.1. CONDENSER MICROPHONE

1. WORKING

2. PCB DESIGN AND FABRICATION

8.1. PCB DESIGN

8.2. PCB FABRICATION

3. COMPONENTS LAYOUT

9.1. TRANSMITTER

9.2. RECEIVER

4. PRINTED CIRCUIT BOARD (PCB) LAYOUT

9.1. TRANSMITTER

9.2. RECEIVER

5. LIST OF TOOLS AND INSTRUMENTS REQUIRED

6. COMPONENTS REQUIRED

12.1. TRANSMITTER

12.2. RECEIVER

7. CONSTRUCTION AND TESTING

13.1. CONSTRUCTION

13.2. TESTING

8. SETTING UP LINK AND PRECAUTIONS

14.1 SETTING UP A

14.2. PRECAUTIONS

14.2.1. LASER SAFETY

9. ADVANTAGES

10.DISADVANTAGES

11.PROBLEM FACED

12.APPLICATIONS

13.CONCLUSION




REFERENCES

APPENDIX




LIST OF FIGURES

1. TRANSMITTER……………………………………………..… xii2. RECEIVER…………………………………………………… xiii3. SYMBOL OF OP AMP…………………………………...…….xiv4. SYMBOL OF IC 741……………………………………...…… xv5. NEGATIVE FEED BACK CONTROL CIRCUIT………….….xvi6. CERAMIC CAPACITOR………………………………….… xvii7. 9V BATTERY………………………………………...……….xviii8. LASER TORCH………………………………………………...xix9. REVERSE BIASED CIRCUIT…………………………...…….xxi10.PHOTODIODE SYMBOL……………………………………..xxii11.PHOTODIODE CIRCUIT………………………………… …xxii12. PHOTOTRANSISTOR SYMBOL……………………….... …xxv13.PHOTOTRANSISTOR USED AS A PHOTODIODE………..xxvi14.COMPONENTS LAYOUT…………………………………..xxxiv15. PCB LAYOUT………………………………………………xxxiv

LIST OF TABLES

1. COMPONENTS REQUIRED FOR TRANSMITTER…xxxvii2. COMPONENTS REQUIRED FOR RECEIVER……….xxxviii




1. INTRODUCTION

Laser as a communication medium can provide a good substitute for the

present day communication systems as the problem of interference faced in

case of electromagnetic waves is not there and high deal of secrecy is

available. Laser communications offers a viable alternative to RF

communications for inter satellite links and other applications where high-

performance links are a necessity. High data rate, small antenna size, narrow

beam divergence, and a narrow field of view are characteristics of laser

communications that offer a number of potential advantages for system

design. The present paper involves the study of wireless, open channel

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 torch can transmit light up to a distance of about 500 meters. 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 laser beam, no

sounds will be heard from the receiver.




2. BLOCK DIAGRAM


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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 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 transmitter section comprises condenser microphone, transistor

amplifier BC548 followed by an op-amp stage built around IC1. The gain of

the op-amp can be controlled with the help of 1-mega ohm pot meter VR1.

The AF output from IC1 is coupled to the base of transistor Bd139, which in

turn, modulates the laser beam. The transmitter uses 9V power supply.

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.




3.3 LASER TORCH

Here we use the light rays coming from laser torch as the medium for transmission.

Laser had potential for the transfer of data at extremely high rates, specific

advancements were needed in component performance and systems engineering,

particularly for space-qualified hardware. Free space laser 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 near infrared region of the spectrum. The laser light across the link is at a

wavelength of between 780 - 920 nm. Two parallel beams are used, one 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 transistor preamplifier and LM386-based audio power amplifier. 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 "speaker") is an electro acoustic transducer that converts an

electrical signal into sound. The speaker moves in accordance with the variations of an

electrical signal and causes sound waves to propagate through a medium such as air or

water.




4. CIRCUIT DIAGRAM

4.1 TRANSMITTER

Fig 4.1. Transmitter




4.2 RECEIVER

Fig 4.2. Receiver




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. OP amps, widely used in computers, can perform mathematical 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 lend themselves readily to IC manufacturing techniques. Improved IC

manufacturing techniques, the op amp's adaptability, and extensive use in the design of

new equipment have brought the price of IC ops amps from very high to 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. A signal 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




An operational amplifier often referred to as op Amp, is a very high gain high

performance amplifier designed to amplify ac and dc signal 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

amplifier with precisely controlled gain characteristics. On the other 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




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):




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)




The DMM is an instrument that is able to measure voltage, 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 some type of load

to the battery using the

wire.

Fig 5.5: 9V Battery

Inside the battery itself, a chemical reaction produces the electrons. The 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 to the positive 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




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 pressure differences in the air around us. What's amazing about it is that the air

transmits those pressure 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 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, 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



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are five different technologies commonly used to accomplish this conversion. We use

condenser mic in our project.

5.7.1 CONDENSER MICROPHONES - A condenser microphone 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, 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 to dark red (700 nm) at the other. Photodiodes also have spectral response

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.

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 slow-

acting direct-coupled light-level sensing applications, while photodiodes are ideal for

use in fast-acting AC-coupled signaling applications. Typical photodiode

applications include IR remote-control circuits.




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 case so that light can reach the base-collector junction. The phototransistor

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




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.

Circuits based on this effect are more sensitive to light than ones based on the

photovoltaic effect.

Avalanche photodiodes have a similar structure; however they are operated with much

higher reverse bias. This allows each photo-generated carrier to be multiplied 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

germanium-based photodiodes, but germanium photodiodes must be used 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 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 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.




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 the base is open circuit, the device is not subjected to significant negative

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




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 of the capacitance of the collector-base 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. Typically, both collector current and collector dark current are measured in

milliamps (mA). Peak wavelength, the wavelength at which phototransistors are most




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

nanoseconds (ns). Collector-emitter breakdown voltage is the voltage at 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 board has an electret 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. Fibre-

optic 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.




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 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 fragile and will not survive electrostatic discharges and momentary surges!

However, if used within specifications, the typical life of one of these lasers is around

20,000 hours. In the transmitter circuit (Fig.1) the laser diode is supplied 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




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 a 9V supply. Increasing the supply voltage will increase the output power

marginally. Incidentally, the photodiode used for this project has a special 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 about it is that the air transmits those pressure 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 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, 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 Condenser microphones - 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 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 varies in time with the first current, because the amount of light that it 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.




8. PCB DESIGN AND FABRICATION

8.1 PCB DESIGN

Designing of PCB is a major step in the 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 layout edition softwares such as Eagle, Orcad or Edwin XP.

Hence a concept, clearly defining all the details of the circuits and partly of the

equipment, is a prerequisite and the actual layout can start. Depending on the

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

best prepared on a 1:1 scale.




8.2 PCB FABRICATION

PCB fabrication involves the following steps.

1) First the layout of the PCB is generated using the software ORCAD.

First step involves drawing the circuit CIS which is a section of ORCAD.

Then the layout is obtained using layout plus. This layout is printed on a

paper.

2) This printed layout is transferred to a Mylar sheet and touched with black

ink.

3) The solder side of the Myler sheet is placed on the shining side of the

copper board and is placed in a frame. It is than exposed to sunlight,

with the Mylar sheet facing the sunlight.

4) The exposed copper board is put in hydrogen peroxide solution. It is

then put in hot water; shook till unexposed region becomes transparent.

5) This is put in cold water and then the rough side is struck in to the skill

screen. This is then pressed and dried well.

6) The plastic sheet of the five - star is removed leaving the pattern on the

screen.

7) A copper clad sheet is cut to the size and cleaned. This is then placed

under the screen.

8) Acid resist ink is spread on the screen, So that the pattern of the tracks

and pad is obtained on the copper clad sheet. It is dried.




9) The dried sheet is then etched using ferric chloride solution till all the

unwanted copper is etched away.

10) The unwanted resist ink is removed using sodium hydroxide solution,

holes are then drilled.

11) The components are soldered neatly on the board without dry soldering




9. COMPONENTS LAYOUT

9.1 TRANSMITTER 9.2 RECEIVER

Fig.14




10. PRINTED CIRCUIT BOARD (PCB) LAYOUT

10.1 TRANSMITTER

10.2 RECEIVER

Fig 15




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 Desoldering wick

11 Petrol

12 Brush

13 Soldering Wire

12. COMPONENTS REQUIRED




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) 111. Semiconductor T1 BC548 112. Semiconductor T2 BD139 113. Condenser MIC 114. IC UA741 115. PCB 1

Table 1

12.2 RECEIVER:

Sl.No. NAME OF THE COMPONENT QUANTITY

1. Resistor (6.8 K) 1

2 Resistor (4.7M) 1




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) 211. Capacitor (1 mf) 112. Capacitor (100 mf) 213. Capacitor(10mf) 114. Capacitor(470 mf) 115. Semiconductor 2N5777 116. Semiconductor BC549 217. LM 386 118. P.C.B 119. 8 ohm Speaker 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 element solders 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




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 different styles of laser pointer, you may have to experiment with 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




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 the photodiode. At close range, there's 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 current adjustments. But the longer the distance between the 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




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.. Remember that, as for any 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 radiation, because they can cause immediate eye damage (and very high

power lasers can cause skin burns or 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 properties to a 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 transmitter schematic, no ballast resistor is shown because most small

LASER power supplies already have one built in. Yours may differ, and a resistor may

be needed.




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 costly

2. Circuit can be easily constructed

3. High data rate

4. No communication licenses required.

5. The laser transmission is very secure because it has a narrow beam.

6. There are no recurring line costs.

7. Compatibility with copper or fiber interfaces and no bridge or router

requirements.

8. Lasers can also transmit through glass, however the physical properties of the

glass have to be considered.

9. Narrow beam divergence

10.Laser transmitter and receiver units ensure easy, straightforward

systems alignment and long-term stable, service free operation,

especially in inaccessible environments, optical wireless systems

offer ideal, economical alternative to expensive leased lines for

buildings.




16. DISADVANTAGES

1. 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.




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.




18. APPLICATIONS

1. Using this circuit we can communicate with our neighbors wirelessly

2. It can be used in inaccessible areas.

3. In future it can be commissioned in satellites for communication.

4. It can be used in conference halls.




19. CONCLUSION

After the successful working of the 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.




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.



Pawan

FYP


APPENDIX










General PurposeSingle Opeartional Amplifier


















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Laser Torch Based Voice Transmitter Amp Receiver

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