Laser torch project

MAJOR PROJECT LASER TORCH BASED VOICE TRANSMITTER AND RECEIVER

CHAPTER -1

INTRODUCTION

1.1 ABOUT PROJECT

LASER TORCH-BASED VOICE TRANSMITTER AND RECEIVER

Using this circuit you can communicate with your neighbors 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 the laser beam, no sound will be heard from the receiver.

The transmitter circuit comprises condenser microphone transistor amplifier BC548

(T1) followed by an opamp stage built around µA741 (IC1). The gain of the op-amp

can be controlled with the help of 1-mega-ohm pot meter VR1. The AF output from

IC1 is coupled to the base of transistor BD139 (T2), which, in turn, modulates the

laser beam. The transmitter uses 9V power supply. However, the 3-volt laser torch

(after removal of its battery) can be directly connected to the circuit—with the body

of the torch connected to the emitter of BD139 and the spring-loaded lead protruding

from inside the torch to circuit ground. The receiver circuit uses an npn

phototransistor as the light sensor that is followed by a two-stage transistor

preamplifier and LM386-based audio power amplifier. The receiver does not need

any complicated alignment. Just keep the phototransistor oriented towards the remote

transmitter’s laser point and adjust the volume control for a clear sound. To avoid

50Hz hum noise in the speaker, keep the phototransistor away from AC light sources

such as bulbs. The reflected sunlight, however, does not cause any problem. But the

sensor should not directly face the sun.

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG,

HARYANA COLLEGE OF TECHNOLOGY AND MANAGEMENT,KAITHAL-136027

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1.2 Laser information page

1. 21 General information

The 'laser' - or - (light amplification by stimulated emission of radiation) was

perfected in 1960, by research scientist Theodore Maiman at the Hughes Laboratory

in Malibu California. Physicists Charles H. Townes and his brother-in-law Arthur

Schawlow were the first to actually apply for a patent on the laser and they were the

first to publish their findings in scientific journals. The He-Ne laser (red beam) was in

commercial use, by 1968. Today many different types of lasers exist, for a wide range

of applications. Lasers are used for surgery, for cutting metal, for determining

distance, for projecting 3-dimensional holographic images, for computer printing and

for entertainment lighting applications. Laser light differs from ordinary light in four

ways. Briefly it is much more intense, directional, monochromatic and coherent. Most

lasers consist of a column of active material with a partly reflecting mirror at one end

and a fully reflecting mirror at the other. The active material can be solid (ruby

crystal), liquid or gas (HeNe, CO2 etc.).

1.22 Why use a laser? A laser as a communications medium has some unique

properties compared to other forms of media. A line-of-sight laser beam is useful

where wires cannot be physically connected to a remote location. A laser beam,

unlike wires, also does not require special shielding over longer distances. Lasers

offer at least an order of magnitude longer distances compared to infrared LEDs.

Although RF transmitters may offer longer distances than line-of-sight lasers, they are

subject to interference from other transmitters. Since the laser medium is line-of-sight

and the beam being only several millimeters in diameter it is very difficult for the

data stream to be tapped. This offers secure communication since any attempts to

intercept the laser beam would be detected at the receiver as a loss in data.



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CHAPTER 2

COMPONENT USED

2.1 Transmitter:.

Table 2.1

S.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



11. Semiconductor T1 BC548 1

12. Semiconductor T2 BD139 1

13. Condenser MIC 1

14. IC UA741 1

15. PCB 1



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2.2 Receiver:

S.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


9. Capacitor(47 pf) 1




13. Capacitor(10mf) 1

14. Capacitor(470 mf) 1

15. Semiconductor 2N5777 1

16. Semiconductor BC549 2

17. LM 386 1

18. P.C.B 1

19. 8 ohm Speaker 1



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CHAPTER 3

COMPONENT STUDY

3.1 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 3.1 symbol of op-amp



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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 3.2 symbol of IC741



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NEGATIVE FEEDBACK CONTROL

Fig 3.3 Negative feedback control ckt

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.

3.2 VR(potentiometer/resistance variac/trimmer):

fig 3.4symbol



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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.

3.3 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 3.5 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.

3.4 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.



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3.5 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 3.6 : 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.



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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.

3.6 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

3.7 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



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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. We use condenser mic in our project.

Condenser microphones - A condenser microphone is essentially a capacitor, with

one plate of the capacitor moving in response to sound waves.

3.8 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.

3.8.1 UA741: GENERAL-PURPOSE OP-AMP

Uses:-

1. Short-Circuit Protection

2. Offset-Voltage Null Capability



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3. Large Common-Mode and Differential Voltage Ranges

4. No Frequency Compensation Required

5. Low Power Consumption

6. No Latch-Up

i.

fig 3.8 UA741

Description

The UA741 is a general-purpose operational amplifier featuring offset-voltage null

capability. The high common-mode input voltage range and the absence of latch-up

make the amplifier ideal for voltage-follower applications. The device is short-circuit

protected and the internal frequency Compensation ensures stability without external

components. A low value potentiometer may be connected between the offset null

inputs to null out the offset voltage as shown in Figure 2. The UA741C is

characterized for operation from 0C to 70C.

3.8.2 LM386

General Description

The LM386 is a power amplifier designed for use in low voltage consumer

applications. The gain is internally set to 20 to keep external part count low, but the



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addition of an external resistor and capacitor between pins 1 and 8 will increase the

gain to any value from 20 to 200.

The inputs are ground referenced while the output automatically biases to one-half the

supply voltage. The quiescent power drain is only 24 milliwatts when operating from

a 6 volt supply, making the LM386 ideal for battery operation.

Features

1. Battery operation

2. Minimum external parts

3. Wide supply voltage range: 4V-12V or 5V-18V

4. Low quiescent current drain: 4mA

5. Voltage gains from 20 to 200

6. Ground referenced input

7. Self-centering output quiescent voltage

8. Low distortion: 0.2% (AV = 20, VS = 6V, RL = 8Ohm, PO = 125mW, f =

1kHz)

9. Available in 8 pin MSOP package

Applications

1. AM-FM radio amplifiers

2. Portable tape player amplifiers



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3. Intercoms

4. TV sound systems

5. Line drivers

6. Ultrasonic drivers

7. Small servo drivers

3.9 Transistors

BC548: NPN general purpose transistors BC548

FEATURES

1. Low current (max. 100 mA)

2. Low voltage (max. 65 V).

APPLICATIONS

General purpose switching and amplification.

DESCRIPTION

NPN transistor in a TO-92; SOT54 plastic package.



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PNP complements: BC556, BC557 and BC558.

BC549: NPN general purpose transistors BC549

FEATURES

1 Low current (max. 100 mA)

2 Low voltage (max. 45 V).

APPLICATIONS

Low noise stages in audio frequency equipment.

DESCRIPTION

NPN transistor in a TO-92; SOT54 plastic package.



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BD 139

Medium Power Linear and Switching

Applications

• Complement to BD136, BD138 and BD140 respectively

3.10 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



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Fig. 3.9 Reverse-baised diode circuit

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. 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. 3

Fig.3.10 Photodiode symbol



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The human eye is sensitive to a range of light radiation, as shown in fig. 4. It has a

peak spectral response to the colour green, which has a wave length of about 550 nm,

but has a relatively low sensitivity to the colour 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. 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 signalling applications. Typical photodiode

applications include IR remote-control circuits.

Fig 3.11 Photodiode circuit with D1-to-V + load

A photodiode is a semiconductor diode that functions as a photodetector. 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.



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Fig. 3.12 Typical spectral response curves of (a) the human eye, (b) a general-purpose photodiode, and (c) an infra-red

photodiode.

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.

3.10.1 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.



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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 bandgap, silicon-based photodiodes generate less noise than

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

wavelengths longer than approximately 1 µm.

3.10.2 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.



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3.11 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. 6, and functions as follows.

Fig. 3.13 Phototransistor symbol.

In fig. 6(a), the base-collector junction of the transistor is effectively reverse biased

and thus acts as a photodiode. The photo-generated currents of the base-collector

junction feed directly into the base of the device, and the normal current-amplifying

transistor action causes the output current to appear (in greatly amplified form) as



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collector current, and in fig. 6(a) R1 causes this current to generate an output voltage

as shown.

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. 6(b) circuit, in which R1 is connected to

Q1 emitter, gives a virtually identical performance to that of fig.

Fig. 3.14 Alternative phototransistor configuration.

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.



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Fig. 3.15 Phototransistor used as a photodiode

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. 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



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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).

CHAPTER 4

CIRCUIT DESCRIPTION AND WORKING

4.1 CIRCUIT DIAGRAM:

4.1.1 TRAMSMITTR:



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fig 4.1:Transmitter

4.1.2 RECEIVER:



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fig 4.2:Receiver

4.2 CIRCUIT DESCRIPTION AND WORKING:-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



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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.

4.2.1Transmitter

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.



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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 heatsink. 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 electret 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.

4.2.2 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.

4.2.3 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.



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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:

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.

How does it do that

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



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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 coneThe 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.

CHAPTER 5

Printed Circuit Board (PCB) Formation

Method Used



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Under the print and etching technique two methods are used:

(a) Manual Method

(b) Screen Printing

Due to certain difficulties the screen-printing method is not used and the printing is

done manually.

The following steps are followed to get final PCB:

1. Printing

2. Painting

3. Scrapping

4. Etching

5. Washing

6. Drilling

7. Masking

These steps are explained bellow in details:

Printing:

First of all the circuit is traced out on the blank PCB plate using a

carbon paper carefully avoiding crossing of lines.



32


Painting:

Paint is now applied carefully on the various numbers in the circuit and

plate kept aside for drying.

Scrapping:

After allowing the plate to dry, the lines are checked for continuity. In

the case of intersection, if any, the unwanted paint is removed with the

help of a scrapper.

Etching:

Now the plate is immersed into the solution of ferric chloride for about

two hours. During this time period the copper from the unwanted is

etched away by the ferric chloride solution leaving behind the copper

under the painted area.

Washing:

The plate is taken out and washes thoroughly and cleans the plate of

ferric solution. It is now allowed to dry for some time. After drying the

reaming paint is removed using a thinner.

Drilling:

The position of all the holes required in the PCB is marked carefully .A

proper drill bit is selected and holed drilled in the PCB.The PCB now is

ready for masking.

:

Masking

Now coasting of protective chemicals are masking the PCB moisture

proof and electrically insulated. The PCB is now ready for component

mounting.



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LIST OF TOOLS AND INTRUMENTS REQUIRED

Following tools and instruments are used for preparing the project

1 Soldering iron

2 Disordering 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

CHAPTER 6

CONSTRUCTION AND TESTING



34


6.1Construction

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 polarised 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 polarised, 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

6.2 Testing



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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.

.

CHAPTER 7

SETTING UP LINK AND PRECAUTIONS DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG,


36


7.1 Setting 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.

7.2 PRECAUTIONS

1. Laser safety:



37


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.



38


4. In order to get any decent amount of modulation, you may need to drive with more

then a watt.

CHAPTER 8

PROBLEMS FACED

Problems faced in the project execution

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.



39


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.

Chapter-9

CONCLUSION

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:-



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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

Electronics device and circuits

By:- S.Salivahanan

N Suresh kumar

A Vallava Raj

Hughes electrical technology

By:- I Mckenzie Smith



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Principles of Electronics

By:- V.K.Mehta

Electronics For You (Magazine)

WEBSITES

www.wikipedia.com

www.google.com

www.answers.com

www.howstuffworks.com

www.efy.com



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http://www.efy.com/

http://www.howstuffworks.com/

http://www.answers.com/

http://www.google.com/

http://www.wikipedia.com/

Documents

Laser torch project