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.
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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. 1. 2 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. NAME OF THE COMPONENT Resistance (8.2 K) Resistance (1.8 M) Resistance (10 K) Resistance (15 K) Resistance (82 ohm) Variable Resistance (1 M) Capacitor (1 mf) Capacitor (0.1 mf) Capacitor (470 mf) Capacitor (1000 mf) Semiconductor T1 BC548 Semiconductor T2 BD139 Condenser MIC IC UA741 PCB QUANTITY 2 1 1 2 1 1 1 1 1 1 1 1 1 1 1

2.2 Receiver:
S.No. 1. 2 NAME OF THE COMPONENT Resistor (6.8 K) Resistor (4.7M) QUANTITY 1 1

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MAJOR PROJECT LASER TORCH BASED VOICE TRANSMITTER AND RECEIVER 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. Resistor (470 K) Resistor (2.2 K) Resistor (1 K) Resistor (10 K) Variable resistor (50 K) Capacitor (0.01 mf) Capacitor(47 pf) Capacitor (0.1 mf) Capacitor (1 mf) Capacitor (100 mf) Capacitor(10mf) Capacitor(470 mf) Semiconductor 2N5777 Semiconductor BC549 LM 386 P.C.B 8 ohm Speaker 1 1 1 2 1 1 1 1 1 2 1 2 1 1 1 2 1

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.
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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 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
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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

NEGATIVE FEEDBACK CONTROL

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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 noninverting 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

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..
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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.

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

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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.

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
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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

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
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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

3. Large Common-Mode and Differential Voltage Ranges 4. No Frequency Compensation Required 5. Low Power Consumption 6. No Latch-Up

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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

addition of an external resistor and capacitor between pins 1 and 8 will increase the gain to any value from 20 to 200.

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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 3. Intercoms 4. TV sound systems 5. Line drivers 6. Ultrasonic drivers 7. Small servo drivers

3.9 Transistors
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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. PNP complements: BC556, BC557 and BC558.

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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

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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

Fig. 3.9 Reverse-baised 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. 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

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 generalpurpose 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
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applications include IR remote-control circuits.

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

1

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.

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

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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. 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.

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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.

3.11 Phototransistors
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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 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.

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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.

Fig. 3.15 Phototransistor used as a photodiode

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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 collectorbase 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 transistortransistor 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).

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CHAPTER 4
CIRCUIT DESCRIPTION AND WORKING 4.1 CIRCUIT DIAGRAM: 4.1.1 TRAMSMITTR:

fig 4.1:Transmitter

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4.1.2 RECEIVER:

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

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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.

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
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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 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.
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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. 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
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MAJOR PROJECT LASER TORCH BASED VOICE TRANSMITTER AND RECEIVER

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 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.

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CHAPTER 5 Printed Circuit Board (PCB) Formation
Method Used
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. 2. 3. 4. 5. 6. 7. Printing Painting Scrapping Etching Washing Drilling Masking

These steps are explained bellow in details:

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MAJOR PROJECT LASER TORCH BASED VOICE TRANSMITTER AND RECEIVER

Printing:
First of all the circuit is traced out on the blank PCB plate using a carbon paper carefully avoiding crossing of lines.

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. :

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Masking
Now coasting of protective chemicals are masking the PCB moisture proof and electrically insulated. The PCB is now ready for component mounting.

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

1 2 3 4 5 6 7 8 9

Soldering iron Disordering pump Drill Machine Multimeter Filer Tweezers Screw driver Dual power supply Flux

10 Desoldering wick 11 Petrol 12 Brush 13 Soldering Wire

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CHAPTER 6 CONSTRUCTION AND TESTING 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
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MAJOR PROJECT LASER TORCH BASED VOICE TRANSMITTER AND RECEIVER

6.2 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.

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MAJOR PROJECT LASER TORCH BASED VOICE TRANSMITTER AND RECEIVER

.

CHAPTER 7 SETTING UP LINK AND PRECAUTIONS
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

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MAJOR PROJECT LASER TORCH BASED VOICE TRANSMITTER AND RECEIVER

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:
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.
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MAJOR PROJECT LASER TORCH BASED VOICE TRANSMITTER AND RECEIVER

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. 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

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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.

Chapter-9 CONCLUSION
Conclusion

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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
Electronics device and circuits By:S.Salivahanan
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MAJOR PROJECT LASER TORCH BASED VOICE TRANSMITTER AND RECEIVER

N Suresh kumar A Vallava Raj Hughes electrical technology By:Principles of Electronics By:Electronics For You (Magazine) V.K.Mehta I Mckenzie Smith

WEBSITES
 www.wikipedia.com  www.google.com  www.answers.com  www.howstuffworks.com  www.efy.com

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