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INDEX

Sr. No. Contents Page No.

1. Certificate of Completion

2. Acknowledgement 1

3. Preface 2

4. Introduction; Laser based Voice Transmitter and Receiver 4-5

5. Electronic Components 6-12

6. Leads Identification 13-14

7. BC 546 15-17

8. BC 548 18-19

9. UA 741 20-22

10. SL 100 23-24

11. LM 386 25-27

12. L14F1 28-29

13. Conclusion 30

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LASER TORCH-BASED VOICETRANSMITTER 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 (Fig. 1) comprises condenser microphone transistor amplifier BC548 (T1) followed by an pomp stage built around μA741 (IC1). The gain of the op-amp can be controlled with the help of 1-mega-ohm potmeter VR1.The AF output from IC1 is coupled to the base of transistor BD139 (T2), which, inturn, modulates the laser beam.

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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 ofThe torch connected to the emitter ofBD139 and the spring-loaded lead protruding from inside the torch to circuit ground. The receiver circuit (Fig. 2) uses an npn phototransistor as the light sensor that is followed by a two-stage transistor preamplifier and LM386-based audioPower amplifier. The receiver does not need any complicated alignment. Just keepthe phototransistor oriented towards the remote transmitter’s laser point and adjustThe 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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A BRIEF DISCRIPTION OF LASER TORCH BASED TRANSMITTER AND RECEIVER

PROJECT DESCRIPTION

 

(a) A. LASER BASED VOICE TRANSMITTER

 

     The circuit is based upon the principle of LIGHT MODULATION where

instead of radio frequency signals; light from a laser torch is used as

the carrier in the circuit.

     Here, the transmitter uses 9V power supply.

      Audio signal or voice is taken as input from the condenser mic,

which is, followed transistor amplifier BC548 along with op-amp

stage built around UA741.

      The gain of the op-amp can be controlled with the help of 1 mega

ohms potmeter.

      The AF output from op-amp UA741 is coupled to the base of the

power transistor BD139, which in turn, modulates the laser.

     However, the three volts laser torch can be directly connected to the

emitter of BD139 and the spring loaded lead protruding from inside

the torch to the ground.

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     In the transmitter circuit, audio signal of the non-sinusoidal waveform

and having a few mV of amplitude is taken as input from condenser

mic.

     Condenser mic is directly followed by the transistor amplifier stage

consist of BC548. Transistor BC548 is connected in common emitter

configuration.

     Resistor R1 is the source resistor, which is directly connected to the

power-supply.

     R2, R3 and capacitor C1 are acting as self-biasing circuits, which is

used for the biasing transistor. These circuit arrangements provide or

establish a stable operating point. The biasing voltage is obtaining by

R2 and R3 resistors network. Self-bias is used for obtaining entire

audio signal as input.

      Capacitor C1 is the coupling capacitor, since audio input signal is

having a non-sinusoidal waveform of different amplitude and

frequency, coupling capacitor is used to reject some of the dc

noise/line as well as level from audio input signal.

      The self-biased circuit is connected with the BC548 in CE

configuration. It is transistor amplifier stage, where the low amplitude

audio signal is amplified to the desired voltage.

      The output is taken from the collector terminal; so inverted audio

input signal is obtained.

      Transistor pre-amplifier stage is coupled with op-amp stage built by

ua741. C2 is the blocking capacitor while R4 is the op-amp stage

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resistor. Op-amp ua741 is easily available general-purpose

operational amplifier.

     Pin configuration of UA741 is shown in the glossary. Here pin no. 1

and 5 are not connected in order to nullify input-offset voltage. Pin

no. 7 and 4 are VCC as well as –VEE supply voltage. Pin no. 3 is non-

inverting input while pin no. 2 is inverting input. Between pin no. 2

and 6, 1 mega-ohm potmeter is connected as voltage series negative

feedback, which control the infinite gain of the op-amp.

      Resistors R5 and R6 of it value acts as a voltage-divider network,

thus it gives a fixed voltage at the non-inverting pin.

      Input inverted audio signal is applied to the inverting pin. Op-amp

works on the differences into the applied two input voltage and

provide a output at pin no. 6. Since, input is applied to the inverting

pin the output is also an inverting one. Thus, again we get in phase

high power and high amplitude level audio signal.

     Capacitors C3, C4 and resistor R7 are acting as diffusion capacitors

and feedback resistor respectively. These diffusion capacitors stored

the carriers like holes and electrons in the base and thus provide

self-biasing of the transistor.

     Power dissipation rate of UA741 is very high, which is not practical

for driving other electronics devices, so heat sink power transistor

BD139 is used.

     Power transistor BD139 absorbs most of the power and supplies the

suitable power to drive the laser torch.

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     This in turns modulates the laser beam, since laser torch acts like a

balanced modulator, where two signals – one is message signal

(audio signal) and carrier laser signal, superimposed. So, laser beam

modulates and transmits the signals to large distances.

B. LASER BASED VOICE RECEIVER

 

      The receiver circuit uses an NPN phototransistor (2N5777) as the

light sensor.

     Here, the phototransistor receives the audio signal of low power and

low amplitude that is followed by a two-stage transistor pre-amplifier.

      In the pre-amplifier stage R8 is a source resistor, which is directly

connected to the power supply.

     The pre amplifier stage is RC coupled amplifier in CE configuration.

      C5, C6 are the junction capacitances, which are taken in to the

account when we consider high frequency response, which is limited

by their presence.

      Resistors R9 and R12 are used to establish the biasing of the

transistor BC549.

     R11 is self-bias resistor, which is used to avoid degeneration.

      C7 is a bypass capacitor, which acts as to prevent loss of

amplification due to negative feedback arrangement.

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     Transistors BC549 are the amplifier transistors, which amplifies the

signal because the signal obtained by the phototransistor is of few

mV.

      C8 is the blocking capacitor, which is connected to the variable

resistor VR2, which in turn followed by audio power amplifier IC

LM386. Pin configuration of LM386 is shown in the glossary.

     Pin no. 1 and 10 is followed by C10, which is an external capacitor,

used to compensate internal error amplifier and thus avoid instability.

     Volume control can be adjusted from variable resistor VR2 of 10 kilo-

ohms.

      LM386 provides suitable power output useful for drive the

loudspeaker of 0.5W.

      From the pin no. 5, the high power as well as suitable amplitude

received audio signal is taken as output.

      R14 and C13 are bypass arrangement used to prevent loss of

amplification.

      C12 capacitor is used for preventing the noise as well as the hum

produced by the ac sources.

     From the loudspeaker, the audio output is heard.

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A BRIEF SUMMARY OF COMPONENTS USED

When a beginner to electronics first looks at a circuit board full of componentshe/she is often overwhelmed by the diversity of do-dads. In these next few sections we will help you to identify some of the simple components and their schematical symbol. Then you should be able to call them resistors and transistors instead of “Whatchamacallits”.

Electronic component are classed into either being Passive devicesOr Active devices.

A Passive Device is one that contributes no power gain (amplification)to a circuit or system. It has not control action and does not require anyinput other than a signal to perform its function. In other words, “Acomponents with no brains!” Examples are Resistors, Capactitors andInductors

Active Devices are components that are capable of controlling voltagesor currents and can create a switching action in the circuit. In otherwords, “Devices with smarts!” Examples are Diodes, Transistors andIntegrated circuits. Most active components are semiconductors.

Resistors:

This is the most common component in electronics. It is used mainly to controlcurrent and voltage within the circuit. You can identify a simple resistorby its simple cigar shape with a wire lead coming out of each end. It uses asystem of color coded bands to identify the value of the component (measuredin Ohms) *A surface mount resistor is in fact mere millimeters in sizebut performs the same function as its bigger brother, the simple resistor. Apotentiometer is a variable resistor. It lets you vary the resistance with a dialor sliding control in order to alter current or voltage on the fly. This is opposedto the “fixed” simple resistors.

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PRESET RESISTORPreset resistor allows adjustments to be arrived out on electronic circuits without the need to make changes to fixed resistors. Various forms of present resistors are commonly available including open carbon tracks skeleton presents and fully encapsulated carbon and multiturn cermet type

s.no parameters Present resistor type

Open skelton carbon

Enclosed carbon

Multiturn cermet

1 Resistance range () 100 to 2.2 470 to 1 100 to 1

2 tolerance + 20 +20 +10

3 Power rating 0.2 0.15 0.25

4 stability Poor Poor good

5 application General purpose General purpose

Small signalAmplifiers,test and measuring equipment.

Condensers/Capacitors:

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Capacitors, or "caps", vary in size and shape - from a small surface mountmodel up to a huge electric motor cap the size of a paint can. It storages electricalenergy in the form of electrostatic charge. The size of a capacitor generally determineshow much charge it can store. A small surface mount or ceramic cap willonly hold a minuscule charge. A cylindrical electrolytic cap will store a muchlarger charge. Some of the large electrolytic caps can store enough charge to killa person. Another type, called Tantalum Capacitors, store a larger charge in asmaller package.

Inductors:You may remember from science class that adding electrical current to a coil of

wire produces a magnetic field around itself. This is how the inductor works. It ischarged with a magnetic field and when that field collapses it produces current inthe opposite direction. Inductors are used in Alternating Current circuits tooppose changes in the existing current. Most inductors can be identified by the"coil" appearance. Others actually look like a resistor but are usually green incolor.

Diodes:Diodes are basically a one-way valve for electrical current. They let it flow in one

direction (from positive to negative) and not in the other direction. This is used to perform rectification or conversion of AC current to DC by clipping off the negative portion of a AC waveform. The diode terminals are cathode and anode and the arrow inside the diode symbol points towards the cathode, indicating current flow in that direction when the diode is forward biased and conducting current. Most diodes are similar in appearance to a resistor and will have a painted line on one end showing the direction or flow(white side is negative). If the negative sideis on the negative end of the circuit, current will flow. If the negative is on the positive side of the circuit no current will flow.

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LEDs (Light Emitting Diodes)

LEDs are simply diodes that emit light of one form or another. They are used asindicator devices. Example: LED lit equals machine on. The general purpose silicondiode emits excess energy in the form of heat when conducting current. If a differentsemiconductor material such as gallium, arsenide phosphide is used, the excessenergy can be released at a lower wavelength visible to human eye. This is thecomposition of LED. They come in several sizes and colors. Some even emit InfraredLight which cannot be seen by the human eye.

Switch :

This is a mechanical part which when pressed makes the current to flow throughit. If the switch is released the current stops flowing through it. This helps to control acircuit.

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Transistors:

The transistor performs two basic functions:

1) It acts as a switch turning current on and off.

2) It acts as a amplifier.This makes an output signal that is amagnified version of the input signal.Transistors come in several sizes dependingon their application. It can be abig power transistor such as is used in power applifiers in your stereo, down toa surface mount (SMT) and even down to .5 microns wide (I.E.: Mucho Small!)such as in a microprocessor or IntegratedCircuit.

NPN Transistor: Bipolar junction perform the function of amplifications wherea small varying voltage or current applied to the base (the lead on the leftside of the symbol) is proportionately replicated by a much larger voltage orcurrent between the collector and emitter leads. Bipolar junction refers to sandwichconstruction of the semiconductor, where a wedge of "P" material is placedbetween two wedges of "N" material. In this NPN construction a small base currentcontrols the larger current flowing from collector to emitter (the lead withthe arrow).

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PNP Transistor: Similar to NPN transistors, PNP's have a wedge of "N" materialbetween two wedges of "P" material. In this design, a base currentregulates the larger current flowing from emitter to collector, as indicatedby the direction of the arrow on the emitter lead. In CED players, PNP transistorsare used less frequently that the NPN type for amplification functions.

PCB’s:

PCB stands for printed circuit board which are used for wiring up of the components of a circuit. PCBs are made of paper phenolic FR2 grade (low cost, for low frequency and low power circuit assembly) and glass epoxy FR4 grade (for high frequency, high power circuits) copper clad laminates (available in 1.6mm, 2.4mm and 3.6mm thickness). Singlesided PCBs have copper foil only on one side while double-sided PCBs have copper foil on both side of the laminate. Thickness of copper foil is 35 micrometer minimum on cheaper PCBs and 70 micrometer on slightly costlier PCBs. Tracks (conductive paths) are made by masking (covering) the track part of copperwith etch-resist enamel paint (you can even use nail polish) and later dipping thelaminate in ferric chloride solutions to dissolve all copper except under the maskedpart. Holes in PCBs are drilled after etching is over. The tracks on two sides of a PCB are joined using printed through hole (PTH) technique, which is equivalent to using slotted copper rivets for joining tracks on both sides. On cheaper PCBs, PTH are not provided, only Pads (i.e. circular copper land with centre hole) are provided and you have to join the tracks on both sides by soldering a copper wire to the pads with a copper wire. In singlesided PCB components are mounted on the side which has no track (called component side). In a double-sided PCB the component side is defined (marked before hand) or it will show component outline (also called silk screen)Green masking is the process of applying a layer of green colour insulation varnish on all parts of tracks except near the holes, to protect the tracks from exposure to atmosphere and thus prolong its life and reliability.

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Batteries:

Symbol of batteries shows +ve terminal by a longer line than the –ve terminal.For low power circuit dry batteries are used.

Speakers:

These convert electrical signals to accoustic viberations. It comprises a permanentmagnet and a moving coil (through which electrical signal is passed). This moving coil is fixed to the diaphram which vibrates to produce sound.

ICs (Integrated Circuits):

Integrated Circuits, or ICs, are complex circuits inside one simple package. Siliconand metals are used to simulate resistors, capacitors, transistors, etc. It is a spacesaving miracle. These components come in a wide variety of packages and sizes.You can tell them by their "monolithic shape" that has a ton of "pins" coming outof them. Their applications are as varied as their packages. It can be a simple timer, toa complex logic circuit, or even a microcontroller (microprocessor with a few added functions) with erasable memory built inside.

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Microprocessors (MPUs):

Microprocessors and other large scale ICs are very complex ICs. At their coreis the transistor which provides the logic for computers, cars, TVs and just abouteverything else electronic. Packages are becoming smaller and smaller as companiesare learning new tricks to make the transistors ever tinier.

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General PurposeSingle Opeartional Amplifier

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SL 100NPN Power Transistor

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CONCLUSION

We got to have a practical overview of what we studied so far in our curriculum. We got a lot of knowledge .