Voice Transmitter and Receiver Based on Laser Torch

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 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 (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, in turn, 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 of The 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 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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A BRIEF DISCRIPTION OF LASER TORCH BASED TRANSMITTER AND RECEIVER PROJECT DESCRIPTION

(a) A. LASER BASED VOICE TRANSMITTER

Ø     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. Ø     Here, the transmitter uses 9V power supply. Ø      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. Ø      The gain of the op-amp can be controlled with the help of 1 mega ohms potmeter. Ø      The AF output from op-amp UA741 is coupled to the base of the power transistor BD139, which in turn, modulates the laser. Ø      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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Ø     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. Ø      Condenser mic is directly followed by the transistor amplifier stage consist of BC548. Transistor BC548 is connected in common emitter configuration. Ø      Resistor R1 is the source resistor, which is directly connected to the power-supply. Ø      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. Ø      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. Ø      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. Ø      The output is taken from the collector terminal; so inverted audio input signal is obtained. Ø      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 resistor. Op-amp ua741 is easily available general-purpose

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operational amplifier. Ø      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 noninverting 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. Ø      Resistors R5 and R6 of it value acts as a voltage-divider network, thus it gives a fixed voltage at the non-inverting pin. Ø      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. Ø      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. Ø      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. Ø      Power transistor BD139 absorbs most of the power and supplies the suitable power to drive the laser torch. Ø      This in turns modulates the laser beam, since laser torch acts like a balanced modulator, where two signals – one is message signal

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(audio signal) and carrier laser signal, superimposed. So, laser beam modulates and transmits the signals to large distances.

B. LASER BASED VOICE RECEIVER

Ø      The receiver circuit uses an NPN phototransistor (2N5777) as the light sensor. Ø      Here, the phototransistor receives the audio signal of low power and low amplitude that is followed by a two-stage transistor pre-amplifier. Ø      In the pre-amplifier stage R8 is a source resistor, which is directly connected to the power supply. Ø     The pre amplifier stage is RC coupled amplifier in CE configuration. Ø      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. Ø      Resistors R9 and R12 are used to establish the biasing of the transistor BC549. Ø     R11 is self-bias resistor, which is used to avoid degeneration. Ø      C7 is a bypass capacitor, which acts as to prevent loss of amplification due to negative feedback arrangement. Ø      Transistors BC549 are the amplifier transistors, which amplifies the signal because the signal obtained by the phototransistor is of few mV.

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Ø      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. Ø      Pin no. 1 and 10 is followed by C10, which is an external capacitor, used to compensate internal error amplifier and thus avoid instability. Ø      Volume control can be adjusted from variable resistor VR2 of 10 kiloohms. Ø      LM386 provides suitable power output useful for drive the loudspeaker of 0.5W. Ø      From the pin no. 5, the high power as well as suitable amplitude received audio signal is taken as output. Ø      R14 and C13 are bypass arrangement used to prevent loss of amplification. Ø      C12 capacitor is used for preventing the noise as well as the hum produced by the ac sources. Ø     From the loudspeaker, the audio output is heard.

A BRIEF SUMMARY OF COMPONENTS USED

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When a beginner to electronics first looks at a circuit board full of components he/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 devices Or 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 any input other than a signal to perform its function. In other words, “A components with no brains!” Examples are Resistors, Capactitors and Inductors Active Devices are components that are capable of controlling voltages or currents and can create a switching action in the circuit. In other words, “Devices with smarts!” Examples are Diodes, Transistors and Integrated circuits. Most active components are semiconductors.

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

Ø

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PRESET RESISTOR Preset 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

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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 signal Amplifiers,test and measuring equipment.

Condensers/Capacitors: Capacitors, or "caps", vary in size and shape - from a small surface mount model up to a huge electric motor cap the size of a paint can. It storages electrical energy in the form of electrostatic charge. The size of a capacitor generally determines how much charge it can store. A small surface mount or ceramic cap will only hold a minuscule charge. A cylindrical electrolytic cap will store a much larger charge. Some of the large electrolytic caps can store enough charge to kill a person. Another type, called Tantalum Capacitors, store a larger charge in a smaller package.

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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 is charged with a magnetic field and when that field collapses it produces current in the opposite direction. Inductors are used in Alternating Current circuits to oppose changes in the existing current. Most inductors can be identified by the "coil" appearance. Others actually look like a resistor but are usually green in color.

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 side is 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 as indicator devices. Example: LED lit equals machine on. The general purpose silicon diode emits excess energy in the form of heat when conducting current. If a different semiconductor material such as gallium, arsenide phosphide is used, the excess energy can be released at a lower wavelength visible to human eye. This is the composition of LED. They come in several sizes and colors. Some even emit Infrared Light which cannot be seen by the human eye.

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

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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 a big power transistor such as is used in power applifiers in your stereo, down to a 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 where a small varying voltage or current applied to the base (the lead on the left side of the symbol) is proportionately replicated by a much larger voltage or current between the collector and emitter leads. Bipolar junction refers to sandwich construction of the semiconductor, where a wedge of "P" material is placed between two wedges of "N" material. In this NPN construction a small base current controls 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" material between two wedges of "P" material. In this design, a base current regulates the larger current flowing from emitter to collector, as indicated by the direction of the arrow on the emitter lead. In CED players, PNP transistors are 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 copper with etch-resist enamel paint (you can even use nail polish) and later dipping the laminate in ferric chloride solutions to dissolve all copper except under the masked part. 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 permanent magnet 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. Silicon and metals are used to simulate resistors, capacitors, transistors, etc. It is a space saving 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 out of them. Their applications are as varied as their packages. It can be a simple timer, to a 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 core is the transistor which provides the logic for computers, cars, TVs and just about everything else electronic. Packages are becoming smaller and smaller as companies are learning new tricks to make the transistors ever tinier.

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General PurposeSingle Opeartional Amplifier

27

28

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SL 100 NPN Power Transistor

30

31

32

33

34

35

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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 .

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