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Laser Torch Based Voice Transceiver Miniproject “11 Jyothi Engineering College 1 Dept. of ECE 1. INTRODUCTION Laser as a communication medium can provide a good substitute for the present day communication systems as the problem of interference faced in case of electromagnetic waves is not there and high deal of secrecy is available. Laser communications offers a viable alternative to RF communications for inter satellite links and other applications where high performance links are a necessity. High data rate, small antenna size, narrow beam divergence, and a narrow field of view are characteristics of laser communications that offer a number of potential advantages for system design. The purpose of the project is to determine the feasibility of replacing microwave communications with laser communications to remote locations. This link is unreliable and can be disrupted in fog or rain. A basic communication system is made up of three main parts being the transmitter, the medium over which the message is being sent, and the receiver. A good example of this is two people communicating from one side of a room to the other. If the person wants to communicate with the other person, he/she speaks words towards the direction of the other individual who receives the voice information and determines the message. This example is much like how any general communication system works. First, the message is determined that needs to be sent to the receiving end. The message is then sent to the transmitter. The transmitter, much like the person’s mouth, is sending the signal containing the message from one person to the other. This can be compared to using an antenna to send out a signal. Laser emission power and the nonlinear distortion depends on the bias current (IO), so the laser light transmitter to set the bias circuit and pre-distortion compensation circuit to ensure that the output of nonlinear index and emission stability. As the value of Laser temperature increases, the threshold will increase, decrease saturated output intensity.

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Page 1: Laser Torch

Laser Torch Based Voice Transceiver Miniproject “11

Jyothi Engineering College 1 Dept. of ECE

1. INTRODUCTION

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

communication systems as the problem of interference faced in case of electromagnetic

waves is not there and high deal of secrecy is available. Laser communications offers a

viable alternative to RF communications for inter satellite links and other applications

where high performance links are a necessity.

High data rate, small antenna size, narrow beam divergence, and a narrow field of

view are characteristics of laser communications that offer a number of potential

advantages for system design.

The purpose of the project is to determine the feasibility of replacing microwave

communications with laser communications to remote locations. This link is unreliable and

can be disrupted in fog or rain. A basic communication system is made up of three main

parts being the transmitter, the medium over which the message is being sent, and the

receiver. A good example of this is two people communicating from one side of a room to

the other. If the person wants to communicate with the other person, he/she speaks words

towards the direction of the other individual who receives the voice information and

determines the message. This example is much like how any general communication

system works.

First, the message is determined that needs to be sent to the receiving end. The

message is then sent to the transmitter. The transmitter, much like the person’s mouth, is

sending the signal containing the message from one person to the other. This can be

compared to using an antenna to send out a signal.

Laser emission power and the nonlinear distortion depends on the bias current (IO),

so the laser light transmitter to set the bias circuit and pre-distortion compensation circuit

to ensure that the output of nonlinear index and emission stability. As the value of Laser

temperature increases, the threshold will increase, decrease saturated output intensity.

Page 2: Laser Torch

Laser Torch Based Voice Transceiver Miniproject “11

Jyothi Engineering College 2 Dept. of ECE

The signal then must travel through some type of medium to reach the receiver. For

the two people talking, this medium would be air. But, sometimes this medium is some

type of cable or wire. The signal is then collected by the receiver, which is comparable to

the person on the receiving end hearing the sound of the person’s voice. Sometimes the

signal can be immediately understood, but other times the signal must first be decoded in

order to perfectly understand what actually the input signal that is fed to the transmitter-

section.

The transceiver basically comprises of a transmitter section and a receiver section

and the input is fed via a condenser mic that gives it to the transmitter and is carried by the

laser beam from the laser torch. The modulated input is received by the receiver at a

maximum possible distance of about 500m and is detected by the phototransistor and then

amplified by the amplifier circuit and is then given to the loud speaker which transforms it

to the original voice message input given initially to the mic . The basic principle

employed here is amplitude modulation where the laser beam is the carrier. A DC power

supply of 9V is used inorder to drive the laser torch and bias the entire transceiver circuit.

An approximate distance of about 500m is the considered range.

1.1 LITERATURE SURVEY

The idea that inspired laser communications is simple. In today’s world, data can

be packaged much more quickly and efficiently without wires and messy connectivity, a

stark contrast to only a few years ago, when wires were the necessity. We had the idea of

doing some communication related project as communication students.

The first thing that striked us was a sort of an FM station that would realize our

dream of a college station. But the problems regarding the outer noise, frequency band of

selection and the licencing and also the topography of our college, we were forced to drop

the idea. Then we adopted a new idea of using some carier for the purpose of modulation

which is highly coherent,directed,energized and reliable which finally took us to the idea

of constructing a laser based voice transceiver. With laser technology, data moves more

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Laser Torch Based Voice Transceiver Miniproject “11

Jyothi Engineering College 3 Dept. of ECE

quickly and with just as much integrity as it would have through a wire. Additionally, a

wire system imposes none of the messy assorted installation costs of a wired system: you

install the system, and then you can create wireless bridges without having to actually run

any more cord.

Laser line of sight communications have never been more practical. They offer

tremendous flexibility, allowing you to choose virtually any communication format you

prefer. They are also secure and more cost effective, even than wireless radio transmissions

over short range. They tend not to be such good choices if you intend to transmit frequent

long distance data bursts as the signal requires a specific line of sight, which can become

impaired over wider expanses.

Laser Communication systems are wireless communications through the

atmosphere. They work similarly to fiber optic links, except the beam is transmitted

through free space. While the transmitter and receiver must require line-of- sight

conditions, they have the benefit of eliminating the need for broadcast rights and buried

cables. Laser Communication systems can be easily deployed since they are inexpensive,

small, low power and do not require any radio interference studies. The carrier used for the

transmission signal is typically generated by a laser diode. Two parallel beams are needed,

one for the transmission and one for reception.

Using this circuit we decided to implement the idea of communicating with our

neighbors wirelessly. Instead of RF signals, light from a laser torch is used as the carrier in

the circuit.

Page 4: Laser Torch

Laser Torch Based Voice Transceiver Miniproject “11

Jyothi Engineering College 4 Dept. of ECE

2. LASER TORCH BASED VOICE TRANSCEIVER

BLOCK DIAGRAM

The basic circuit includes a transmitter and a receiver section. The above shown is

a schematic representation of LASER torch based voice transceiver. The input is fed to a

condenser mic which is modulated at the transmitter end and then detected by a photo

transistor at the receiver end and then it’s fed to a loud speaker where the voice is

regenerated.

Figure 2.1 Transceiver Block Diagram

2.1 TRANSMITTER

The transmitter circuit (Fig.2.2) comprises condenser microphone transistor

amplifier BC548 (T1) followed by an op-amp stage built around µA741 (IC1). The gain of

the op-amp can be controlled with the help of 1-mega-ohm potentiometer VR1.The AF

output from IC1 is coupled to the base of transistor BD139 (T2), which, inturn, 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. The torch connected

to the emitter ofBD139 and the spring-loaded lead protruding from inside the torch to

circuit ground.

CONDENSER

MIC

LOUD SPEAKER RECEIVER

TRANSMITTER LASER TORCH

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Laser Torch Based Voice Transceiver Miniproject “11

Jyothi Engineering College 5 Dept. of ECE

Figure 2.2.Transmitter circuit

2.2 RECEIVER

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.

The receiver section comprises of a receiving phototransistor which will detect the

received laser beam containing the input information and then it is amplified by the two

stage amplifier section that is fed to the input terminals of the comparator circuit of the

receiver section as per the figure shown.

Page 6: Laser Torch

Laser Torch Based Voice Transceiver Miniproject “11

Jyothi Engineering College 6 Dept. of ECE

The comparator is driven by the positive power supply and the terminal number

four of the audio power amplifier is then grounded and its output is the amplified version

of the input that is detected by the receiver. The actual input is then made ripple free by a

capacitor that is connected to the amplifier section and is fed to capacitor that is parallel to

the resistor at the load section as shown in the diagram below.

The above shown receiver receives signal which is carried by a laser beam that acts

as the carrier for this amplitude modulation technique employed. The received signal may

contain noises which will be filtered of using a pi filter.

Figure 2.3.Receiver circuit

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Laser Torch Based Voice Transceiver Miniproject “11

Jyothi Engineering College 7 Dept. of ECE

3. CIRCUIT COMPONENTS

When a beginner to the electronics first looks at a circuit board full of circuit

components he/ she 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 schematic

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, Capacitors 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.

3.1 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.

Page 8: Laser Torch

Laser Torch Based Voice Transceiver Miniproject “11

Jyothi Engineering College 8 Dept. of ECE

Figure 3.1.1.Resistor bands

3.2 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 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.

Figure 3.2.1.Capacitors

Page 9: Laser Torch

Laser Torch Based Voice Transceiver Miniproject “11

Jyothi Engineering College 9 Dept. of ECE

3.3 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.

Figure 3.3.1. Diode

3.4 LED

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.

Page 10: Laser Torch

Laser Torch Based Voice Transceiver Miniproject “11

Jyothi Engineering College 10 Dept. of ECE

Figure 3.4.1.LED

3.5 SWITCH

This is a mechanical part which when pressed makes the current to flow through

that. If the switch is released the current stops flowing through that. This helps to control a

circuit.

Figure 3.5.1.Switch

3.6 TRANSISTORS

The transistor performs two basic functions: It acts as a switch turning current on

and off and it acts as an amplifier. This makes an output signal that is a magnified version

of the input signal. Transistors come in several sizes depending on their application. It can

be a big power transistor such as is used in power amplifiers 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 Integrated Circuit.

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Laser Torch Based Voice Transceiver Miniproject “11

Jyothi Engineering College 11 Dept. of ECE

Figure 3.6.1.Transistors

3.6.1 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 with the arrow).

Figure 3.6.2.NPN Transistor

3.6.2 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

Page 12: Laser Torch

Laser Torch Based Voice Transceiver Miniproject “11

Jyothi Engineering College 12 Dept. of ECE

CED players, PNP transistors are used less frequently that the NPN type for amplification

functions.

Figure 3.6.3.PNP Transistor

3.7 PCB

PCB stands for printed circuit board which is 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).

Single sided 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.

Page 13: Laser Torch

Laser Torch Based Voice Transceiver Miniproject “11

Jyothi Engineering College 13 Dept. of ECE

In single sided 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.

Figure 3.7.1.PCB

3.8 SPEAKER

These convert electrical signals to acoustic vibrations. It comprises a permanent

magnet and a moving coil (through which electrical signal is passed). This moving coil is

fixed to the diaphragms which vibrate to produce sound.

Figure 3.8.1.Speaker

Page 14: Laser Torch

Laser Torch Based Voice Transceiver Miniproject “11

Jyothi Engineering College 14 Dept. of ECE

3.9 ICs

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.

3.9.1 uA741

Of the different types of opamps produced, type 741 has achieved a very wide

popularity. It is available in 14- pin dual-in line, 8 – pin dual-in line or in TO- style

packages. Integrated circuit type 747 accommodates two type 741 operational amplifiers in

a single package.

The op-amp needs a dual symmetrical power supply. With its center tap grounded.

This enables the op-amp to amplify dc signals of both polarities, positive or negative, with

respect to ground. The circuit is so designed that if both inputs are connected to ground, the

dc output voltage is zero. However, because of small internal unbalances, a small dc

voltage may appear at the output. It is too small to be objectionable in normal applications.

For critical applications, the output voltage can be set precisely to zero by

connecting a 10K potentiometer between terminals marked “offset-null”. It is possible to

operate the 741 on a single rail supply also.

This is usually done by raising the standing dc input voltage to the non-inverting

input terminal to approximately half the supply voltage by a voltage divider network. The

output dc voltage in such cases stands at half the supply voltage. Bt this does not matter

because the dc can be easily blocked by a capacitor allowing only the ac signal to be

passed on to the next stage.

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Laser Torch Based Voice Transceiver Miniproject “11

Jyothi Engineering College 15 Dept. of ECE

Figure 3.9.1.1.uA741C Opamp

The operational amplifier type 741 has many features that have made it so popular.

It has an in-built circuitry that provides full protection against output overloads or even

shorts to ground for any length of time. The 741 does not need any external component for

phase compensation or adjusting its frequency response. This simplifies its circuit design

and minimizes the number of components used.

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Laser Torch Based Voice Transceiver Miniproject “11

Jyothi Engineering College 16 Dept. of ECE

4. CONDENSER MICROPHONE

The condenser microphone, invented at Bell Labs in 1916 by E. C. Wentes also

called a capacitor microphone or electrostatic microphone. Here, the diaphragm acts as

one plate of a capacitor, and the vibrations produce changes in the distance between the

plates. There are two types, depending on the method of extracting the audio signal from

the transducer: DC-biased and radio frequency (RF) or high frequency (HF) condenser

microphones. The voltage across the capacitor plates change according to the vibrations of

the plates in response to air variations. Voltage across the plates changes as (C= Q / V),

where Q = charge in coulombs, C = capacitance in farads and V = potential difference

in volts. The capacitance of the plates is inversely proportional to the distance between

them for a parallel-plate capacitor. (See capacitance for details.) The assembly of fixed and

movable plates is called an "element" or "capsule."

A nearly constant charge is maintained on the capacitor. As the capacitance

changes, the charge across the capacitor does change very slightly, but at audible

frequencies it is sensibly constant. The capacitance of the capsule (around 5 to 100 pF) and

the value of the bias resistor (100 mega ohms to tens of giga ohms) form a filter that is

high-pass for the audio signal, and low-pass for the bias voltage. Note that the time

constant of an RC circuit equals the product of the resistance and capacitance.

Within the time-frame of the capacitance change (as much as 50 ms at 20 Hz audio

signal), the charge is practically constant and the voltage across the capacitor changes

instantaneously to reflect the change in capacitance. The voltage across the capacitor varies

above and below the bias voltage. The voltage difference between the bias and the

capacitor is seen across the series resistor. The voltage across the resistor is amplified for

performance or recording. In most cases, the electronics in the microphone itself contribute

no voltage gain as the voltage differential is quite significant, up to several volts for high

sound levels. Since this is a very high impedance circuit, current gain only is usually

needed with the voltage remaining constant. The circuit is therefore often called an

"impedance converter" or "follower" because no voltage gain is provided]

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Laser Torch Based Voice Transceiver Miniproject “11

Jyothi Engineering College 17 Dept. of ECE

RF condenser microphones use a comparatively low RF voltage, generated by a

low-noise oscillator. The signal from the oscillator may either be amplitude modulated by

the capacitance changes produced by the sound waves moving the capsule diaphragm, or

the capsule may be part of a resonant circuit that modulates the frequency of the oscillator

signal. Demodulation yields a low-noise audio frequency signal with a very low source

impedance. The absence of a high bias voltage permits the use of a diaphragm with looser

tension, which may be used to achieve wider frequency response due to higher compliance.

The RF biasing process results in a lower electrical impedance capsule, a useful by-product

of which is that RF condenser microphones can be operated in damp weather conditions

that could create problems in DC-biased microphones with contaminated insulating

surfaces. The Sennheiser "MKH" series of microphones use the RF biasing technique.

Figure 4.1.Condenser microphone

Condenser microphones span the range from telephone transmitters through

inexpensive karaoke microphones to high-fidelity recording microphones. They generally

produce a high-quality audio signal and are now the popular choice in laboratory and

studio recording applications. The inherent suitability of this technology is due to the very

small mass that must be moved by the incident sound wave, unlike other microphone types

that require the sound wave to do more work. They require a power source, provided either

via microphone inputs on equipment as phantom power or from a small battery. Power is

necessary for establishing the capacitor plate voltage, and is also needed to power the

microphone electronics (impedance conversion in the case of electret and DC-polarized

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Jyothi Engineering College 18 Dept. of ECE

microphones, demodulation or detection in the case of RF/HF microphones). Condenser

microphones are also available with two diaphragms that can be electrically connected to

provide a range of polar patterns (see below), such as cardioid, omnidirectional, and figure-

eight. It is also possible to vary the pattern continuously with some microphones, for

example the Røde NT2000 or CAD M179.

4.1 ELECTRET CONDENSER MICROPHONE

An electret microphone is a relatively new type of capacitor microphone invented

at Bell laboratories in 1962 by Gerhard Sessler and Jim West. The externally applied

charge described above under condenser microphones is replaced by a permanent charge in

an electret material. An electret is actually a ferroelectric material with the main feature of

having a permanently electrically charged or polarized state. This special characteristic

name comes from electrostatic and magnet; a static charge is embedded in an electret by

alignment of the static charges in the material, much the way a magnet is made by aligning

the magnetic domains in a piece of iron.

Due to their good performance and ease of manufacture, hence low cost, the vast

majority of microphones made today are electret microphones; a semiconductor

manufacturer estimates annual production at over one billion units. Nearly all cell-phone,

computer, PDA and headset microphones are electret types. They are used in many

applications, from high-quality recording and lavaliere use to built-in microphones in

small sound recording devices and telephones.

Though electret microphones were once considered low quality, the best ones can

now rival traditional condenser microphones in every respect and can even offer the long-

term stability and ultra-flat response needed for a measurement microphone. Unlike other

capacitor microphones, they require no polarizing voltage, but often contain an

integrated preamplifier that does require power (often incorrectly called polarizing power

or bias). This preamplifier is frequently phantom powered in sound reinforcement and

studio applications.

Page 19: Laser Torch

Laser Torch Based Voice Transceiver Miniproject “11

Jyothi Engineering College 19 Dept. of ECE

Microphones designed for personal computer (PC) use, sometimes called

multimedia microphones, use a stereo 3.5 mm plug (though a mono source) with the ring

receiving power via a resistor from (normally) a 5 V supply in the computer; unfortunately,

a number of incompatible dynamic microphones are fitted with 3.5 mm plugs too. While

few electret microphones rival the best DC-polarized units in terms of noise level, this is

not due to any inherent limitation of the electret. Rather, mass production techniques

needed to produce microphones cheaply don't lend themselves to the precision needed to

produce the highest quality microphones, due to the tight tolerances required in internal

dimensions. These tolerances are the same for all condenser microphones, whether the DC,

RF or electret technology is used.

Figure 4.1.1.Small diaphragm mic

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Laser Torch Based Voice Transceiver Miniproject “11

Jyothi Engineering College 20 Dept. of ECE

5. CONSTRUCTION OF LASER TORCH

A laser is constructed from three principal parts:

� An energy source (usually referred to as the pump or pump source),

� A gain medium or laser medium, and

� Two or more mirrors that form an optical resonator.

5.1 PUMP SOURCE

The pump source is the part that provides energy to the laser system.

Examples of pump sources include electrical discharges, flash lamps, arc lamps, light

from another laser, chemical reactions and even explosive devices. The type of pump

source used principally depends on the gain medium, and this also determines how the

energy is transmitted to the medium. A helium-neon (HeNe) laser uses an electrical

discharge in the helium-neon gas mixture, a Nd:YAG laser uses either light focused

from a xenon flash lamp or diode lasers, and excimer lasers use a chemical reaction.

5.2 GAIN MEDIUM

The gain medium is the major determining factor of the wavelength of operation,

and other properties, of the laser. Gain media in different materials have linear spectra or

wide spectra. Gain media with wide spectra allow tune frequency of laser. First

wide tunable crystal laser with tunability more octave represent on photo

3 http://spie.org/x39922.xml . There are hundreds if not thousands of different gain media

in which laser operation has been achieved (see list of laser types for a list of the most

important ones). The gain medium is excited by the pump source to produce a population

inversion, and it is in the gain medium that spontaneous and stimulated emission of

photons takes place, leading to the phenomenon of optical gain, or amplification.

Examples of different gain media include:

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Laser Torch Based Voice Transceiver Miniproject “11

Jyothi Engineering College 21 Dept. of ECE

Liquids such as dye lasers. These are usually organic chemical solvents, such as

methanol, ethanol or ethylene glycol, to which are added chemical dyes such

as coumarin, rhodamine and fluorescein. The exact chemical configuration of the dye

molecules determines the operation wavelength of the dye laser.

Gases, such as carbon dioxide, argon, krypton and mixtures such as helium-neon.

These lasers are often pumped by electrical discharge. Solid host materials include

YAG (yttrium aluminium garnet).

5.3 OPTICAL RESONATOR

The optical resonator, or optical cavity, in its simplest form is two parallel mirrors

placed around the gain medium which provide feedback of the light. The mirrors are

given optical coatings which determine their reflective properties. Typically one will be

a high reflector, and the other will be a partial reflector. The latter is called the output

coupler, because it allows some of the light to leave the cavity to produce the laser's output

beam.

Light from the medium, produced by spontaneous emission, is reflected by the

mirrors back into the medium, where it may be amplified by stimulation. The light may

reflect from the mirrors and thus pass through the gain medium many hundreds of times

before exiting th In more complex lasers, configurations with four or more mirrors forming

the cavity are used. The design and alignment of the mirrors with respect to the medium is

crucial to determining the exact operating wavelength and other attributes of the laser

system.

Other optical devices, such as spinning mirrors, modulators, filters, and absorbers,

may be placed within the optical resonator to produce a variety of effects on the laser

output, such as altering the wavelength of operation or the production of pulses of laser

light.

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Jyothi Engineering College 22 Dept. of ECE

Some lasers do not use an optical cavity, but instead rely on very high optical gain

to produce significant amplified spontaneous emission (ASE) without needing feedback of

the light back into the gain medium. Such lasers are said to be super luminescent, and emit

light with low coherence but high bandwidth. Since they do not use optical feedback, these

devices are often not categorized as lasers.

Figure 5.1.Nd:YAG solid-state laser

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Jyothi Engineering College 23 Dept. of ECE

6. RESULTS AND DISCUSSIONS

LASER TORCH BASED VOICE TRANSCEIVER is well built and the output is

verified. Basically the circuit comprises of a transmitter and a receiver section. The input

voice signal is fed through a condenser mic and the same is modulated via Am at the

transmitter section. The input only if uninterrupted will be heard properly at the receiver

end.

There is a coupling capacitance at the input end that couples the ac and removes the

dc components. That means the input if given only, are there the transistor will be forward

biased. Else the transistor will be reversed biased and there will be high voltage at the

collector terminal. This high voltage will be coupled by the capacitance and is given to the

comparator which is an op-amp here. The noise from the power supply is filtered out by

the pi filter. The output of the comparator is given to the transistor which act as the heat

sink and the emitter voltage drives the LASER that acts as the carrier for the amplitude

modulation.

The input that is given to the mic at the transmitter section will be carried to the

receiver photo transistor by the laser beam and is detected by the photo transistor and then

given to the two stage amplifier. Then this is fed to the audio power amplifier that

amplifies the output that is intended at the receiver section. This amplified voice signal is

then detected by the RC circuit and is fed to the loudspeaker. During the experiment it has

been identified that the terminals between which the phototransistor when shorted yields a

drip sound. This is one of the methods to determine whether the circuit is right or wrong.

If the above mentioned method is employed, then the circuit can be checked almost

accurately.

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Laser Torch Based Voice Transceiver Miniproject “11

Jyothi Engineering College 24 Dept. of ECE

7. CONCLUSION

The “LASER TORCH BASED VOICE TRANSCEIVER” is successfully

completed with proper output. Even though similar projects are available, it is arguable for

us that our project is most convenient circuit with comparatively high throughput at

minimum cost. We have done our maximum to update appropriate changes in the hardware

to make our project a standard one. We are happy to put our entire effort open source and

are interested to promote among people who make use of highly accurate signal analysis in

the lab experiments. While pursuing our mini project, we have learned a lot new things

both in circuit designing and debugging. Even a beginner looks at this circuit, it is easier

for him to recognize what the circuit actually comprises of. The voltage drops are such that

the total voltage of 9V is obtained across the biasing resistors. The potential of 0.6V is

obtained across the base to emitter junction. There is a volume control which is a pot and

that is fed to the input of the op-amp which is an audio power amplifier and then fed to the

speaker. It is evident that the project has helped us lot to understand the basic concept of

amplitude modulation. The laser which acts as the carrier for the modulation process is

well modulated during the transmission process.

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Laser Torch Based Voice Transceiver Miniproject “11

Jyothi Engineering College 25 Dept. of ECE

BIBLIOGRAPHY

1. www.seminarprojects.com

2. www.electronicsforu.com/efylinux/circuit/jan2002/circuit

3. www.elxproject.com

4. www.fullinterview.com

5. epfy.blogspot.com/2009/02

6. sourceforge.net/projects

7. qt.nokia.com

8. en.wikipedia.org

9. www.matni.com

10. www.howstuffworks.com

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Laser Torch Based Voice Transceiver Miniproject “11

Jyothi Engineering College 26 Dept. of ECE