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   Using this circuit you can communicate with your neighbours wirelessly. Infrared

signals are used as the carrier in the circuit. The infrared signals can transmit up to a

distance of about 5 meters. The phototransistor of the receiver must be accurately oriented

towards the IR beam. If there is any obstruction in the path of the IR beam, no sound will

be heard from the receiver.



           1.1   Objective of the Project
           1.2   Circuit Diagram
           1.3   Principle of Operation
           1.4   System Features
           1.5   Equipments

           2.1   Block Diagram Description
           3.1   Resistors
           3.2   Capacitirs
           3.3   Use of Infrared Detector Basics
           3.4   Condenser Microphones
           3.5   IC UA741
           3.6   IC LM386
           3.7   BD139 Heat Sink
           3.8   Power Supply

                         LIST OF FIGURES

Fig. No.    Name                           Page No.

 1         Transmitter                       4

 2         Receiver                          5

                      CHAPTER 1

   IR-BASED VOICE TRANSMITTER AND RECEIVER, the aim of the project is to

communicate wirelessly. In this project we are using various transistors, LED, op-amp etc.

and we made a detailed study on this so as to effectively use them in our project.

   The aim of the project is to communicate between any 2 distant places wirelessly

which are at a distance of 5meters apart. The distance can be improved to even 500meters

using Laser light devices. This communication is very much use full when you want to

communicate with some one which is very important.

   This kind of technology is very much useful in Army and Defence, since their

communication is very much crucial.



   When ever you want to transmit information from one person to another our equipment

i.e. transmitter and receiver is very much useful.

   You may also think a mobile does the same work, but it charges for every second you

talk, our system just works like a walky-talky. As a mobile, you need not press the number

whom u need to call, if the other person had this equipment, by just pressing a key you will

get connected.

1.4 System Features:
                      Easy operation
                      Convenient
                      Affordable

             Required Skills:
                      Understanding of IC’s
                      Understanding op-amps and all

                  Design and Fabrication of PCB

      Project phase: -
                  Schematic design and drawing of PCB
                  Preparation of PCB
                  Assembling and Testing of Interfacing Circuits
                  Project Report

              Printed Circuit Board

              9volts Power Supply

              IC UA741 and LM386

              Transistors BC547 and BC548

              Resistors and Capacitors

              BD139 Heat Sink

              Mike and Speaker

              2N5777/L14F1 Photo Transistor

              IR led’s

                                 CHAPTER 2


   Using this circuit you can communicate with your neighbours wirelessly. The IR led

can transmit light up to a distance of about 5 meters. The phototransistor of the receiver

must be accurately oriented towards the IR beam. If there is any obstruction in the path of

the laser beam, no sound will be heard from the receiver. The tra nsmitter circuit (Fig. 1)

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. 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


                                   CHAPTER 3

   A Resistor is a two-terminal electronic component designed to oppose an electric

current by producing a voltage drop between its terminals in proportion to the current,

that is, in accordance with Ohm's law:

V = IR

   Resistors are used as part of electrical networks and electronic circuits. They are

extremely commonplace in most electronic equipment. Practical resistors can be made

of various compounds and films, as well as resistance wire (wire made of a high-

resistivity alloy, such as nickel/chrome).

   The primary characteristics of resistors are their resistance and the power they can

dissipate. Other characteristics include temperature coefficient, noise, and inductance.

Less well-known is critical resistance, the value below which power dissipation limits

the maximum permitted current flow, and above which the limit is applied voltage.

Critical resistance depends upon the materials constituting the resistor as well as its

physical dimensions; it's determined by design.

   Resistors can be integrated into hybrid and printed circuits, as well as integrated

circuits. Size, and position of leads (or terminals) are relevant to equipment designers;

resistors must be physically large enough not to overheat when dissipating their power.

                       Fixed and Variable Resistors

      There are two kinds of resistors, FIXED and VARIABLE. The fixed resistor will

have one value and will never change (other than through temperature, age, etc.). The

resistors shown in A and B of figure 1-29are classed as fixed resistors. The tapped

resistor illustrated in B has several fixed taps and makes more than one resistance value

available. The sliding contact resistor shown in C has an adjustable collar that can be

moved to tap off any resistance within the ohmic value range of the resistor.

   There are two types of variable resistors, one called a POTENTIOMETER and the

other a RHEOSTAT (see views D and E of fig. 1-29.)An example of the potentiometer

is the volume control on your radio, and an example of the rheostat is the dimmer

control for the dash lights in an automobile. There is a slight difference between them.

Rheostats usually have two connections, one fixed and the other moveable. Any variable

resistor can properly be called a rheostat. The potentiometer always has three

connections, two fixed and one moveable. Generally, the rheostat has a limited range of

values and a high current-handling capability. The potentiometer has a wide range of

values, but it usually has a limited current-handling capability. Potentiometers are

always connected as voltage dividers.


                                                                Preset              Symbol

   These are miniature versions of the standard variable

resistor. They are designed to be mounted directly onto the circuit board and adjusted only

when the circuit is built. For example to set the frequency of an alarm tone or the

sensitivity of a light-sensitive circuit. A small screwdriver or similar tool is required to

adjust presets.

   Presets are much cheaper than standard variable resistors so they are sometimes used

in projects where a standard variable resistor would normally be used.

   Multiturn presets are used where very precise adjustments must be made. The screw

must be turned many times (10+) to move the slider from one end of the track to the

other, giving very fine control.

           Preset            Presets
                                                        Multiturn preset
           (open style)      (closed style)



   Capacitors store electric charge. They are used with resistors in timing circuits because

it takes time for a capacitor to fill with charge. They are used to smooth varying DC

supplies by acting as a reservoir of charge. They are also used in filter circuits because

capacitors easily pass AC (changing) signals but they block DC (constant) signals.


   This is a measure of a capacitor's ability to store charge. A large capacitance means that

more charge can be stored. Capacitance is measured in farads, symbol F. However 1F is

very large, so prefixes are used to show the smaller values.

       Three prefixes (multipliers) are used, µ (micro), n (nano) and p (pico):

       µ means 10-6 (millionth), so 1000000µF = 1F

       n means 10-9 (thousand- millionth), so 1000nF = 1µF

      p means 10-12 (million- millionth), so 1000pF = 1nF

   Capacitor values can be very difficult to find because there are many types of

capacitor with different labeling systems!

   There are many types of capacitor but they can be split into two groups, polarised

and unpolarised. Each group has its own circuit symbol.

Polarised capacitors (large values, 1µF +)

Electrolytic Capacitors

                                                  +    -

   Electrolytic capacitors are polarised and they must be connected the correct way

round, at least one of their leads will be marked + or -. They are not damaged by heat

when soldering.

   There are two designs of electrolytic capacitors; axial where the leads are attached to

each end (220 µF in picture) and radial where both leads are at the same end (10µF in

picture). Radial capacitors tend to be a little smaller and they stand upright on the circuit


   It is easy to find the value of electrolytic capacitors because they are clearly printed

with their capacitance and voltage rating. The voltage rating can be quite low (6V for

example) and it should always be checked when selecting an electrolytic capacitor. If the

project parts list does not specify a voltage, choose a capacitor with a rating which is

greater than the project's power supply voltage. 25V is a sensible minimum most battery


3.3 Use of Infrared Detectors Basics

IR emitter and IR phototransistor

   An infrared emitter is an LED made from gallium arsenide, which e mits near- infrared

energy at about 880nm.

   The infrared phototransistor acts as a transistor with the base voltage determined by the

amount of light hitting the transistor.

   Hence it acts as a variable current source. Greater amount of IR light cause greater

currents to flow through the collector-emitter leads.

   As shown in the diagram below, the phototransistor is wired in a similar configuration

to the voltage divider.

   The variable current traveling through the resistor causes a volta ge drop in the pull- up


This voltage is measured as the output of the device

   IR reflectance sensors contain a matched infrared transmitter and infrared receiver pair.

These devices work by measuring the amount of light that is reflected into the receiver.

Because the receiver also responds to ambient light, the device works best when well

shielded from abient light, and when the distance between the sensor and the reflective

surface is small(less than 5mm).

   IR reflectance sensors are often used to detect white and black surfaces. White surfaces

generally reflect well, while black surfaces reflect poorly. One of such applications is the

line follower of a robot.

Schematic Diagram for a Single Pair of Infrared Transmitter and Receiver

3.4 Condenser Microphones
   Condenser means capacitor, an electronic component which stores energy in the form of

an electrostatic field. The term condenser is actually obsolete but has stuck as the name for

this type of microphone, which uses a capacitor to convert acoustical energy into electrical


   Condenser microphones require power from a battery or external source. The resulting

audio signal is stronger signal than that from a dynamic. Condensers also tend to be more

sensitive and responsive than dynamics, making them well-suited to capturing subtle

nuances in a sound. They are not ideal for high- volume work, as their sensitivity makes

them prone to distort.

How Condenser Microphones Work:

   A capacitor has two plates with a voltage between them. In the condenser mic, one of

these plates is made of very light material and acts as the diaphragm. The diaphragm

vibrates when struck by sound waves, changing the distance between the two plates and

therefore changing the capacitance. Specifically, when the plates are closer together,

capacitance increases and a charge current occurs. When the plates are further apart,

capacitance decreases and a discharge current occurs.

   A voltage is required across the capacitor for this to work. This voltage is supplied

either by a battery in the mic or by external phantom power.

Cross-Section of a Typical Condenser Microphone

The Electret Condenser Microphone

   The electret condenser mic uses a special type of capacitor which has a permanent

voltage built in during manufacture. This is somewhat like a permanent magnet, in that it

doesn't require any external power for operation. However good electret condenders mics

usually include a pre-amplifier which does still require power. Other than this difference,

you can think of an electret condenser microphone as being the same as a normal


Dynamic Loudspeaker Principle

A current-carrying wire in a magnetic field experiences a magnetic force perpendicular

to the wire.

Loudspeaker Basics

                  The loudspeakers are almost always the limiting element on the fidelity

                  of a reproduced sound in either home or theater. The other stages in

                  sound reproduction are mostly electronic, and the electronic components

                  are highly developed. The loudspeaker involves electromechanical

                  processes where the amplified audio signal must move a cone or other

mechanical device to produce sound like the original sound wave. This process involves

many difficulties, and usually is the most imperfect of the steps in sound reproduction.

Choose your speakers carefully. Some basic ideas about speaker enclosures might help

with perspective. Once you have chosen a good loudspeaker from a reputable manufacturer

and paid a good price for it, you might presume that you would get good sound

reproduction from it. But you won't --- not without a good enclosure. The enclosure is an

essential part of sound production because of the following problems with a dir ect

radiating loudspeaker:

Loudspeaker Details

                                           An enormous amount of engineering work has

                                        gone     into   the design of today's dynamic

                                        loudspeaker. A light voice coil is mounted so that

                                        it can move freely inside the magnetic field of a

                                        strong permanent magnet. The speaker cone is

                                        attached to the voice coil and attached with a

                                        flexible mounting to the outer ring of the speaker

                                        support. Because there is a definite "home" or

                                        equilibrium position for the speaker cone and there

is elasticity of the mounting structure, there is inevitably a free cone resonant frequency

like that of a mass on a spring. The frequency can be determined by adjusting the mass and

stiffness of the cone and voice coil, and it can be damped and broadened by the nature of

the construction, but that natural mechanical frequency of vibration is always there and

enhances the frequencies in the frequency range near resonance. Part of the role of a good

enclosure is to minimize the impact of this resonant frequency.

3.5 IC UA741:

3.6 IC LM386:-

   It’s a low voltage Audio power amplifier. The gain is internally set to 20 to keep

external part count low, but the addition of an external capacitor and resistor between pins

1 and 8 will increase the gain to any value from 20 to 200.

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

supply voltage. The quiscent power drain is only 24milliwatts when operating froma a

6volts supply, making the LM386 ideal for battery operation.


      Wide supply voltage range: 4v-12v or 5v-18v

      Low quiscent current drain: 4mA

      Voltage drains from 20 to 200

      Ground referred input

      Self-centering output quiscent voltage

      Low distortion: 0.2%

      Available in 8 pin MSOP package


      AM-FM radio amplifiers

      Portable tape player amplifiers

      TV sound systems

      Line drivers

      Ultra sonic drivers

      Small servo drivers

      Power converters

3.7     BD139 Heat Sink:-


         TRANSISTOR, NPN, SOT-32

         Transistor Polarity: NPN

         Collector-to-Emitter Breakdown Voltage:80V

         Power Dissipation Pd:1.25W

         DC Collector Current:1.5A

         DC Current Gain hFE:40

         Transistor Case Style:SOT-32

         No. of Pins:3

         Case Style:SOT-32

         Current Ic hFE:0.5A

         Full Power Rating Temperature:25°C

         Max Current Ic:3A

         Max Current Ic Continuous a:1.5A

         Max Power Dissipation Ptot:12.5W

         Max Voltage Vce Sat:0.5V

         Min Hfe:25

         Power Dissipation:1.25W

         Termination Type: SMD

         Transistor Type: Bipolar

         Voltage Vcbo:80V


      Here we use a 9volts DC external battery to avoid complexity of the circuit.

                                 CHAPTER 4

   Hence we conclude that by using our equipment there is easy transmission and

receiving of information for a short range of distance. The distance can be increased by

using lasers instead of IR devices. More over we got to have a practical overview of what

we studied so far in our curriculum.

                                 CHAPTER 5

5.1 Books Referred:
     Jacob Millman Christos C. Halkias.: “Electronic Devices And Circuits”, Tata
       McGraw-Hill Publishing Company Ltd. Sep, 2003.

      “Electronic Devices and Circuits” by SALIVAHANA, Tata McGraw-Hill
       Publishing Company.

5.2 Sites visited:

5.3 Data Sheets:
        IC UA741 and LM386

          Transistors BC547 and BC548

          BD139 Heat Sink

          2N5777/L14F1 Photo Transistor

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