multipurpose listening device

5/19/2018 Multipurpose Listening Device

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A Mini Project Report

on

MULTIPURPOSE LISTENING DEVICEsubmitted as a partial fulfillment towards Mini-project for third year

of Electronics Engineering, University of Pune, Pune.

Submitted By

Abhijit Kolatkar

Kaustuv Sen

Department of E&TC Engineering

G. H. Raisoni Institute of Engineering & Technology, Wagholi,

Pune-412207

2013-2014

C E R T I F I C A T E


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This is to certify that Abhijit Kolatkar (Roll no. 25),

Kaustuv Sen (Roll no. 24) of G.H. Raisoni Institute of

Engineering & Technology, Wagholi, Pune-412207 have

submitted Mini Project report on Multipurpose Listening

Deviceas a partial fulfillment of T.E. (Electronics/E&TC) for

satisfactory completion of term work of University of Pune,

Pune, during the academic year 2013-14.

ProjectGuide H.O.D.

Mr. N.B. Hulle Prof. A.D.Bhoi

ACKNOWLEDGEMENT


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First and foremost, I would like to thank our respectedH.O.D Prof. A.D. Bhoi for being a source of insipiration and

providing a nice and encouraging environment for us to study.

I would also like to express my sincere gratitude to ourguide Mr. N.B. Hulle for his valuable help during the development

of this project. Without whose priceless suggestion and timely help

we would not be able to complete our project.

Last but not the least we would like to thank Mr. HemantSir and Mr. Siddharth Sir who have helped us during the designing

of the project.


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Table of content

List of Figures

Abstract

S.NO. TITLE PG.NO

CHAPTER 1 INTRODUCTION 1

CHAPTER 2 LITERATURE SURVEY 2

CHAPTER 3 BLOCK DIAGRAM & DESCRIPTION 3

CHAPTER 4 METHODOLOGY 5

CHAPTER 5 SIMULATION & TESTING 15

CHAPTER 6 COST OF PROJECT 16

CHAPTER 7 CONCLUSION 17

CHAPTER 8 REFERENCES 18

APPENDICES


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List of Figures

1.

Block diagram of circuit - Fig. 3.1 Pg 3

2.

Circuit Diagram - Fig. 3.2 Pg 4

3. Condenser microphone - Fig. 4.1 Pg 5

4.

Internal Diag. of mic. - Fig. 4.2 Pg 7

5. Sound propagation - Fig. 4.3 Pg 7

6.

Transistor Configuration - Fig. 4.4 Pg 8

7.

Transistor Working - Fig. 4.5 Pg 8

8.NPN transistors - Fig. 4.6 Pg 9

9.

Transistor BC548 - Fig. 4.7 Pg 10

10.Diode Symbol - Fig. 4.8 Pg 11

11.Solder side PCB design - Fig. 4.9 Pg 13

12.

Component layout - Fig. 4.10 Pg 13

13.

Final PCB layout - Fig. 4.11 Pg 14

14.

Simulation of circuit - Fig. 5.1 Pg 15

15.

Testing on Breadboard - Fig. 5.2 Pg 15


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ABSTRACT

In this report we have presented a circuit which allows us to faint

and unclear sounds with the help of a listening device. This device is a

basic amplifier circuit that receives the audio signal from the condenser

microphone and amplifies the signal to audible human range. We can use

this device by simply connecting a stereo headset and listening to it. This

circuit uses basic electronic technologies such as a transducer and

amplifiers using NPN transistors.

Keywords Listening Device, Amplifier, Condenser microphone, stereo

headset, NPN transistors.


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1 Introduction

Listening devices include track and trace devices where one has to physically track

the conversation using a microphone and a transmitting and receiving unit which are

used as a source of detection and investigation. While other such devices may be

required by journalist for keeping track and recording information, these types of

devices may include a magnetic tape or a disc.

Listening devices of any sort that can be traceable or recordable are illegal unless

and until used by law enforcement agencies for investigative purpose or for providing

evidence in the court of law. The only way it can be legal that before using such

devices the concerned party is informed before using it and permission is granted for

example an interview.

Listening devices, be it a radio listening device that captures the radio frequency

signalling or be it covert listening devices are generally used for audio surveillance by

means of electronic signal transfer or direct digital recording both methods are form

of one way communication.

A listening device may be a smart audio tracker i.e. the voice operated device that

gets activated once it detects a sound. It may also be manual voice transmitter which

can be controlled using a remote switch which overrides the control of the transmitter.

Listening devices are controlled manually or can be controlled automatically using

smart trans-receivers. They may be connected to a basic surveillance recorder, audio

transmitter which may either store the devices or may broadcast the audio to those

who need to hear the data without being present.

Hidden listening devices can be made into mobile configurations to carry on a

person, while covert listening devices may be built into stationery objects or left

covered so no one would notice them

Here we have built a multipurpose listening device to convert the weak and

unclear audio to audible and clear audio which can have multiple usage according to

their field of use.


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2 Literature Survey

Of course listening is one of the best ways to obtain information, it has been one

of the most important ways to communicate and these communications between people

often reveal important facts. These facts may be of a lot of importance when we look at

the legal perspective; overhearing such facts may strengthen a case against a criminal.

Even if we acknowledge the fact that such evidences can be often forged or even

doctored but still for many instances and lodging complaints it is a definitely an

important aspect when we provide evidence.

Laws in many countries state that it is illegal for a person or group to avail audio

surveillance against any other person or group unless and until he is figure/body of

law and has a warrant against the complaint.

Esp. US federal laws clearly prohibits anyone other than the law enforcement

officials from installing, using and maintaining listening devices to hear or evenrecord audio data without any prior consent.

Before any one even considers setting up an audio surveillance device he must

first take advice from his lawyers because in some states the law regarding audio

tracking varies.

Recording voice is extremely crucial when it comes to providing evidence at the

court of law. Nowadays digital voice recorders are so small and sensitive which could

be worn or left in target areas for recording voice for many hours.

This provides an easy buffer for giving judgement whenever such cases come tolight and the prosecution with the audio data definitely have the leverage above

others. Many a times innocent people are victimized of heinous crimes such times

audio surveillance is of mighty importance.

Not all devices are illegal and not allowed some of them such as the parabolic

microphones and SAT recorders are some of the available commercial available audio

surveillance device that is available to the common public. Nature photographers, bird

watchers and even wildlife journalists use devices such as the parabolic microphone

to hear the sound of different creatures from a faraway place such that they may get a

particular location of such species.

Some of the devices used by the military such as the SATNAV, SAT recorders are

similar kind of listening devices that they use to find the strategic location of the

enemies. It has a range of more than 300m and can be used by pointing the antenna to

a desired location. Simple headphones are typically plugged in and recorders can be

hooked up to record the audio. Due to their long range, listeners can remain

undetected while spying on the others.


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3 Block Diagram & Description

Fig. 3.1 Block Diagram of Multipurpose listening device

The fig. 3.1 shows the basic block diagram of a multipurpose listening device the

heart of the block is the amplifier circuit the amplifier circuit we have used in this

circuit is the common emitter type amplifier that uses an NPN transistor.

The input of audio is provided by microphone. In our circuit we will use an

electret microphone that provides audio input to the amplifier. The electret microphone

is a condenser that is used to transmit audio signalling.

The volume control is provided by the potentiometer that will control the sound

i.e. the volume of the audio by changing the resistance values of the biased resistors

connected to the transistors.

The power source for this circuit is a 1.5V AA battery as most of the components

do not have high ratings. The receiver end of the circuit is a simple headphone that

receives the amplified signal from the amplifier.


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3.1 Circuit Diagram & Description

Fig. 3.2 Circuit Diagram of multipurpose listening device

The fig. 3.2 shows the circuit diagram for multipurpose listening device. This

circuit can detect very faint and remote sounds with good clarity. An eletret

microphone is used to pick up sounds.

The advantages of using it are its excellent frequency range and highsensitivity. Four transistors and a few resistors are required for fabricating this circuit.

The circuit operates off two 1.5v AA size cells. Transistors T1 and T2 amplify

sound picked up by the microphone. Transistors T3 with resistor R2 in feedback path

ensures constant level.

This is necessary because faint sounds should be amplified by the device and

strong sounds should be attenuated to a safe level, to match the quality of the sound

that must be audible to the human ear.

Wire the stereo headphones socket such that both the right and left earphones

are connected to the circuit. Multi-turn trimmer VR1 is used to control the sensitivity

of the microphone.


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4 Methodology

The methodology used in this project is based on the basic applications of the

transistors be it NPN or PNP type both can act as amplifiers and when connected infeedback can act as constant level source.

The various hardware components that we will require for building this circuit are

listed below:

Electret Microphone

TransistorsBC548, BC337

Diodes - 1N448

Resistors

Capacitors

Stereo Headset

Below we have discussed the important components that were required to build this

circuit.

4.1 Electret/Condenser Microphone

The condenser microphone is a transducer that uses a capacitor to convertacoustical energy to electrical energy. The sound waves result into electric signals that

can be used to drive a circuit or a device. The pin connected to the case of the mic. is

connected to the ground and the other pins gives an output. Fig. 4.1 shows a condenser

type microphone.

Fig. 4.1 Condenser microphone


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A simple capacitor has two plates with a voltage difference between them. The

capacitance depends on the distance between them. In a condenser mic. a diaphragm

acts as a plate.

When the plates come closer they increase the capacitance and a current is

generated. When sound waves strike the diaphragm, the distance between the plate

changes and so does the capacitance, so as in the case when they come closer, when

they move apart they discharge current as the capacitance between the two plates

changes.

There are two methods for extracting sound signals from the microphone, namely

DC biased and Radio frequency (RF). In DC biased method the charge within the

capsule remains constant. The voltage variations across the capsule are passed through

a series resistor and are further amplified.

In RF method either the capacitance change modulates the amplitude of a low RF

oscillator or the capsule is part of a resonant circuit which modulates the frequency of

the output. Both these methods are feasible options when it comes to extract a sound

signal.

The electret microphone is a special type of condenser. A moving diaphragm and a

fixed back plate form a parallel-plate capacitor, which is charged with a DC voltage.

The sound waves hitting the microphone make the diaphragm vibrate. As a result the

capacitance varies, which causes voltage variations across the resistor that are

proportional to the incident sound signal.

In the electret microphone, an electric charge is permanently frozen in the

diaphragm or in the back plate. This removes the need for a DC voltage for the

transducer, so an electret microphone only needs a power supply for its output

amplifier.

The electret microphone has the merits of good signal quality and low cost. Electret

microphones are extremely small, making them easy to fit even into compact housings.

They are relative1y insensitive to electromagnetic interference, which is particularly

important for digital radio communications.

Fig. 4.2 shows the internal diagram of an electrets microphone.


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Fig 4.2 Internal Block Diagram of Condenser Mic.

The condenser microphone also has different properties of sound propagation which

are defined below:

Omni-directional:

A microphone that has an Omni-directional pattern is equally sensitive to sound

waves from any direction it will pick up all ambient sounds.

It is used, for example when several people are speaking from different positions,

or if a microphone is fixed and the person speaking has the freedom to move anywhere

in the room.

The Omni-directional microphone is relatively insensitive to pop and wind noise. Inaddition it does not exhibit the proximity effect common to directional microphones.

Pop and wind noise:

Plosive sounds such as p or t produce pop noise when speaking close to the

microphone. Wind noise occurs during outdoor use if wind hits the microphone

diaphragm directly. The stronger the directivity of a microphone, the more prone it is

to pop and wind noise. An Omni-directional microphone is barely susceptible, whereas

hypercardioid or figure-of-eight microphones are very sensitive to pop and wind

effects. Pop and wind noise can be muffled effectively by foam shield (pop shield).

The sound propagation of different microphones are shown below in fig. 4.3

Fig. 4.3 Sound propagation through different microphone


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4.2 Transistors

There are basically two types of transistors PNP and NPN. The letters define the

material of semiconductor used as layers. Most transistors today are NPN transistors as

they are easy to make using Silicon. The fig. 4.4 shows the two transistor

configuration.

Fig. 4.4 Transistor configurations

The following diagram and description explains the basic working of the NPN

transistors:

Fig. 4.5 Transistor working

When the switch is closed a small current flows through the base (B) of the

transistor, it is just enough to make LED B glow dimly. The transistor amplifies this

small current to allow a larger current to flow through from the collector (C) to its

emitter (E). This collector current is large enough to make LED C brightly.


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When the switch is open no base current flows, so the transistor switches off the

collector current. Both LEDs are off no transistor currents flows

A transistor amplifies current and can be used as a switch.

This arrangement where the emitter (E) is in the controlling circuit (base current) at

controlled circuit (collector current) is called Common emitter mode. It is the most

used arrangement for transistors.

Functional model of NPN transistor:

The Fig. 4.6 shows the functional model of the NPN transistor that we will use in

our circuit.

Fig. 4.6 NPN transistor functional model

The operation of a transistor is difficult to explain and understand in terms of its

internal structure. It is more helpful to use this functional model:


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The base-emitter junction behaves like a diode.

A base current Ib flows only when the voltage VBE across the base-emitter junction

is O.7V or more.

The small base current Ib controls the large collector current Ic.

Ic= hFE x Ib (unless the transistor is full on and saturated)

hEE is the current gain (strictly the DC current gain), a typical value for hFE is 100 (it

has no units because it is a ratio)

The collector-emitter resistance RCE is controlled by the base current Ib:

o Ib = 0 ; RCE = ; transistor off

o Ib small ; RCE reduced; transistor partly off

o Ib increased ; RCE = 0; transistor ON (saturated)

We have two types of transistors in our circuit and both are explained below:

Transistor BC548:

BC548 is general purpose silicon, NPN bipolar junction transistor. It is used for

amplification and switching purposes. The current gain may vary between 110 and

800. The maximum DC current gain is 800.

Its equivalent transistors are 2N3904 and 2SC1815. These equivalent transistors

however have different lead assignments. The variants of BC548 are 548A, 548B and548C which vary in range of current gain and other characteristics.

The transistor terminals require a fixed DC voltage to operate in the desired region

of its characteristic curves. This is known as the biasing. For amplification

applications, the transistor is biased such that it is partly on for all input conditions.

The input signal at base is amplified and taken at the emitter. BC548 is used in

common emitter configuration for amplifiers. The voltage divider is the commonly

used biasing mode. For switching applications, transistor is biased so that it remains

fully on if there is a signal at its base. In the absence of base signal, it gets completely

off.

Fig. 4.7 Transistor BC548


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Transistor BC337:

A BC337 transistor is a three-terminal current control device used for general

purposes. Based on the semiconductor type used, the BC337 transistor is a negative-positive-negative (NPN) type because the main terminals use a negative (N)-type

material. The transistor schematic for an NPN transistor indicates the emitter with an

arrow pointing away from the transistor.

It is known as a bipolar junction transistor because it uses a positive (P)-type

semiconductor material sandwiched between two N-type materials. Semiconductors,

such as silicon and germanium, are used in the BC337 transistor to control the flow of

electrical charges by allowing a control current to vary the conductivity of the

transistors main terminals.

This NPN transistor has a base or the control terminal, an emitter terminal, and acollector terminal. On the base-to-emitter junction, the amount of current flow

determines the collector current. The ratio of the collector current to the base current is

known as beta.

If a transistor has a beta of 100, a current of 1 milliampere (mA) through the base-

emitter produces a current of 100 mA in the collector. The collector could be

connected to a passive load, which could be a resistor, a tuned circuit, an audio

transformer, or a relays electromagnetic coil, onto a direct current (DC) power supply.

4.3 Diodes

Diodes allow e1ectricity to flow in only one direction. The arrow of the circuit

symbol shows the direction in which the current can flow. Diodes are the electrical

version of a valve and early diodes were actually called valves. Fig. 4.8 shows the

basic diode symbol.

Fig. 4.8 Diode symbol


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A diode operates in two regions namely forward and reverse regions both of them

are explained below:

Forward Biasing Operation:

When we apply an external voltage more than the barrier potential, the negative

terminal of battery pushes the electrons against barrier from N to P region. Similarly

positive terminal pushes the holes from P to N region.

Thus holes get repelled by positive terminal and cross the junction against barrier

potential. This reduces the width of depletion region. As forward voltage increased, at

a particular value the depletion region becomes very much narrow such that large

number of charge carriers can cross the junction. In this way the flow of charge

increases through the diode by increasing the applied voltage.

Reverse Biasing Operation:

In reverse bias, the negative terminal attracts the holes in the P region away from

the junction. The positive terminal attracts the free electrons in the N region and no

charge carrier is able to cross the junction. In this way the depletion width increases

highly and acts as a resistor, but it does not exist for more time. In steady state,

majority current ceases the holes and electrons stop moving away.

In this way there is a reverse current flow due to minority charge carriers which are

small in number.


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4.4 PCB Design

We have designed the PCB of the circuit in PROTEUS, which is a cross platform

simulation and design software. The PCB design of the circuit if given below:

Fig 4.9 Solder side PCB design in 3D view

The component layout for the circuit is given below:

Fig 4.10 Component layout in 3D view


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The final PCB layout is shown in Fig 4.11

Fig. 4.11 Final PCB layout


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5 Simulation and Testing

Similar to the PCB design we have simulated the circuit in PROTEUS and the

screenshot of the simulation is given in Fig. 5.1

Fig. 5.1 Screenshot of simulation of Multipurpose listening device

We have also tested the circuit on a breadboard and got positive results for it. The

snapshot of the breadboard is given in Fig. 5.2

Fig 5.2 Testing on breadboard


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6 Cost of Project

S.NO COMPONENT QTY PRICE/pc

1. Transistor 4 3

2. Resistor 7 5

3. Capacitor 3 2

4. Electrolytic Capacitor 2 4

5. Condenser Mic. 1 5

6. Diode 1 5

7. Potentiometer 1 5

8. Headphone socket 1 10

9. Switch 1 5

10. Copper Clad 1 25

11. FeCl3 1 25

12. Connecting wires 1 4

13. 1.5V cell battery 1 20

TOTAL Rs. 165 /-


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7 Conclusion & Future Scope

Listening devices, be it a radio listening device that captures the radio frequency

signalling or be it covert listening devices are generally used for audio surveillance bymeans of electronic signal transfer or direct digital recording both methods are form

of one way communication.

As far as the as the applications are concerned it can be varied according to various

situations but for the basic applications are listed below:

Hearing Aid

Audio Surveillance

Tracking and Tracing

As far as the future scope goes it can be used as the following:

o Used by the defence forces for spying and surveillance.

o Can be attached to a recording device and provided as evidence.

Thus this circuit can detect very faint, remote sounds with a good clarity. It is useful

for conference halls, auditoria, cinema halls, lecture rooms etc. It can be housed in a

small plastic box and kept in a shirt pocket. It is especially useful watching TV

programmes at a low volume so as not to disturb other family members in the house.


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8 References

multipurpose listening device

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