Multipurpose Listening Device 1q7345

A Mini Project Report on

MULTIPURPOSE LISTENING DEVICE submitted 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

CERTIFICATE 1

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 Device” as 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 Mr. N.B. Hulle

H.O.D. Prof. A.D.Bhoi

ACKNOWLEDGEMENT 2

First and foremost, I would like to thank our respected H.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 our guide 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. Hemant Sir and Mr. Siddharth Sir who have helped us during the deg 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

4

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 15. Testing on Breadboard

-

Fig. 5.1 Fig. 5.2

Pg 15 Pg 15

5

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 even record 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 to light 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 high sensitivity. 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 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 in can act as constant level source. The various hardware components that we will require for building this circuit are listed below:      

Electret Microphone Transistors – BC548, 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 convert acoustical 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 ed 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. In addition 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 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 and 548C 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 16

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 negativepositive-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 transistor’s main terminals. This NPN transistor has a base or the control terminal, an emitter terminal, and a collector 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 baseemitter produces a current of 100 mA in the collector. The collector could be connected to a ive load, which could be a resistor, a tuned circuit, an audio transformer, or a relay’s 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 19

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 21

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

22

Rs.

165 /-

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 by means 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 in the house.

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8 References

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