Title page………………..i




Table of content……...…v


1.1     introduction / background

1.2     statement of the problem

1.3     significance of the study

1.4     scope of the study

1.5     definition of some terms


2.1    operational     principles     of     an    intercom system

2.2     make up of an intercom system

2.3      master and remote stations of an intercom system

2.3.1 intercom master station

2.3.2 intercom remote station

2.4     transmission lines

2.4.1    fundamentals of transmission lines

2.4.2 characteristic   impedance

2.4.3 losses in transmission lines

2.5      filters

2.5.1       high pass filter ( h p f )

2.5.2   low pass filter (lpf)

2.6     electro-acoustical devices

2.7      amplifiers and amplification 2.7.1 amplifiers

How transistor amplifies

2.7.2 transistors amplification

2.8     amplifier coupling

. 2.8.1        capacitive coupling

2.8.2        direct amplifier coupling

2.9     voltage gain in coupled stages

2.10   transistor audio power amplifier

2.10.1     performance         qualities         of         power ~ p 3? " amplifiers

CHAPTER THREE: DESIGN AND CONSTRUCTION 3.1      choice of 1c (lm 386)

3.1.1   pin configuration and block diagram

3.1.2 gain control of an lm 386 op-amp

3.1.3 lm 386 input biasing

3.1.4 electrical characteristics of the lm 386

3.1.5 features necessitatng the choice of lm 386.

3.2     design of the intercom circuit

3.2.1 design of the input rc network (input filter) to the amplifier lm386.

3.2.2 design of the gain network

3.2.3 design at the amplifier's output

3.3      design of an optional tone circuit

3.3.1 how it work s>

3.3.2 calculating the on and off time for the two transistors

3.4     9v dc power supply

3.5       design of a simple microphone preamplifier

3.6      parts list

3.7      construction of the simple intercom system

3.8     installation and application


4.1      discussion of result

4.1.1        function(s)  of  some  components  as they are used in the circuit

4.2      suggestions for further study

4.4     conclusion

4.3      recommendation




Human to human communication is an essential part of our every day activities. Advances in communication technology have enabled any-time-anywhere connection between people. There are a wide variety of devices to communicate with family, friends and workgroup members who are distributed geographically. Cellular phones are used at the working places, stores etc. to call home to be reminded of what is needed or when one will be late to  work, meeting etc.

The purpose of a communication system is to transfer information between two or more points.  In general, a communications system  requires three things namely;  a transmitter, a  propagation  medium, and a  receiver. The information  to   be  transferred   may  be   in  analog   form (example, voice or audio) as in the case of this study where voice   is  the   source   of  information,   or   in   digital   form (computer   data,   digitalized   voice,   and    so   on).   The transmitter must have some means of superimposing this information   onto   its   output   signal    (modulation).   The modulated    transmitter    signal    must    pass    through    a propagation medium to a receiver. The propagation medium may be free space, an RF transmission line, or a fiber optic

cable. The function of the receiver is to select (filter) the desired signal, amplify and extract the information. A simplified block diagram of a typical communication system is shown on figure 1. 0 below

Interest arises in promoting communication within a building which is difficult and stressful going from one point to another within the building in the name of delivering information. In a domestic setting, family members will be collocated within the same building, but often will be unable to be heard using a normal speaking voice. Many households utilize intercom systems to facilitate conversations between remote parts of the home, but these systems have their drawbacks. They require one or both parties to go to specific parts of the house or hold onto a handset in order to communicate. They also offer little ability to direct communication to a person, within the house when the location of the person is initially unknown.

An intercom system is therefore a private telecommunication system that allows typically two or more locations to communicate with each other like telephone does. An intercommunication system may be a simple two- station system or a multi - station system. Many productions which need co-operation of more than a few people need special intercoms that cover many users. Intercom systems used in TV and stage productions are usually headsets type intercoms connected to one line using party line arrangement. The   primary  use  of this  type  of system   is   in   lives  or  media productions where (for example) the video director speaks to the camera operators, or where the stage manager speaks to the stage hands  and   lighting   operator,   etc.   A  simple  two  station  wired intercom system can be used as an intercom and door phone.  One can use an intercom system from the house to house gate to screen visitors to the house.

In this project work, discussions will base on design and implementation of a two station intercom system that will result in the instrumentation of a home to facilitate flexible one and two-way audio conversations. The intercom circuit will consist of two intercom stations linked to each other with a push - to - talk (PTT) switch. This kind of circuit, generally consist of only one or two amplifiers and generally uses loudspeakers as normal speaker and microphones (how it is used depends on talk switch position). Depending on the circuit design, there could be one PTT switch on one end, or separate PTTs on both stations (as in this case).

1.2     Statement of the problem

In a business organization, shops, stores, homes etc, many people find it difficult and stressful in relating/ delivering information between each other within a particular building. The servant finds it laborious going to the boss always whenever he / she has information to deliver or wants to carry out action(s) with respect to the master's authority. Parents within a home are even forced to walk down to the child's room whenever need arises. As a matter of fact, reaction to information is delayed.

Therefore, this project work is to address this issue, looking forward to eliminating the stressful manpower involved and to facilitate information delivery, making communication easier.

1.3     Significance of the study:

The aim and objective of this project is to design and construct a simple intercom system (mainly two communication stations) as a means to replace man's labour and stress of walking over a given premises for information delivery.

These station wired intercom system can be used as a door phone, connecting from the house to the door to screen visitors to your house. The wife, after preparing dinner, can through this system call on the husband in his bedroom to the dinner table.

Generally, intercom system can be used for message broadcast (in case of multi- channel intercom), as door phone, monitoring etc.

1.4     Scope of the study:

The scope of this project work is limited to the design, construction and test on a two station simple intercom system with specification that;

i       The   demodulator   should   operate   with a   minimum distortion while producing an adequate output

ii.         The small signal amplifier must provide an undistorted signal into the buffer amplifier to be able to drive a speaker of 8Q. Impedance.

iii.        A 9 volt d.c power supply is to be design and employed to power the simple intercom for operation.-, at each station.

iv.     The mater and the remote stations will be built individually.

v.     Result'  shall fully be discussed while suggestions for further study made.

1.5     Definition of some terms

In this study of simple intercom system, we shall come across terms and some of these are listed below with their meaning(s)-

DESIGN: Refers to

i       A goal directed problem solving activity.

ii      Relating product with situation to give satisfaction.

iii      The imaginative jump from the present facts to future possibilities,

iv     The conditioning factor for those parts of the product which come into contact with people.

MODULATION: This is the process

i      Of placing baseband information (voice, data etc) onto also called "carrier" signal so that it can be transmitted to are receiver   that   demodulate  or   extracts   the   original information .

ii.       By which some parameter of one wave is varied in accordance with some parameter of another wave.

FULL DUPLEX: Refers to the simultaneous transmission of two or more messages in either or both directions over a carrier channel. It may as well be of a telegraph or other data channel, able to operate in both directions simultaneously.

HALF- DUPLEX: Is a directional transmission of messages over a

carrier channel. Here, one person speaks at a time while the other


DEMODULATION:   To   recover  the   modulating   wave   from   a modulated carrier. Also known as decode; which is the translation of coded characters into a more understandable form.

MULTIVIBRATOR:  Is a  relaxation oscillator using two tubes, transistors; or other electron devices, with the output of each coupled to the input of the other through resistance - capacitance elements or other elements to obtain in phase feedback voltage.

 ASTABLE MULTIVIBRATOR: Is a multivibrator in which each active device alternately conducts and is cut off for intervals of time determined   by  circuit  constants,   without  the  use  of external triggers. It is also known as free - running multivibrator.

. FILTER: A device or program that separates data or signals in accordance with specified criteria.

FILTER CAPACITOR: Is a capacitor used in a power supply filter system to provide a low - reactance path for alternating currents and therefore suppressing ripple currents, without affecting direct


ACCOUSTIC:  Relating to,  containing,  producing,  arising  from,

actuated by, or carrying sound.

SIGNAL: Refers to

i      A  visual,  aural,   or  other  indication   used  to  convey information

ii      The intelligence, message, or effect to be conveyed, over

a communication system.

RAREFIED AIR: Air whose pressure is much less than atmospheric pressure; thin.

DIELECTRIC: A material which is an electrical insulator or in which an electric field can be sustained with a minimum dissipation in power.

AMPLIFIER: A device capable of increasing the magnitude or power level of a physical quantity such as an electric current or a hydraulic mechanical force, that is varying with time, without distorting the wave shape of the quantity.

TRANSISTOR AMPLIFIER: Is an amplifier in which one or more transistors provide amplification capable to that of electron tubes. PREAMPLIFIER: Is an amplifier whose primary function is boosting the output of a low - level audio- frequency, radio -frequency, or microwave source to an intermediate level so that the signal may be further processed without appreciable degradation of the signal to noise ratio of the system. Also known as preliminary amplifier.

FEED BACK: Refers to the return of a portion of the output of a circuit or device to its input. It is also the control of input as a function of output by returning a portion of the output to the input.

DECIBEL:  Is a  unit for describing the ratio of two  powers or intensities, or the ratio of a power to the reference power; in the measurement of sound intensity.

DECADE:  Is defined as the ten- fold  increase or decrease in frequency of a network.

MARK SPACE RATION: Is the ratio in a pulse wave of pulse duration to the time between successive pulses.

COMPATIBILITY: Is the ability of a device to accept data handled by another device without conversion or modification of the code.

COUPLING:   A   mutual   relationship   between   two   circuits   that permits energy transfer from one circuit to another through a wire, resistor, transformer, capacitor, or other device. COUPLING CAPACITOR: A capacitor used to block the flow of direct current while allowing alternating or signal current to pass. It is also called blocking capacitor or stopping capacitor.

TRANSISTOR BIASING: maintaining a direct current voltage between the base and some other element of a transistor.



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