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CHAPTER ONE
1.0 INTRODUCTION
Intercom is a private telecommunication system that
allows typically two or more locations to communicate with
each other like telephone does.
This project (two-way wired intercom) consists of two
intercom stations linked to each other using wire. It uses full
duple mode ! a process where"y one can transmit and
receive voice calls simultaneously which made the
transmission medium to "e four wires "etween the two
nodes and typical headsets on the nodes (separate speaker
and microphone# good isolation from sound coming from
speaker to the mic element).
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1.1 POINTS RELATED TO INTERCOM SYSTEM
1.1.1 POINT-TO-POINT (P-P)
$oint-to-$oint is a direct# one-way# simple
communication "etween two intercom stations or "etween
stations and interfaces. It is the "asis of communication in
matri intercom systems and is esta"lished "y activating a
station key. The receiving station does not have to take any
action to receive the incoming call. %owever# a talk key at
the receiving station must "e pressed to answer.
1.1.& PARTY LINE (PL)
$arty-line communication (sometimes called
'onference# line# net# or ring) is a group of two or more
stations communicating with each other in a two-way# full
duple mode. ach station must activate the listen key to
the desired party line to listen and the talk key to talk. $arty
line re uires two actions to esta"lish a communication path
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(e.g.# activating "oth a talk key on the sending station and a
listen key on the receiving station). *tations are dynamically
added and su"tracted from a party line as users activate thetalk and listen keys. + party line is intended for use in a
conference with a signi,cant amount of "ack and forth
communication "etween users.
1.1. INTERRUPTIBLE FOLD BACK (IFB )
Interrupti"le old "ack allows a user to hear one audio
source# which may then "e interrupted "y another audio
source. + typical application is in "roadcasting where talent
hears program audio in his ear piece and a producer can
interrupt that audio to o/er the talent information and
instructions.
1.1. ISOLATION (ISO)
The isolation (I* ) function allows a user to speak
privately to another user. I* is generally used to provide
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private communication "etween two mem"ers of a party
line. In "roadcasting# I* is often used "y a video operator to
speak privately to one cameraman who is one of manycameramen on the camera party line. This was called
'amera Isolate as it was ,rst used to remove an individual
camera from a conference to allow private communications.
Typically# this works in the following way2 The person who
needs to interrupt presses a "utton or a key# and there is
esta"lished a private two person conversation with the
desired person. 3pon releasing the key# the two participants
are returned to whatever conversation(s) they were a part of
previously.
1.1.4 FIXED GROUP
+ i ed 5roup is a group of intercom stations and
interfaces. + user who has a talk key programmed for a , ed
group is a"le to simultaneously talk to everyone who is part
of that , ed group. + , ed group di/ers from a party line in
that the group6s mem"ership is set "y the con,guration of
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the intercom system (not changed dynamically). + , ed
group is intended to "e used for one-to-many type
communication.
1.1.7 TALLY
Tally is a signal sent for the purpose of indicating status
for a particular purpose. The sound of your telephone ringing
can "e descri"ed as a tally. In an intercom panel with
multiple channels# it can "e a visual signal to indicate to
which station a calling voice "elongs. It can "e used to
indicate that a particular function is not availa"le due to a
con8ict - just like the "usy signal you get when calling the
radio station to try and "e the tenth caller and win a year
long supply of cat litter.
1.& APPLICATION
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Two-way wired intercom system can "e used as an
intercom and door phone. ne can use an intercom system
from house to house# or from house to gate9door to screenvisitors.
:any productions which needs co-operation of more
than a few people need special intercoms that cover many
users. Intercom systems used in T; and stage productions
are usually headset type intercoms connected to one line
using party line arrangement. The primary use of this type of
system is in live or media productions where (for e ample)
the video director speaks to the camera operators# or where
the stage manager speaks to the stage hands and lighting
operator# etc.
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CHAPTER TWO
&.0 LITERATURE REVIEW
In the 1ell "oth independently designed devices that could
transmit speech electrically (the telephone). >oth men
rushed their respective designs to the patent o?ce within
hours of each other. +le ander 5raham >ell patented his
telephone ,rst. lisha 5ray and +le ander 5raham >ell
entered into a famous legal "attle over the invention of the
telephone# which >ell won.
The telegraph and telephone are "oth wire-"ased
electrical systems through which this project ( Two-way
w !"# $%"!&o' ) comes into "eing.
;arious types of components are used in "uilding the
intercom. This chapter e plains "rie8y some of their "asicfeatures.
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&.1 RESISTORS
+ resistor is an electronic component that opposes the
8ow of electric current "y producing a voltage drop across its
terminals# in accordance with ohm6s law. There are & types of
resistors@
i ed resistor ;aria"le resistor
>oth of them have their value of resistance written on
them using colour coding representation.
&.1.1 FIXED RESISTORS
i ed resistors maintain constant values of resistance
with small percentage of variation. The values can "e
determined through colour coding. ;arious colours are
chosen to represent various values. The ,rst and second
colours show the ,rst and second digits respectively. The
third colour shows the powers of ten while the last colour is
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the tolerance "and which shows the tolerance range of the
resistor.
&.1.& RESISTOR COLOUR CODING
Ta " *
' A 3B 1 st CI5IT & nd CI5IT D .
E B *
T A B+D'
>lack --- --- 0 --->rown 1 1 10 1FBed & & 10 & &Frange 10 ---
Gellow 10 ---5reen 4 4 10 4 --->lue 7 7 10 7 ---;iolet = = 10 = ---5ray < < 10 < ---Hhite 10 ---5old --- - --- 4F*ilver --- --- --- 10FDo colour --- --- --- &0F
Tolerance
1 st digit & nd digit no. of Jeros
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ig &.0a S&+"'a% & Sy' o o, a R" %o!
ig &.0 (") & !&. % y' o o, a !" %o!
&.1. VARIABLE RESISTORS
They can "e altered to give di/erent values from
particular resistors as desired. The variations in resistance
can either change current value or voltage settings.
ig &.1 Sy' o o, Va! a " R" %o!
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&.& DIODES
+ diode is a semiconductor component that restricts the
direction of 8ow of charge carriers in one direction.
ssentially# it allows an electric current to 8ow in one
direction "ut "locks it in the opposite direction# until when it
reaches the reverse "reakdown voltage of the diode. Their
various reverse voltage ($I;) that can force conduction in the
opposite direction are always contained in a semiconductor
data "ook. ample# diode ID4 & has a $I; of 700;# +
ma imum forward current.
There are some diodes that emit light in colours of red#
yellow# and green when they are "iased in the forward
direction. They are called Aight mitting Ciode (A C).
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Fig. 2.2a Schematic symbolof a diode
Fig. 2.2b Circuit symbol of a diode
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&. CAPACITOR
+ capacitor is an electronic component that has two
conductors separated "y a dielectric and it is used to store
charges in an electric ,eld. There are two types of
capacitors.
The polariJed capacitor The non ! polariJed capacitor
&. .1 POLARI/ED CAPACITOR
The polariJed capacitor (like electrolytic capacitor) has
two terminals marked positive and negative respectively.
'apacitors allow the +' signal to pass through them easily.
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+ -
Fig. 2.2c Circuit symbol of Light Emitting Diode (LED)
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Hhen connected in series with the signal line# their
reactance to the +' signal disappears as the fre uency
increases.
&. .& NON 0 POLARI/ED CAPACITOR
The non ! polariJed capacitor (like ceramic capacitor)
have either side marked. Their values are always very low.
&. TRANSFORMER
The transformer is an electric component that has two
separate windings (the primary and secondary windings)
used in transferring +' voltage from the primary coil to the
secondary coil. The transformer can either "e a step up or a
step down. The one used in this design is a step down
transformer.
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Fig. 2.3a Circuit symbol of olarized ca acitor
Fig. 2.3b Non polarizedcapacitor
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The step ! up transformer changes the voltage to a high
value from a low source input while the step ! down
transformer changes the voltage to a low value from a highsource input. $ower remains the same in "oth types of
transformation at the primary and secondary coils. The step
! down transformer has "een used in this design to reduce
the &&0; +' to a 1&; +' output value.
&.4 SPEAKER
It is a mechanical transducer that converts an electrical
signal to sound. It has a magnetic core that forms the "ase
of the speaker and a mechanical part called the diaphragm.
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*econdary coil$rimary
Fig. 2.4 Circuit diagram of a transformer
+' Input ;oltage
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In any electrical system# changing signal esta"lishes a
changing magnetic ,eld. This principle is used in pushing
the core at a speaker to move the diaphragm in or outdepending on the applied signal direction.
&.7 TRANSISTORS
+ transistor is an active component# a device capa"le of
producing an output signal with more power in it than the
input signal. It is the essential ingredient of every electronic
circuit# from the simplest ampli,er or oscillator to the most
ela"orate digital computer.
+ transistor has three terminals and it is divided into
two namely (D$D and $D$)# with properties that meet the
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+
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following rules for D$D transistors (for $D$ simply reverse all
polarities).
This collector must "e more positive than the emitter. The "ase ! emitter and "ase ! collector "ehaves like
diodes.
Dormally the "ase ! collector diode is reverse ! "iased#
i.e.# this applied voltage is in the opposite direction to easy
current 8ow. +ny given transistor has ma imum values of I'#
I># and ;' that cannot "e e ceeded. There are also limits
such as power dissipation (Ic ;c )# temperature# ;> # etc
that one has to keep in mind. Hhen the a"ove rules are
o"eyed# I' is roughly proportional to I> and can "e written as
I' K hf I> K >Is
Hhere hf # the current gain (also called "eta) is typically
a"out 100.
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collector
emitter
"ase
emitter
"ase
collector
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&.= INTEGRATED CIRCUIT (IC)
The evolution of I' reduces the comple ity of electronic
circuit. *ome active and passive discreet components are
integrated into a module called chip. There are three types
of I' used in this work# namely2 the 444 timer I' which is
used to generate the tone# 01= which are used to generate
a triggering pulse to the transistor and TC+&00 which is the
audio ampli,er.
>elow is the diagram showing the pin con,guration of
the I'
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$D$ transistorD$D
Fig. 2.6 Transistor symbol
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Fig 2.7 P $ Co$12.!a% o$ o, 34*5 #"&a#"
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&.< RELAY
The relay is an electromechanical switch that makes
and "reaks contact when current passes through the coil.
Belays are rated in operating voltage and the contact
current. The contact is for *$C= (single pole# dou"le thro).
There are many varieties# including Llatching6 and stepping
relays. The latter provided the cornerstone for telephoneswitching stations. Belays are availa"le for C' and +'
e citation and coil voltage from 4 volts up to 110 volts are
common.
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Coil
'onta
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CHAPTER THREE
.0 RESEARCH6METHODOLOGY
.1.0 PRINCIPLE OF OPERATION
The principle of operation of this work (Two-Hay Hired
Intercom) will "e properly understood "y taking the analysis
of the stages that make up the entire system. The complete
circuit is run on 7 volts .4+% "attery. The "attery is where
all the su" circuits such as the audio ampli,er and the tone
generator took their source. Hhen the circuit is powered "y
turning on the switch# all the su" circuits will "e set to
receive power from the source as the phone handle is lifted
up e cept the tone generator circuit which is powered
immediately so that any tone received from UNIT 7 will "e
ampli,ed to tell the user of UNIT * that UNIT 7 wants to
esta"lish conversation. +ny of the users will tell the second
user that he wants to esta"lish conversation "y pressing the
ringer "utton. This sends a tone signal to the phone and
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conversation is successfully esta"lished when the two users
must have picked up the phone.
The handle of the phone is where the microphone (mouth
piece) and the ear piece are located. +ny sound or speech
made "y any of the users is picked up "y the microphone as
mechanical vi"ration and converted to electrical signal that
has a voltage which its amplitude varies linearly with time
(alternating)# and a fre uency which is a replica of the
fre uency of vi"ration. This electrical signal has low
amplitude and it is ampli,ed "y the pre ampli,er "uilt with
the transistor ('1
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CHAPTER FOUR
.0 DESIGN AND CONSTRUCTION
This chapter e plains the set of methods# principles and
standards used to accomplish this project (Two-way wired
intercom). It should "e noted that there are a"out nine
"locks that make up the entire system of this project. our of
these "locks make up an individual unit (i.e. four "locks for
unit 1 and another four for unit &)# while the remaining one is
"eing shared "y the two as shown "elow.
UNIT * UNIT 7
22OUTPUT
DE !CE
TONE
"ENE#$TO#
$UD!O!NPUT%OU#CE
$UD!O
$&P'!F!E#&ED!U&
TONE
"ENE#$TO#
$UD!O!NPUT%OU#CE
OUTPUT DE !CE
$UD!O
$&P'!F!E#
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ig .0 B o&8 D a2!a' O, Two-Way W !"# I$%"!&o'9
The e planation of standards used and calculations
made as regards to the e ecution of this project will "e
properly understood "y taking it "locks.
.1 TONE GENERATOR
This "lock generates the ringing tone of the intercom.
Binging tone is an audio tone that signals the user of one of
the two phones (e.g. unit 1) that the other user (unit &)
wants to esta"lish a conversation in the network. The
process of generating the ringing tone involves researching
on the datasheet of top semiconductor manufacturers# to get
a suita"le I' (integrated circuit) for it. The semiconductor
guide of Toshi"a shows that the popular timer I' D 444 is
suita"le since it can "e connected to function as an asta"le
multivi"rator and generate an audio tone. This I'# D 444 as
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used in this project is connected as an asta"le multivi"rator
to generate an audio fre uency tone of 40%E. The pin out
of the I' and the test circuit is as shown "elow.
ig .1 P $ O.% o, N"::: T '"! IC
.1.1 APPLICATION CIRCUIT
ig .& A;; &a% o$ C !&. % ,o! %+" ::: T '"!
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CC
Di(c)a*g
T)* ()ol,
Con *ol ol ag
"*o n,
T*igg *
O /
# (
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The 444 monolithic timing circuits is a highly sta"le
controller capa"le of producing accurate time delays# or
oscillation. In the time delay mode of operation# the time isprecisely controlled "y e ternal resistors (B + and B >) and
capacitor ('). or a sta"le operation as an oscillator# the free
running fre uency and the duty cycle are "oth accurately
controlled with two e ternal resistors and one capacitor. The
circuit may "e triggered and reset on falling waveforms# and
the output structure can source or sink up to &00m+.
*peci,cation from producer shows that the I' operates at a
voltage range of 4 to 14 volts.
'+A'3A+TI D
The fre uency of oscillation is calculated "y using the
formula "elow.
T1 K Time f D K 0.7 (B A MBB) '
T& K Time f K 0.7 (B B) '
Total Time(T) K T1 M T&
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K 0.7 (B A M &BB) '.
rom the a"ove e pression# one can easily compute the
fre uency since it is the inverse of time. %ence K 19T
(%ertJ).
K 0.7 (1000 M & (14000) 100 10 -
T K &.1 < 10 -
K N.1 < 10-
K 74. < 74%J
The fre uency of the oscillator is set to 74 hertJ "y
using 1000 ohm as the B A and 14000 ohms as B B and 100
nanofarad as '.
.& AUDIO INPUT SOURCE
This "lock contains the transducer which converts the sound
(speech) in form of mechanical vi"ration to electrical signal.
The electrical signal has a voltage which its amplitude varies
linearly with time. The transducer in this "lock is a capacitor
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(condenser) microphone. 'ondenser :icrophones are
electronic component which stores energy in the form of an
electrostatic ,eld. The term condenser is actually o"solete"ut has stuck as the name for this type of microphone# which
uses a capacitor to convert acoustical energy into electrical
energy. 'ondenser microphones re uire power from a
"attery or e ternal source. The resulting audio signal is a
stronger signal than that from a dynamic. 'ondensers also
tend to "e more sensitive and responsive than dynamics#
making them well-suited to capturing su"tle nuances in a
sound. They are not ideal for high-volume work# as their
sensitivity makes them prone to distort.
.&.1 HOW CONDENSER MICROPHONE WORKS
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ig . C!o -S"&% o$ o, a Ty; &a Co$#"$ "!
M &!o;+o$"
+ capacitor has two plates with a voltage "etween
them. In the condenser microphone# one of these plates is
made of very light material and acts as the diaphragm. The
diaphragm vi"rates when struck "y sound waves# changing
the distance "etween the two plates and therefore changing
the capacitance. *peci,cally# when the plates are closer
together# capacitance increases and a charge current occurs.
Hhen the plates are further apart# capacitance decreases
and a discharge current occurs. + voltage is re uired across
the capacitor for this to work. This voltage is supplied either
"y a "attery in the microphone or "y e ternal phantom
power.
.&.& MICROPHONE IMPEDANCE
Hhen dealing with microphones# one consideration
which is often misunderstood or overlooked is the
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microphoneOs impedance rating. $erhaps this is "ecause
impedance isnOt a PcriticalP factor@ that is@ microphones will
still continue to operate whether or not the "est impedancerating is used. %owever# in order to ensure the "est uality
and most relia"le audio# attention should "e paid to getting
this factor right.
.&. WHAT IS MICROPHONE IMPEDANCEuilding this project successfully has "een a great
challenge to me# though it has e posed me to proper under
standing of "asic principles electronics components. This
project %wo - way w !"# $%"!&o' is a communication
system which can "e used as an intercom and door phone.
ne can use an intercom system from house to house# or
from house to gate9door to screen visitors# and also in o?ces
(e.g. "etween the o?ce of the % C and the C +D). It o/ers
a great advantage running with a rechargea"le "attery
which reduces cost. +lso# the system is very easy to install.
Its disadvantages include "eing a stationary unit# which
means not to "e carried a"out like we do with our 5*:
phones. +lso# it is wired and therefore any mechanical
damage to the wire interrupts esta"lishment of conversation.
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4.10 REMARK
The model design in this project is compara"le with
those from the market and could "e in organisations.
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4.&0 REFERENCES
4.&.1 Telecommunication ngineering "y ;. .
Idigo# I.I Dsionu# '. . haneme (&00 )
4.&.& + Te t"ook of lectrical Technology >.A. Theraja and +. . Theraja. ThoroughlyBevised Twenty ! Third Bevised dition &00&
4.&. lectronic and lectrical ngineering#$rinciples and $ractice (*econd dition) >yAionel Harnes 1 -1