What are the main TELEPHONE INSTRUMENTS ?| Types of Telephones

  What are the main TELEPHONE INSTRUMENTS ?| Types of Telephones 

To fulfill the need for communication, a device that is used is a telephone instrument. Various types of instruments are available according to the type, the function they perform features available with them, and much more.

telephone instruments used in Railways.

Following are the types of telephone instruments used in Railways.

Push-button Telephones:

Push Button Telephone

 push-button telephones use a dial pad containing a button to press digits from 0 to 9 and special characters * and #.

they come in the variant of single line and double line. 

A picture of the line cord is shown below. it is a 2 wire line cord with an RJ11 connector at both ends.

line cord  RJ 11

Main & Extension (1+1) type Telephones:

Main  Extension 1 plus 1 type Telephones

 these telephones are used to connect between boss and secretary or may be between operator as the main phone and other users as extension.

Hand-free Telephone with caller ID (CLIP phone): 

these telephones come with the inbuilt speaker to enable the user to talk without lifting the handset and to display the identity of the caller, a display unit is also provided.

Two lines with speaker phone facility: these telephones come with 2 lines selectable. 2

different dial tones can be connected on it.

Two lines CLIP with speaker phone facility: 

as the name indicates these telephone instruments are available with 2 lines terminated on it with hands free and caller identity display unit.

Cord less phone: 

Hand-free Telephone with caller ID CLIP phone
Cord less phone

these telephones come with a handset connected to base telephone by a radio transmission. Uses trans-receiver frequencies for Trans and receive.

Control telephones:

 this phone is used for train traffic control at various stations which are supposed to intimate the information of all the passing trains through that station to the central control at control office. 

These phones are connected in Omni bus fashion to achieve all-time communication between stations with central control and selective calling facility.

Magneto Telephones:

Magneto Telephones:

 magneto Telephones are used in Indian Railways for connecting point to point communication without talking the help of any exchange switch.

 Generally used for Level crossing gate communication with the nearest cabin. Now a day’s other communication ways are available so this type of magneto Telephones are not in fashion.


A simple push-button telephone circuit consists of the following stages:

a) Ringer stage – produces audible ringing sound.

b) Key board matrix stage – to dial the required number or to perform the function of dialing

the digits.

c) Dialer stage – to generate the tone or pulse corresponding to the key dialed.

d) Sound amplifier stage – to produce the speech sounds during transmitting and receiving.

e) Voltage dropper stage – to drop the from -48V DC to 5V to 12V when the hand set is


f) Rectifier and protection stage – to give voltage of proper polarity, to limit any high

voltage peaks due to induction on lines.

All the above stages are inter connected as shown in the given block diagram 



a) Ringer Stage:

It consists of a Ringer IC, which operates an “Piezo electric buzzer’ or ‘Speaker’. The IC

operates on AC ringing signal of 20Hz frequency with its associated circuit.

Ringer IC: It consists of 8 terminals IC. It includes bridge rectifier circuit, power supply control circuit, output amplifier, low and high frequency oscillator circuit.

Piezo electric buzzer – It gives audible sound. It is operated by the tone produced by the output of the oscillator. It is connected between the output terminal of output stage of IC, and ground (+ve) through a volume control that regulates the sound volume.

Cradle hook switch – When the hand set is on the cradle the entire telephone circuit is

disconnected from the lines and only ringer section is kept connected to the lines L1 and L2.

When the hand set is OFF the cradle hooks, the ringer section is disconnected by the cradle

switch contacts and the bridge rectifier is connected to the voltage dropper stage, and further

to dialer, sound amplifier stage.

Other components: The other components of ringer section are, - Capacitor (C1) of value 0.1uF. It is to change the frequency of oscillator. - Resistance (R5) of value about 212Ω. It decides the frequency of oscillations. 

To change the frequency of oscillator, the resistance value to be changed. - Capacitor (C4) of value 10Uf 63V, is used to filter the rectifier output.

Description of ringer stage circuit used in GCEL 501 telephone is given below. 

The ringer section consists of LS 1240 (8 pin) IC, which consumes less current.

 This IC has an internal oscillator to generate two tone frequencies and connects them across the output amplifier.

The tone and oscillator frequencies can be adjusted externally with the help of resistor R3

and capacitor C2. 

This stage consists of a ringer switch having S1, S2, and S3 switches.


When the hand set is on the cradle hooks, switch S1 and S2 are OFF and S3

contact on ‘B’ side. So the ringing current of 20 Hz is applied to the Bridge rectifier inside the IC as input. This AC ringing current is rectified by the bridge rectifier and the DC output is

used for the working of oscillator IC. The oscillations produced by the oscillator are

connected between pin.No.5 and 2 via output stage in the IC. These are connected to the

piezo electric buzzer via volume control. The volume of the buzzer can be adjusted by the

volume control. IC pin.No.3 and 4 are connected with a capacitor of 0.1uF and a resistance

of 212Ω which decides the frequency of oscillations (tone). 

The 10 uF, 63V capacitor is a filter capacitor which filters the output of the bridge rectifier. As soon as the hand set is lifted,

the ringer switch operates and S3 changes over its position to ‘A’ side and the input to IC

pin.No1 is disconnected and ring stops

b) Key board matrix stage:

It is provided with a “Key pad” to enter the subscriber’s telephone numbers.

It has 4 Rows R1, R2, R3 & R4 and 3C columns C1, C2 & C3. The key pad is connected to

dialer IC. Minimum 12 numbers of keys are provided on key pad.

Number buttons -10 No’s (0-9 numbers)

Special keys – 2 to 6 No’s

Function of some special buttons is:  Redialing ® - To repeat the last dialed number.  Tone-Pulse dialing

 Star key(*) – or Ashtreik key (#) – To get the Tone mode dialing.  Mute (M) – To prevent outgoing speech.  Pause (P) to get multiples of 2.2 seconds delay between two digits of a number.

A row and the column are shorted when a number on the key pad is pressed. This generates

equal number of pulses or DTMF tone pair which will be available at dialer IC.

Refer Fig.3.3 for key board matrix connection diagram

Fig.3.3 Key-board matrix connection diagram

c) Dialer Stage:

This section is connected to the key board matrix stage on one side, and other side it is

connected to rectifier and protection stage, and sound amplifier.

When any key on the key board matrix is pressed the corresponding pins of dialer IC are

stored and pulses are produced. These are connected with the lines via rectifier protection

It sends the dial pulses at speed of 10 IPS. It can store the dial pulses, when the keys are

operated fast to an extent of 1720 digits.

The dialer section has the following parts:

(i) Dialer IC: - It is 16 pin IC. It consists of, Key board interface circuit, Oscillatory circuit,

Transmit Amplifier circuit, Mute circuit, Memory circuit etc.

Working of stage: When ay key of the keyboard is pressed corresponding pins are shorted.

Thus the corresponding pulses are generated by UIC via keyboard interface, control logic

these are conned to pin.5 of IC. Then these are connected to telephone lines, through Q10

Transistor and ringer switch contacts. These generated pulses are stored in 20 bit memory if

the dialing is fast. If any key is pressed in ON-HOOK position, it is not considered and the

oscillator will not function.

FIG.3.4 Dialer stage

d) Sound Amplifier Stage:

This is also called as “Speech stage”. It consists of a speech IC having 18 or 20 pins in

special cases. This IC performs al interfacing functions between the Microphone, the ear

phone, the dialer IC and the Telephone lines.

This stage performs 3 functions:

 It receives the incoming speech signals, amplifies and are connected to the loud

speaker, which reconverts them to sound signals by sound output circuit in the IC.

 The outgoing speech sounds are converted to corresponding speech currents by

microphone, amplified by microphone amplifier and connected to telephone lines.

 Side tone suppression is done by “Side Tone’ circuit in the IC.

 The gain of the sound amplifier is controlled by “Automatic gain control (AGC) circuit”

provided in the IC.

 Impedance matching between the Telephone lines, IC circuit, Mike, and Loud

speaker is done by “Equalization Amplifier” provided in the IC. These IC’s are bipolar

integrated circuits performing all speech and line interface functions required in fully

electronic circuits.

 The circuits internally perform electronic switching between dialing and speech.

Working of Sound Amplifier:

Speech Transmission: The microphone output is connected to input of microphone amplifier.

The output of the amplifier is connected to telephone lines.

Speech Reception: The incoming signal received on Pin.No1 is internally applied to pin11

and pin18. The output of the speech output amplifier is connected to loudspeaker.

e) Voltage Dropper Section:

Telephone instrument gets 40 to 50V DC supply from the telephone exchange through

telephone lines. Till the ring comes, this high voltage is maintained. But as soon as the

receiver is lifted, with two way switch, this high voltage goes to voltage dropper section,

where it is converted into +9V or +12V as per the telephone needs. Then dial tone signals

are given and are heard after amplification.

f) Rectifier / Protection Section (fig.3.7):

The rectifier section is also called as “Polarity guard”.

Functions of this section are,

1. To supply voltage of the proper polarity to the input of the telephone circuit for proper

functioning and avoids damage to electronic circuit. This is done by bridge rectifier.

2. A Metal Oxide Veristor (MOV) or Voltage Dependent Resistor (VDR) of 95V rating is

connected across the input of the bridge circuit to limit any high peaks of voltages due to

induction on telephone lines.

3. The output of the bridge rectifier is connected to the dialer section, voltage dropper section

and Ringer section. IN4004 diodes are used in bridge rectifier.

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