TELECOMMUNICATION(Rotary dial telephone system)

Rotary Dial Telephone System

Introduction:

Rotary dial telephone system was a switching system made by Almon B. Strowger to replace the human operator by automatic exchange. In an automatic exchange, informal communication between operator and calling subscriber is not possible and a formal numbering plan is required to identify the subscribers.  The mechanism to transmit the identity of the called subscriber to the exchange can be sent by:

1.       Pulse Dialling

2.       Multi Frequency Dialling

Operating Principle:

In pulse dialling, a train of pulses is required to represent a digit in the subscriber number. The number of pulses in a train is equal to the digit value it represents except in the case of zero which is represented by 10 pulses. Two successive trains are distinguished from one another by a pause in between them, known as inter-digit gap.

Construction:

A rotary dial telephone uses the following for implementing pulse dialling:

(a)    Finger plate and spring

(b)   Shaft, gear and pinion wheel

(c)    Pawl and ratchet mechanism

(d)   Impulsing cam and suppressor cam/ trigger mechanism

(e)   Impulsing  contact

(f)     Centrifugal governor and worm gear

(g)    Transmitter, receiver and bell by pass circuits

The arrangement of the finger plate is shown below:

Working:

The dial is operated by placing a finger in the hole appropriate to the digit to be dialled, drawing the finger plate round in the clockwise direction to the finger stop position and letting the dial free by withdrawing the finger. Now dial will come back to its initial rest position and the dial pulses are produced during the return travel of the finger plate.

Internal mechanical arrangement of rotary dial telephone is as shown below:

Cam mechanism:

1.       When the dial is in the rest position, the impulsing contacts are kept away from the impulsing cam by the suppressor cam.

2.       When the dial is displaced from its rest position (off normal position),impulsing contacts come near the impulsing cam.

3.       Rotation of finger plate causes rotation of the main shaft. Pawl slips over the ratchet during clockwise rotation.

4.       The ratchet, gear wheel, pinion wheel and the governor are all stationary during the clockwise movement of the dial.

5.       When the dial returns, the pawl engages and rotates the ratchet. The gear wheel, pinion wheel and the governor all rotate.

6.       The governor helps to maintain a uniform speed of rotation. The impulsing cam attach to the pineal shaft now breaks and makes the impulsing contacts which in turn causes the pulses in the circuit.

7.       When the dial reaches the rest position, the suppressor cam moves the impulsing contacts away from the impulsing cam.

8.       This action provides the required inter-digit gap timing independent of the pause that may occur between 2 successive digits due to human dialling habit.

Trigger mechanism:

Trigger is sprung away from the impulse contacts during the clockwise motion of the dial, thus preventing pulsing at this stage.The trigger is sprung back to the operative position during the initial return motion of the dial and thereafter operates the pulse contacts.

Advantages of Trigger Dial over Cam Dial:

The precision of operation in the cam dial is affected by the wear and tear of the cam elements and other friction members in the mechanism. The trigger dial design eliminates friction members and helps to achieve more uniform impulse ratio, larger inter-digit pause and better stabilisation of the return speed of the dial.