2-Line Intercom-Telephone Line Changeover Circuit

The circuit presented here can be used for
connecting two telephones in parallel and also as a 2-line intercom.
Usually a single telephone is connected to a telephone line. If another
telephone is required at some distance, a parallel line is taken for
connecting the other telephone. In this simple parallel line operation,
the main problem is loss of privacy besides interference from the other
phone. This problem is obviated in the circuit presented here. Under
normal condition, two telephones (telephone 1 and 2) can be used as
intercom while telephone 3 is connected to the lines from exchange. In
changeover mode, exchange line is disconnected from telephone 3 and gets
connected to telephone 2. For operation in intercom mode, one has to
just lift the handset of phone 1 and then press switch S1.

As a result, buzzer PZ2 sounds. Simultaneously, the side tone is
heard in the speaker of handset of phone 1. The person at phone 2 could
then lift the handset and start conversation. Similar procedure is to be
followed for initiation of the conversation from phone 2 using switch
S2. In this mode of operation, a 3-pole, 2-way slide-switch S3 is to be
used as shown in the figure. In the changeover mode of operation, switch
S3 is used to changeover the telephone line for use by telephone 2. The
switch is normally in the intercom mode and telephone 3 is connected to
the exchange line. Before changing over the exchange line to telephone
2, the person at telephone 1 may inform the person at telephone 2 (in
the intercom mode) that he is going to changeover the line for use by
him (the person at telephone 2).

As soon as changeover switch S3 is flipped to the other position,
12V supply is cut off and telephones 1 and 3 do not get any voltage or
ring via the ring-tone-sensing unit. Once switch S3 is flipped over for
use of exchange line by the person at telephone 2, and the same (switch
S3) is not flipped back to normal position after a telephone call is
over, the next telephone call via exchange lines will go to telephone 2
only and the ringtone-sensing circuit will still work. This enables the
person at phone 3 to know that a call has gone through. If the handset
of telephone 3 is lifted, it is found to be dead. To make telephone 3
again active, switch S3 should be changed over to its normal position.

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