Schematics > Telephone related > Two line intercom plus a telephone changeover switch
Two line intercom plus a telephone changeover switch
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 ring-tone-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.
Title: Two line intercom plus a telephone changeover switch
Source: www.electronic-circuits-diagrams.com
Published on: 2005-02-09
Reads: 273
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