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Two Line
Intercom Push A Telephone Change Over Switch |
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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.
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Click
Here For Circuit Diagram
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