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Two line intercom plus a telephone changeover
switch Click here for the circuit diagram
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
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
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