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2-Line
Intercom-Cum-Telephone Line Changeover Circuit
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The
circuit presented here can be used for connecting two
telephones in parallel and also as a 2-line intercom.
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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.
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| 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. |
| The
sensitivity depends on the current-limiting resistor in
series with the infrared LED. It is observed that with
in-circuit resistance of preset VR1 set at 20 ohms, the
object at a distance of about 25 cms can be sensed. |
| This
circuit can be used for burglar alarms based on beam interruption,
with the added advantage that the transmitter and receiver
are housed in the same enclosure, avoiding any wiring
problems. |
