Specifications

CIRCUIT IDEAS
change line to telephone
2, the person at tele-
phone 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
S.C. DWIVEDI
A
myriad of circuits have appeared
in EFY for protection of refrig-
erators and air-conditioners
against voltage fluctuations and brown-
outs. Here is a useful and economic cir-
cuit blending three features, namely, un-
der-/over-voltage protection, switch ‘on’
delay, and regulation.
The circuit with commonly available
components is a combination of familiar
building blocks. The ladder resistance-
trimpot configuration in conjunction with
modified bridge comparator ensures reli-
able sequential operations of boosting,
low-voltage cut-in, bucking, and high-volt-
age cut-off.
When the input line voltage is above
140V, relay RL2 energises and the boosted
voltage appears at the N/O contact RL1(b)
of relay RL1. However, relay RL1 remains
de-energised under two conditions: first,
if the input is below 170V threshold volt-
age (being controlled by trimpot VR1), and
second, due to the initial shunting effect
of capacitor C6 at the base junction of
THOMAS SEBASTIAN
GUARD FOR REFRIGERATORS
AND AIR-CONDITIONERS
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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