This
circuit enables automatic switching-on of the tape recorder
when the handset is lifted. The tape recorder gets switched
off when the handset is replaced. The signals are suitably
attenuated to a level at which they can be recorded
using the ‘MIC-IN’ socket of the tape recorder.
Points X and Y in the circuit are connected to the telephone
lines. Resistors R1 and R2 act as a voltage divider.
The voltage appearing across R2 is fed to the ‘MIC-IN’
socket of the tape recorder. The values of R1 and R2
may be changed depending on the input impedance of the
tape recorder’s ‘MIC-IN’ terminals. Capacitor C1 is
used for blocking the flow of DC.
The second part
of the circuit controls relay RL1, which is used to
switch on/off the tape recorder. A voltage of 48 volts
appears across the telephone lines in on-hook condition.
This voltage drops to about 9 volts when the handset
is lifted. Diodes D1 through D4 constitute a bridge
rectifier/polarity guard. This ensures that transistor
T1 gets voltage of proper polarity, irrespective of
the polarity of the telephone lines.
During on-hook
condition, the output from the bridge (48V DC) passes
through 12V zener D5 and is applied to the base of transistor
T1 via the voltage divider comprising resistors R3 and
R4. This switches on transistor T1 and its collector
is pulled low. This, in turn, causes transistor T2 to
cut off and relay RL1 is not energised.
When the
telephone handset is lifted, the voltage across points
X and Y falls below 12 volts and so zenor diode D5 does
not conduct. As a result, base of transistor T1 is pulled
to ground potential via resistor R4 and thus is cut
off. Thus, base of transistor T2 gets forward biased
via resistor R5, which results in the energisation of
relay RL1. The tape recorder is switched ‘on’ and recording
begins.
The tape recorder should be kept loaded
with a cassette and the record button of the tape recorder
should remain pressed to enable it to record the conversation
as soon as the handset is lifted. Capacitor C2 ensures
that the relay is not switched on-and-off repeatedly
when a number is being dialled in pulse dialing mode.
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