|
Telephone
Conversation Recorder
|
|
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. |
