Basically the circuit consists of a bridge network which is used to ensures that transistor T1 gets voltage of proper polarity, irrespective of the polarity of the telephone lines. To this bridge circuit the telephone line is connected.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 second part of the circuit which consists of two transistors T1 and T2 that controls relay RL1, which is used to switch on/off the Tape recorder. The tape recorder should be in recording mode. The values of R1 And R2 depends on input impedance of the tape recorder’s MIC-IN terminals.
A voltage of 48 v appears across the telephone lines in on hook condition. This voltage drops to about 9v when the handset is lifted. Diodes D1 through D4 constitute a bridge rectifier/polarity guard. This ensures that transistors T1 gets voltage of Proper polarity. During on hook condition, the output of the bridge (48 v DC) passes through 12 v zenor 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 energized.
When the telephone handset is lifted, the voltage across points X and Y Falls below 12v and so zener diode D5 does not conduct. As a result, base of the transistor T1 is pulled to ground potential via resistor R4 and thus is cut off. Thus, base of the 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 handset is lifted. Capacitor C2 ensures that the relay is not switched on and off repeatedly when a number is being dialled in pulse dialling mode.