Simple Telephone Call Recorder
This circuit can be used to record the conversation on the phone in a cassette recorder. You can connect this circuit between the telephone and a cassette recorder. Whenever the handset is lifted then the green LED is turned ON and it results in a reduced value of the resistance. In this way IC 555 is triggered and the relay connected in the circuit is switched ON through a transistor connected at its output. This causes the supply to reach the cassette recorder. Both side recording in the cassette is possible by using this circuit.

When the telephone rings on an incoming call then both the LED s turn ON/ OFF. In this way the LDR receives continuous light by which the IC 555 gets the triggering volts. The transistor switches ON the relay. If the call remains unattended then the relay will switch OFF automatically after 10 seconds. If the telephone is attended during this time then the conversation will be recorded.
It is to be ensured that the cassette is loaded in the cassette recorder and the recorder is kept in recording position.