D1-D4 diodes serves as a device to convert the alternating current from the telephone line to become direct current in order to be used as a voltage activator. 12V zener diode lowers the voltage to 12V. When the phone receiver has not been appointed, the T1 voltage value will be low. This causes the transistor T2 on its cut off state and deactivate the relay. If the phone receiver is picked up, the voltage flows from T1 to T2. This activates the relay and turn the tape recorder on.
Components list :
Resistor R1 : 8k ohm
Resistor R2 : 2k2 ohm
Resistor R3 : 1M ohm
Resistor R4 : 100k ohm
Resistor R5 : 4k7 ohm
Diode D1-D4, D6 : 1N4001
Zener diode D5 : 12V
Transistor T1-T2 : BC548
Polar capacitor C1 : 0.001 uF/150 V
Polar capacitor C2 : 100 uF/25 V
Relay : 6V, 100 ohm
