This audio activated relay switch circuit can be used for variety of audio related tasks. For example it can be used to start a recorder only when an audio signal is received, switch on an amplifier on small audio signal etc. This little circuit can save a lot of power and also it is ideal for battery operated projects.
The circuit requires a small audio signal to switch on the relay such as equal to the audio output of a headphone jack of any equipment.
The circuit can be operated with 12 volt DC but you can also operate it with 5, 6 or 9 volts, make sure to use the relay according to the voltage. For example if you are operating the circuit with 6 volt then use a 6 volt relay with the circuit. A 12 volt relay will not work on 6 volt.
Working of the circuit
When the two 1N4148 diodes receive an audio signal through the 1uF capacitor then they convert this audio signal in to DC signal and send it to the base of the transistor due to which the transistor become switch ON and activates the relay. 1N4001 diode across the relay is used to safe the transistor from the back EMF of relay. You can also use 1N4148 diode in the place of 1N4001. The 1000uF capacitor across the relay is used to make the relay activated for few seconds. The 20K variable resistor is used to control the sensitivity of the circuit.