Wiring Schematic Diagram

Here is a circuit that allows you to control a relay in response to sound from a non-amplified sound source (computer, CD player, or my Digital Sound Recorder board).

This circuit accepts audio input from any non-amplified sound source and when the volume reaches a certain level, it actuates a relay. This relay can be used to switch another device (light bulb, etc.) on and off in sync with the volume of the input.

The circuit can work from any 5–12 VDC regulated power supply provided a relay with the suitable coil voltage is used. When you first connect the supply voltage to the sound activated switch circuit, the relay will be energised because of the effect of capacitor C2. Allow a few seconds for the relay to be switched off. You can increase or decrease the ‘on’ period by changing the value of C2. A higher value results in a longer ‘on’ period, and vice versa. Do not use a value greater than 47 μ F.

Biasing resistor R1 determines to a large extent the microphone sensitivity. An electret microphone usually has one internal FET inside which requires a bias voltage to operate. The optimum bias level for response to sound has to be found by trial and error.