Control by sound may be very useful, not just on a robot but also for a bit of home automation, for example, a sound-activated light responding to a knock on the door or a hand clap. The light will be automatically switched off after a few seconds. An alternative use is burglar protection — if someone wants to open the door or break something the light will come on, suggesting that someone’s at home.
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 circuit, the relay will be energized 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 pF. Biasing resistor R1 determines to a large extent the microphone sensitivity. An electret microphone usually has one internal FET which requires a bias voltage to operate. The optimum bias level for response to sound has to be found by trial and error. All relevant electrical safety precautions should be observed when connecting mains-powered loads to the relay contacts.