This is a relay circuit that detect the presence of sound to activate the relay. You can use this sound-controlled relay as voice operated switch, light control, toys, or any other application that can be controlled by a switch. Just replace the switch with this relay and now you have a sound-controlled device. This circuit uses a 741 operational amplifier to amplify the signal form microphone with the gain about 100 times. This circuit must use a dynamic circuit, or an electret microphone which has a battery inside. A condenser microphone can’t be used since this circuit has no biasing circuitry. Here is the schematic diagram of the circuit:
After the signal from microphone is amplified, a rectifier-doubler circuit built with D1 and D2 convert the AC signal to DC. After rectified, the AC ripple is filtered by C4 capacitor to smooth the signal. Finally, a potentiometer R5 is used to adjust the sensitivity of this circuit, calibrating the point of how loud the sound would turn on the relay.