This sound triggered timer uses an operational amplifier and the NE555 timer. The timer is triggered by sound captured from a condenser microphone. The 555 timer is used in monostable mode and the time delay can be set from a few seconds to about 30 minutes. The circuit may also be used as a sound sensitive burglar alarm.
The circuit diagram is quite straightforward. Any sound signal captured by the condenser microphone MK1 is amplified by the U1A which operates as a high gain inverting amplifier. The amplified audio signal is fed from the output pin 1 of U1A to the pin 2 of NE555 (U2) through the capacitor C3. C3 is used for AC-coupling.
The audio signal triggers the 555 timer and the output pin 3 of the 555 goes high. Thus the relay K1 goes on for some time set by the RV2 potentiometer. There is also a LED indicator, the D3 LED, which indicates when the relay is in its energized state.
After the preset time, the relay goes off and the timer relaxes at its stable state until its next energized cycle. NE555 can source up to 200mA, thus an ordinary relay can be directly connected to U2’s output.
RV1 is used to bias the non-inverting input of U1A. Since we use only one power supply voltage, normally for getting a maximum dynamic range, the voltage on pin 3 of U1A should be adjusted at a voltage equal to about half that of the power supply voltage. However, since distortion is not important, biasing at another level alters the dynamic range and brings the op-amp at saturation for less or more input signal amplitude, thus adjusting the sensitivity of the sound triggered timer.