This is a simple circuit in which the glowing LED can be switched OFF just by a puff. A condenser mic (M1) is used to sense your puff. When the push button S1 is pressed, the transistors Q2 and Q3 wired as latching pair gets activated and drives the LED to glow. The LED remains in this condition. When you puff on the condenser mic, the sound pressure is converted into a voltage signal at its output. This voltage signal will be amplified by the transistor Q1.Since the collector of the Q1 is coupled to the emitter of the latching pair, the pair will stop conducting when ever there is a signal from the condenser mic due to puffing and the LED will go OFF. The push button switch S1 has to be pressed again to switch the LED ON.
Circuit diagram with Parts list.
- The circuit can be powered from a 3V battery.
- The M1 can be a general purpose condenser microphone.
- The switch S1 can be push button switch.
- The circuit can be assembled on a good quality PCB or common board.
- Instead of the LED, you can also try a low power 3V bulb.