This is circuit of Clap Triggered Switch. When the circuit detects tap, or short whistle or a clap, this circuit will toggle the LEDs. 33k and 5K6 charge The second 10u. The negative excursion of the waveform takes the positive end of the 10u towards the 0v rail, when a sound is detected. the two 1N4148 diodes will be pulled by The negative end of the 10u because The negative end of the 10u will actually go below 0v. So the anode ends will have near to zero volts on them. Here is the schematic diagram of the circuit:
Clap Triggered Switch circuit schematic
When the transistor that is turned on, the voltage drops is 0.6v on the base and have zero volts on the base when the transistor that is turned off. the transistor that is turned on, will begin to turn off because of the anodes of the two signal diode are brought lower. So, the other transistor will begin to turn on via its 47k and 100u. The two switch over very quickly. to a buffer transistor to operate a relay or other device The collector of the third transistor is used. [Source: talkingelectronics.com]