Monday, September 04, 2006


Clap Switch
Several circuits for sound operated switches have been published earlier. The circuit shown here for clap operated switch is inexpensive and easy to assemble by hobbyists of any level. Clap sound signals picked up by condenser microphone are first amplified by transistor T1, which is a simple common-emitter amplifier. Amplified signals are rectified by diode D1. Positive half cycle of clap signal is applied to a 3-stage DC amplifier formed by transistors T2, T3 and T4. Output from transistor T4 is used as a clock for flip-flop IC1 (7472). For each clap, the output of IC 7472 toggles alternately to on/off state.

Transistor T5 functions as relay driver since output level of IC1 is insufficient to drive a relay directly. The entire circuit, except the relay driver, operates at 5 volts regulated. The 5-volt supply is derived from 12V supply (used for relay driver transistor T5) using a zener diode with series resistor R11 (150-ohm, 1-watt). Using this circuit any electrical or electronic load can be controlled by just clapping in front of the microphone. The microphone should be housed inside a suitable funnel shaped enclosure to improve sensitivity. Potmeter VR1 is used as sensitivity control.

The entire circuit can be powered from simple 12-volt unregulated supply using 12-volt step-down transformer, followed by full-wave rectifier and a filter capacitor of about 1000µF, 25V.



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