The figure below shows a very useful and interesting project of sound controlled toggle switch circuit. The circuit is quite sensitive and can detect low sound signals easily. It can be used to perform variety of tasks for example it can be used to control AC home appliances with the sound of snapping fingers, whistle

or clap from distance. The sensitivity of the circuit can be controlled with 100K variable resistor.

The transistor Q1 and Q2 are used to amplify the audio signals. The diode D1 and D2 rectify audio signals received from filter capacitor C3 and making Q3 switch ON. When Q3 become switch ON it provide positive pulses at the clock input pin 14 of the CD4017 IC. The CD 4017 is wired as a toggle switch here therefore when it will receive positive pulses on pin14 its output goes high and low at pin2 and pin3. Therefore when the microphone receives a first sound signal then the relay and LED1 will become activated till another sound signal is received and when it will receive sound again the relay and LED1 will become deactivated. The transistor Q4 is used to drive the relay. The operating voltage of the circuit is 9 to 12 volt DC.
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Sound Controlled Toggle Switch Circuit Diagram