Clap Relay

Posted in Sound sensor, on 2015-07-18

Circuit diagram:

Clap Relay


  • R1,R6___________12K 1/4W Resistors
  • R2_______________1M 1/4W Resistor
  • R3,R9____________6K8 1/4W Resistors
  • R4_____________220K 1/4W Resistor
  • R5_______________2M2 1/4W Resistor
  • R7,R10_________100K 1/4W Resistors
  • R8______________22K 1/4W Resistor
  • C1,C3__________220nF 63V Polyester Capacitors
  • C2,C4,C5________22nF 63V Polyester Capacitors
  • C6______________47µF 25V Electrolytic Capacitor
  • D1,D2_________1N4148 75V 150mA Diodes
  • IC1_____________7555 or TS555CN CMos Timer IC
  • Q1_____________BC550C 45V 100mA Low noise High gain NPN Transistor
  • Q2,Q3__________BC328 30V 800mA PNP Transistors
  • MIC1___________Miniature electret microphone
  • SW1_____________SPST Switch
  • RL1_______________5V DIL Reed-Relay SPDT (See Notes)
  • B1________________3V Battery (2 x 1.5V AA, AAA Cells in series etc.)

Device purpose:

This circuit was intended to activate a relay by means of a hand clap. A further clap will turn-off the relay. An interesting and unusual feature of this project is the 3V battery operation. The circuit's sensitivity was deliberately reduced, in order to avoid unpredictable operation. Therefore, a loud hand clap will be required to allow unfailing on-off switching.

Circuit operation:

Q1 acts as an audio amplifier. IC1 timer, wired as a monostable, provides a clean output signal and a reasonable time delay in order to allow proper switching of the following bistable circuit. A discrete-components circuit formed by Q2, Q3 and related parts was used for this purpose, in order to drive the Relay directly and to allow 3V supply operation.


  • A small DIL 5V reed-relay was used in spite of the 3V supply. Several devices of this type were tested and it was found that all of them were able to switch-on with a coil voltage value comprised in the 1.9 - 2.1V range. Coil resistance values varied from 140 to 250 Ohm.
  • Stand-by current consumption of the circuit is less than 1mA. When the Relay is energized, current drain rises to about 20mA.

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