Clap Sensitive On-Off Relay


This circuit was intended to activate a relay by means of a hand clap. Further claps 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. 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.

Circuit Diagram:

Circuit Diagram


  • R1 = 12K
  • R2 = 1M
  • R3 = 6.8K
  • R4 = 220K
  • R5 = 2.2M
  • R7 = 100K
  • R8 = 22K
  • R9 = 6.8K
  • R10 = 100K
  • Q1 = BC550C
  • Q2 = BC328
  • Q3 = BC328
  • C1 = 220nF-63V
  • C2 = 22nF-63V
  • C3 = 220nF-63V
  • C4 = 22nF-63V
  • C5 = 22nF-63V
  • C6 = 47uF-25V
  • D1 = 1N4148
  • D2 = 1N4148
  • B1 = 3V Battery
  • IC1 = 7555 CMos IC
  • RL1 = DIL Reed-Relay SPDT
  • SW1 = SPST Switch
  • MIC1 = Electret Mic


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.