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How to build Clap Sensitive On-Off Relay

2018-11-13 08:44  
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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 = 12KR2 = 1MR3 = 6.8KR4 = 220KR5 = 2.2MR7 = 100KR8 = 22KR9 = 6.8KR10 = 100KQ1 = BC550CQ2 = BC328Q3 = BC328C1 = 220nF-63VC2 = 22nF-63VC3 = 220nF-63VC4 = 22nF-63VC5 = 22nF-63VC6 = 47uF-25VD1 = 1N4148D2 = 1N4148B1 = 3V BatteryIC1 = 7555 CMos ICRL1 = DIL Reed-Relay SPDTSW1 = SPST SwitchMIC1 = 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.

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