Clap Sensitive On-Off Relay

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3V Battery operated, Small portable unit

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.

Clap Sensitive on off Relay Circuit

Clap Sensitive on-off Relay 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

Source: Red Free Circuit Design