Sound Operated Timer

Several timer circuit have been published in, but this is a unique timer circuit with no timer IC in use. It is transistorized timer having a 2-stage audio amplifier and a relay driving unit.
The incoming sound waves are transformed into a low audio signal by the condenser (mic). The direct coupled audio amplifier built around transistors T4 and T5 amplifier the tiny signal. The amplified signal is rectified by Diode D1 and is fed to the base of T3. Capacitor C1 starts charging through T3. The charge of C1 switches on transistors T2 and T1 simultaneously. T1 energizes the relay and the load gets power through it.
The time period is maintained by C1, whose value can be in between 10 µF to 220 µF. It is found that the delay time is about one sound with 10 µF capacitors and it is about 20 seconds with 220 µF capacitor.

circuit diagram of sound operated timer

The circuit easily be mounted on a small veroboard. You can control radio, TV, taperecorder or any other electronic equipment with this timer.
Resistors (all ¼-watt, ± 5% Carbon unless stated otherwise)
R1 = 2.7 KΩ
R2 = 100 Ω
R3, R10 = 100 KΩ
R4 = 220 KΩ
R5 = 27 KΩ
R6 = 3.3 KΩ
R7 = 1 KΩ
R8, R11 = 10 KΩ
R9 = 270 KΩ
C1 = 10 µF/12V
C2 = 0.04 µF
C3 = 0.1 µF
T1 = AC128 or AC188
T2 – T5 = BC148
D1 = 1N4001
MIC1 = Condencer Mike
RL1 = 6V 100Ω relay

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