Sound-Operated Timer

By Pradeep G.


This sound-operated timer is based on LM324 quad-operational amplifier and NE555 timer. Time delay can be set from a few seconds to 30 minutes. It can also be used as a sound-sensitive burglar alarm.

Circuit and working

Circuit diagram of the sound-operated timer is shown in Fig. 1. Output pin 1 of LM324 (IC1) is fed to pin 2 of NE555 (IC2) through capacitor C3. IC2 is configured as a monostable multivibrator. Sound signal, like clapping, sensed by condenser microphone MIC1 is amplified by IC1.

Fig. 1: Circuit diagram of the sound-operated timer

Amplified signal is fed to trigger pin 2 of IC2. Output pin 3 of IC2 goes high and relay RL1 gets energised for a time delay set by potmeter VR2. Time delay can be set from a few seconds to 30 minutes. Energisation of the relay is indicated by the glowing of LED1.

Relay RL1 is de-energised after the time delay. NE555 can source 200mA current; so, an ordinary relay can be directly connected to IC2’s output without an extra transistor driver. Use a good-quality microphone for maximum sensitivity.

Construction and testing

An actual-size, single-side PCB layout for the sound-operated timer is shown in Fig. 2 and its component layout in Fig. 3. After assembling the circuit on the PCB, enclose it in a suitable plastic box.

Fig. 2: Solder-side PCB layout of the sound-operated timer
Fig. 3: Component layout of the PCB

Download PCB and component layout PDFs: click here

The circuit needs a 6V-12V DC power supply. When MIC1 captures the sound made by an intruder, RL1 gets energised. An electric lamp may be connected via N/O contacts of RL1 along with a bell or an alarm.

Pradeep G. is B.Sc. (Physics) and a regular contributor to international magazines. He is also a small-business owner in south India.



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