Hum-Sensitive Touch Alarm

Pradeep G.


Radiation signals from mains wiring can travel a few metres of distance. These can be induced by the electromagnetic field in the human body also.

This touch-sensitive alarm is based on generation of the AC hum signal. When someone touches the touch plate, low-power AC hum (the same as induced from AC wiring of the house) is generated on the touch plate. This signal is first amplified by the high-gain preamplifier built around IC 741 (IC1) and then fed to another op-amp CA3140 (IC2) that is wired as a voltage comparator.

When IC2 receives the amplified signal at its input pin 3, its output goes high. As a result, transistor T1 conducts to sound the buzzer. At the same time, LED1 glows. For satisfactory working of this unit, power it from mains- derived 12V.

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Assemble the circuit on a general-purpose PCB and enclose in a small cabinet. Use a shielded wire to connect the touch plate to the circuit keeping the length of the wire and the size of the touch plate as short as possible.