Touch Sensitive Alarm

By Pradeep G.

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Security has been a major concern for people for long. Any personal belonging kept out of personal reach is prone to attack and sometimes, even the ones we are using, get attacked. Presented here is a touch sensitive alarm that alerts you whenever touches any of your belongings. The concept used for the touch-based alarm is using radiation signals.

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 the generation of an 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.

Touch sensitive alarm circuit

Touch sensitive alarm circuit
Touch sensitive alarm circuit

The low power AC hum 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.

Construction & testing

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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.


The project was first published in May 2010 and has recently been updated.

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