Highly sensitive touch alarm made using uA741+CA3140

The electromagnetic field radiation signal of the AC power supply of the mains can reach a distance of several meters, and these radiation signals can also be sensed by the human body. By using this electromagnetic field radiation signal sensed by the human body, a touch alarm can be made, as shown in the attached figure.

When the human body touches the touch pad in the attached figure, this electromagnetic induction signal will also be generated on the touch pad. After the signal is amplified by the high-gain operational amplifier IC1 (741) through capacitor C1 and resistor R2, it is output from pin 6 and then enters the voltage comparator composed of the operational amplifier IC2 (CA3140) through C2 and D1. When the input pin 3 of IC2 receives the signal amplified by IC1, transistor T1 is turned on. The piezoelectric buzzer PZ1 makes a sound. At the same time, the light-emitting tube LED1 lights up to remind people to pay attention. The circuit power supply is +12V, and it can be powered by an AC adapter.

Install the circuit on a common PCB. Then put the PCB into a small box. Put the touch panel outside the small box and connect it to the circuit input stage with a shielded wire of a certain length. The size of the touch panel should not be too large. The circuit can still work normally.