3 touch alarm circuit using 555 IC

This touch alarm circuit will sound when somebody touches the trigger plate. This plate can be place in any metal such our doors or in metal knobs. It can be use as your simple burglar alarm or security circuit that can place in your doors.

touch alarm schematic

Touch alarm circuit operation
The heart of this alarm circuit is the 555 timer  that is operated in monostable mode. The timer produces an output sound to buzzer with the time delay constant equal to 1.1RC.When the voltage at the trigger pin of 555 IC (pin 2) reaches below 1/3 of the supply voltage, the output will be high with the duration of 1.1RC. For 3 seconds duration, R is 12K resistor while 22k for 5 seconds. You can use 555 timer calculator for your delay options. The circuit is very similar to Simple Touch alarm and sensor.

Touch alarm materials:

Part list