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L28, Touch Activated Alarm System

This is a basic electronic project of Touch Activate Alarm System using a 555 timer. The 555 can be a LM or NE type they are all pin-compatible.


  • R1 = 100K

  • D1 = 1N4004

  • U1 = 555 Timer

  • R2 = 56K

  • C1 = 47μF/16V

  • R3 = 10M

  • C2 = 33μF/16V

  • R4 = 220K

  • T1 = 2N3904, or equivalent

  • P1 = 100K

  • RE1 = 9v RelayList R1

Rule of thumb:

The working voltage of capacitors are at least double the supplied voltage, in other words, if the powersource is 9Volt, your capacitor(s) is at least 18V. Transistor T1 can be any approximate substitute.Use 9v relay to handle the alarm set-up.

Metal casing should not ground. But as the schematic shows you can basically hook it up to any type of metal surface. And use a 9vdc power source.

The trigger (touch) wire is connected to pin 2 of the 555 and will trigger the relay, using your body resistance, when touched. It is obvious that the 'touching' part has to be clean and makes good contact with the trigger wire.

In certain noisy conditions a small ceramic capacitor is placed between pin 5 and ground. It does no harm to add one or leave it out.


There is an approximate 5-second delay build-in before activation of the relay to avoid false triggering, or a 'would-be' thief, etc.


Make sure the latch is not touching anything 'ground' or the circuit just keeps resetting itself and so will not work. If you use on wooden doors so it will work fine. If you cannot get yours to work, check the trigger input, verify there is some sort of signal coming from output pin 3 play with the value of C2 = 33μF/16V, R4 = 220K, T1 = 2N3904, or equivalent P1 = 100K

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