Touch Activated Alarm System using 555 IC

The 555 can be LM,NE or MC(cmos) type.C1/C2's working voltage should be increased to 25V if you decide to go with a 12V power source. Rule of thumb: the working voltage of capacitors are at least double the supplied voltage, in other words, if the power source is 9Volt, your capacitor(s) is at least 18V.T1 can be any approximate substitute.But make sure that metal casing was not grounded.

Circuit Diagram :

But as the schematic shows you can basically hook it up to any type of metal surface. I used a 12-vdc power source. Use any suitable relay to handle your requirements. A 'RESET' switch (Normally Closed) can be added between the positive and the 'arrow-with-the-+'. 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. This particular circuit may not be suitable for all applications. Just in case you wonder why pin 5 is not listed in the schematic diagram; it is not really needed. 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.


AGAIN, make sure the latch is not touching anything 'ground' or the circuit just keeps resetting itself and so will not work. My shed has wooden doors so works fine. If you can not 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 R3



Parts List


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 = Relay

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