Touch Activated Alarm System
by Tony van Roon
Parts List
R1 = 100K D1 = 1N4004 U1 = 555 Timer*
R2 = 56K C1 = 47uF/16V**
R3 = 10M C2 = 33uF/16V**
R4 = 220K T1 = 2N3904
P1 = 100K Re1 = Relay***
Additional Notes
Not much to tell here as the circuit speaks for itself. *The 555 can be a LM, NE, or MC(cmos) type, they are all pin-compatible. **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 powersource is 9Volt, your capacitor(s) is at least 18V. Transistor T1 can be any approximate substitute. ***Use any suitable relay for your project and if you're not tight on space, use any size. I've build this particular circuit to prevent students from fiddling with the security cameras in computer labs at the University I am employed. I made sure the metal casing was not grounded. 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 pin 1 of the 555 (U1) and ground.
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Copyright © Tony van Roon, CET
