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# A touch activated alarm system circuit

2016-02-19 12:29
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This article describes is a touch-activated alarm system circuit theory. The circuit 555 can be almost any type, they are pin compatible. While some CMOS types may not have enough power to drive the transistor, in this case with an ordinary 555. C1 operating voltage should be increased to 25 v If you decide to go with a 12 v power supply. Changing the value of the desired output, C1 pulse. For timed using this equation: T = 1.1 * (R1 P1) * C1 Assuming R1 P1 = 150 k, and the following of C1: C1 = 6μf for each second pulse width. For example, if you want the pulse width is 5 seconds, C1 should be 30 ultrafiltration or the nearest value like 22 or μf 33. In addition, P1 can be adjusted to rest. Thumb rule: the capacitor voltage is a voltage at least doubled, in other words, if the power supply is a 9-volt, your capacitor (s) is at least 18 v. The  transistor  T1 may be any approximation instead. Any suitable relay for your project if you are not tight space, use any size. I created this particular circuit to prevent students from fiddling with security cameras in the computer lab at the university I hired. I'm sure the metal casing is not grounded. As the diagram shows, but can be connected to essentially any type of metal surfaces. I use a 12 v DC power supply. Using any suitable relay to handle your requirements. A "reset" switch (normally closed) can add betw1een positive and "arrows - ." Trigger (touch) wire connected to the pin 2555, will trigger the relay, with your body resistance, when touched. Obviously, the "touch" part must be clean and full contact trigger line. This particular circuit may not be suitable for all applications. In case you wonder why pin 5 is not listed in the diagram, it does not really need. In certain conditions a little noisy 0.01μf; ceramic capacitors betw1een placed pin 5 and ground. It does not affect add one or leave it.

The Parts List

R1 = 100K D1 = 1N4004, general purpose diode
R2 = 4K7 C1 = 47μF/16V
R3 = 10M IC1 = 555 Timer
P1 = 100K Q1 = 2N3904, 2N2222, or similar
Ry = Relay

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