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R1 = 100K D1 = 1N4004, general purpose
diode
R2 = 4K7 C1 = 47�F/16V
R3 = 10M IC1 = 555 Timer
P1 = 100K Q1 = 2N3904, 2N2222, or similar
Ry = Relay
Not much to tell here as the circuit
speaks for itself. The 555 can be almost any type, they
are all pin-compatible. Although some CMOS types may not
have enough power to drive the transistor, in that case
use an ordinary 555. C1's working voltage should be
increased to 25V if you decide to go with a 12V power
source. Change the value of C1 for the desired output
pulse.
For the timing use this equation:
T=1.1*(R1+P1)*C1 assuming R1 + P1 = 150K, then select C1
as follows: C1 = 6�F for each 1-second pulse width. For
example, if you want the pulse width to be 5 seconds, C1
should be 30uf or nearest value like 22 or 33�F.
Additionally, P1 can adjust the rest.
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. 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 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 0.01�F;
ceramic capacitor is placed between pin 5 and ground. It
does no harm to add one or leave it out. |
