Here are two circuits from Discover Circuits - a site created by David Johnson, P.E. I urge you to visit his site and check out his library! The italicized text has been reproduced from his site.
Here are two circuits from Discover
Circuits - a site created by David Johnson, P.E. I urge you to
visit his site and check out his library! The italicized text has
been reproduced from his site.
Touch Activated Switch - The circuit is powered from a
standard +5v supply. It has both a source and sink output that
change state whenever a metal button connected to the circuit is
touched. An earth ground reference is required.
Doorknob Alarm - Many companies offer simple alarm devices
for personal use in bedrooms or hotel rooms. A metal chain attached
to a box holding the electronics is placed around the inside
doorknob of a wood door. Anyone grabbing the knob from the outside
is detected by the electrical capacitance change that occurs from
the human hand contact between the knob and the box. Almost all of
the commercial devices sold use a more expensive and power
consuming radio frequency circuit approach to detect the
capacitance change. But, a very inexpensive and micro power
technique can also work. This circuit schematic should dramatically
reduce the cost of the device and allows it to operate for many
years from one set of batteries.