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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.
