@▷ DOORKNOB ALARM | Diagram for Schematic

DOORKNOB ALARM


DOORKNOB ALARM…[more]

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 betw1een
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.

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