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Circuits designed by David Johnson, P.E.
Last Updated on:
Saturday, December 23, 2017 03:21 PM
List of Dave's Circuit Designs
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More
Alarm Circuits
Capacitance Touch Switches |
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Door Knob 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.
Also see
Completed Project by Matthew C. Ruschmann |
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Click
on Drawing Below to view PDF version of Schematic |
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