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Dave's Circuits
Circuits designed by David Johnson, P.E.

Master Category List - Dave's Circuits
 Last Updated on: Friday, July 18, 2008 11:34 AM

Alarm Circuits   Capacitance Touch Switches
 
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.


Also see Completed Project  by Matthew C. Ruschmann

Click on Drawing Below to view PDF version of Schematic

Doorknow Touch Alarm designed by Dave Johnson, P.E. (May 29, 2000)

Alarm Circuits   Capacitance Touch Switches
Master Schematic Category List - David A. Johnson, P.E.
 

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