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Last Updated on:  06/19/2009 02:47:28 PM

Door Knob Touch Alarm Circuit
designed by David Johnson, P.E.
I designed this electronic circuit, hobby circuit many years ago and it seems be a very popular construction project.  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 electronic circuit schematic should dramatically reduce the cost of the device and the power drain is so small that it will operate for many years from one set of AA or AAA batteries. 
This device will only work with wood doors. It is suggested that the hobby circuit be housed in a metal box although some people have successfully used a plastic box with a large metal plate inside.  The plate needs to be positioned so it is close to the door when the device is hanging from the doorknob.  Two insulated holes in the box, allow a standard metal key chain to emerge.  The chain is looped over and around metal doorknob.  A sensitivity adjustment allows the circuit to be fine tuned to cope with different doors and component variations.

Also see Completed Project  by Matthew C. Ruschmann

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

Click on Schematic below to view PDF version of this Circuit
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