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DiscoverCircuits.com -- Hobby Corner
Last Updated on:
Tuesday, June 01, 2021 03:06 PM
Hobby Circuits List
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Alarm Circuits
Beepers & Buzzers
Door Bell Circuits |
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Door Knob Touch Alarm Circuit
designed
by David Johnson, P.E. |
| I designed
this 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. |
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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. |
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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. |
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Click on Circuit Below to view PDF of Schematic |
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Alarm Circuits
Beepers & Buzzers
Door Bell Circuits
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eMail David A.
Johnson, P.E. about this circuit |
