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DiscoverCircuits.com -- Hobby Corner
Last Updated on:
Wednesday, June 02, 2021 04:17 AM
Hobby Circuits List
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Hidden Panic Switch (5/1/2011) |
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Problem: Design an electronic switch which would be positioned behind a wall
at several key positions0 and would be activated when an adult human hand was pressed against the wall at that position.
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Hidden Switch Behind Wall |
Panic Button |
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Solution: The best way to detect the human hand against a wall was with a capacitance switch. The QTFS3X from Faraday switches looked like a perfect match. The switch could be powered by a DC voltage down to 3v. It required only 30ua of current. It was quite small and would easily fit behind the wall. |
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The circuit is shown below. A
signal diode and capacitor routed power to the switches V+ terminal. The
momentary output terminal of the capacitance switch module was connected to the
alarm circuit’s sensor wire. The other wire from the alarm cable connected to
module’s circuit ground. When a hand was placed over the section of wall
containing the aluminum foil, the capacitance would increase. The switch module
would detect the sudden capacitance increase and would pull the sensor wire signal
to circuit ground. The energy stored in the capacitor would supply enough
power to the module for several seconds of operating, even as the input power was
shorted to ground. |
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DC Magazine, May 2011, Page 7:
Hidden Panic Switch |
