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Last Updated on: Wednesday, June 02, 2021 04:17 AM

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Hidden Panic Switch (5/1/2011)

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.
Hidden Switch Behind Wall Panic Button
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.
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. 

Hidden Panic Switch, 5/1/2011

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DC Magazine, May 2011, Page 7:  Hidden Panic Switch

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