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Wily Widget

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Hidden Panic Switch

Wily was eating lunch when he got a call from an alarm company.  They were installing more and more home alarm systems. Business was good.  But, a customer of theirs had an unusual request.  The homeowner wanted an invisible panic button.  The alarm company owner described the device as an electronic switch which would be positioned behind a wall and activated when an adult human hand was pressed against the wall at that position.  Several such switches would be installed at key locations around the house.  The caller wanted to know if Wily could design something like that for them.  Wily had some ideas how he might do this so he agreed to take on the project.
 
Security System Control Panel  Panic Button
Burglar   Hidden Switch Behind Wall
The security company’s alarm system had multiple inputs, which could be configured as panic switches.  A typical switch interface circuit is shown below.  A 4.7K “pull-up” resistor is connected to the alarm’s +5v supply. The remote switches are connected to the resistor and to circuit ground.  When activated, the switch “sinks” the +5v signal to ground.  Based on the component values, the current through the switch was about 1 milliamp and the threshold voltage for a valid switch closure was about 1.0v above ground.  Wily was told that this two wire configuration was the standard in the industry and was the alarm’s company’s preference.  But, this configuration made it a bit more difficult for any remotely activated panic button circuit to operate without a secondary power source.  A battery powered unit behind a wall would not be practical.  Also, some other power source, like an AC power adapter, would equally be undesired.  What Wily needed was a system which could detect a human hand pressed against a wall and demanded so little power that it could draw power from the alarm’s pull-up circuit. Wily figured that a device which required 100ua or less of current would be best.
Alarm Sensor Interface Circuit
Wily figured the best way to detect the human hand against a wall was with a capacitance switch.  The concept would be to mount the capacitance switch behind the wall in key locations. An aluminum foil patch about 6 inches on a side would be glued to the inside sheetrock wall, between the two by four studs.  The patch would be positioned about 6 feet off the floor, so small children or pets could not accidentally trigger the alarm.  Using earth ground as a reference, the capacitance between the metal patch and ground would increase when the human hand was placed over the patch area.  Some quick calculations told Wily that the capacitance change would be about 10 picofarads.  This was not much but with the right switch, it would be detectable.
After doing some searches on the Internet.  Wily found a very nice capacitance switch to do the job.  The QTFS3X from Faraday switches looked like a perfect match for what Wily needed.  The switch could be powered by a DC voltage down to 3v.  It required only 30ua of current.  It was self calibrating so any changes in the installation capacitance caused by humidity or temperature would be compensated for.  It could easily detect the 10pf of capacitance change. It was quite small and would easily fit behind the wall.
The circuit Wily used for this application 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. 
 

A few weeks after Wily submitted his design to the alarm company, they reported that the system worked perfectly.  The alarm company indicated that they would be offering this hidden switch as an option to future customers.

May 2011     Issue 17

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