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Thread: touch switch?
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face BY : Mike Hardwick email (remove spam text)



>Does anyone have any good clues as to how to impliment touch switches?

Alan,

Drive a sensor pad with a square wave through a series resistor. Connect
the sensor to one input of a CMOS XOR gate. Connect the other XOR input to
the same square wave source through an equal resistance. The XOR output is
a series of narrow spikes that get wider due to capacitive loading when you
touch (or come near) the sensor. Adjust frequency and R values for best
results. If the sensor is remote, add balancing capacitance at the second
XOR input to minimize pulse width when the sensor is unloaded.

You could integrate this signal with a simple R/C network and, if
necessary, use a comparator to insure perfect logic output. Or, you could
measure pulse width with a PIC timer. The XOR gate could even be
eliminated, at lower operating frequencies, if you're willing to use
another PIC I/O pin and do that function in software.

RF emissions are minimized because the sensor is fed through enough
resistance that most high-frequency harmonics from the square wave source
are removed by stray capacitance.

You could do something similar using the PIC for a signal source, much like
the canonical R/C measuring technique.

Unfortunately, with any of these methods, the sensor can still bring
destructive ESD jolts into your system. It's difficult to balance
sensitivity against multi-KV insulation. You may be compelled to add
protection circuitry...

Mike Hardwick, for Decade Engineering -- <http://www.decadenet.com>
Manufacturer of the famous BOB-II Serial Video Text Display Module!

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