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Hi,

Circuit idea attached with sketches of waveforms to show how it
works.

The micro toggles a port pin to generate a square wave. It is
capacitively coupled to the probe as DC is not a good thing on a
probe for electrolysis reasons.

The probe is an uknown resistance to ground. This forms a voltage
divider with the series resistor. The amplitude of the waveform is
now a function of the resistance of the probe.

Two diodes and two caps make a rectifier / voltage doubler. The
input to the ADC is now in the range of 0 to about 3.8V. The parallel
resistor (large value) discharges the filter capacitor when the probe
is open circuit again.

Hope this gives you some ideas.

Brent Brown
Electronic Design Solutions
16 English Street
Hamilton, New Zealand
Ph/fax: +64 7 849 0069
Mobile/text: 025 334 069
eMail:  brent.brown@clear.net.nz

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