Hi,
I am trying to use a LM339 comparator to tell me when two probes are shorted by water.  I have one probe connected to Vs and the other going into the +input of the comparator.  The -input is connected to a pot between the supply and gnd, which should let me set the threshold at which it decides the probes are shorted.  I have a 1M pulldown on the input pin, and a 3.3K pullup resistor on the output.

This seems to be working fine, but there is some sort of oscillation on the output that is puzzling me... essentially, when the probes are open, it sits at zero, but when they are shorted, it sits at about 3.1V with a few mV of wiggle.  I thought the pullup would eliminate this, but apparently not.

I've posted the schematic here:
http://www.moosecraft.org/comparator.html

thanks for any suggestions.

cheers,
mike


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