Thanks philba! What voltage would this circuit run on? The other circuit is attractive because it will run on very low voltage, and also the probe polarity alternates, as Alec pointed out above.You can program an ATTiny85 with an arduino. Smaller than the schematic you posted.
Also, I would look at using a comparator or opamp to do a non microcontroller based circuit. Very straightforward. Here's one example. I'd replace the 47K/10K divider with a trimmer pot to allow for adjusting the trip point. Also, I'd add hysteresis and a 100n bypass cap. The 358 could drive maybe a 10mA load (ie an led).