motorola (well, onsemi, or is it freescale, the morons keep changing the
name, not helpful!) have a very nice capacitive sense ac circuit chip,
go over to circuitcellar.com and look at the results of the design
contest they had in the last couple of months.  i think it's even cheap,
and you an have up to 10 electrodes, or just 2 and get an analog output
(voltage proportional to capacitance), all you need then is a
comparator.  the electrodes will have a higher capacitance when in
water, and can actually be completely insulated from it, though
preferably by thin insulation, but you could even have them on opposite
sides inside a small piece of pvc pipe and have it all completely
sealed.  always look for chips that make your life easier.  it's worth
going to the major chip makers sites and looking around at analog chips,
especially special use or "misc." functions.  maxim.com is also a great
place to find some very useful and unusual chips.  note that often chips
are very useful outside of the originally intended purpose, and with
some chips you may only want to use a small part of them but it's still
worth buying the chip and just "wasting" most of the silicone.

Michael O'Donnell wrote:
>
> Hi Dave,
>
> Thanks a lot!  You nailed it with the low supply voltage.  I switched my
> power supply for a 7805 and things seem happy now.  Actually, it was
> working on 3.3 V too, but I realized that my trigger circuit (output from
> my computer) was only putting out 2.1 V, which put it right at the
> sort-of-working threshold.
>
> The feedback is a good idea too, though, because the water might drain away
> slowly, which causes the voltage to drift down rather than chopping, which
> would give the problem you mentioned.
>
> This might be another reason to use AC as the other poster suggested.
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