Water Alarm - my adaptation of Tony van Roon's fluid-level sensor

Tony's schematic and notes are here:
~~http://www.sentex.ca/~mec1995/circ/sensor3.htm~~
https://web.archive.org/web/20161112101748/http://www.sentex.ca/~mec1995/circ/sensor3.htm

Version 1

This is the first version I built, in 2003. Click on the thumbnails for full-sized pictures:

Version 2

Ambient noise spikes (like lightning) were causing the alarm to "chirp" occasionally, so I modified the circuit slightly, by adding a "test point," so that I could easily adjust R2 to reduce the sensitivity, by adding another resistor in parallel, without opening the box.