That's cool. If you read the comments you'll see some links, one to a $15 unit on amazon and a $10 unit at Home Depot. That is, if you don't want to build it yourself. I sure enjoy a good DIY.

http://www.amazon.com/Leak-Frog-LF00.../dp/B000WMSTUO
http://www.homedepot.ca/product/water-alarm/925817