Easiest water alarm, period.

Floats are more reliable than oxidizeable electrical contacts. Float activated relays are widely used in both industrial and home situations. We have several pumps at work that have built in floats for exactly this purpose. You just put them in a hole in the floor where water drains and give it enough space on all sides and as long as you check the mechanicals every 6 months to make sure nothing rusted or was blocked you never have to worry about it.

Set me straight ............

Sceadwian said:

Floats are more reliable than oxidizeable electrical contacts. Float activated relays are widely used in both industrial and home situations.[snip]

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Are you referring to the sensing contacts in the original circuit? They should not be subject to much humidity like a switch in a partial water filled hole.

If your float brings a magnet up to a glass enclosed reed switch, then there is no possibility of getting crud on the contacts.

Gaston said:

why not just use a float with a mechanical switch? it seems like that would be the simplest and most reliable

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I designed a detector for my brother last year using a mosfet. It was to monitor the basement sump level in the event the power went out.
One night, it was raining heavily and he heard the alarm downstairs, so he went to check on it. The float for the pump switch had stuck in the down position and the sump was nearly full.
It was $10 well spent (mostly on the case and 9v battery).
Jeff

ROLF, .01 inches? You can do better with humidity and temperature sensing in a known enviornment. physical contacts are a good backup, but no reliable enough on their own. Guess it goes to show no matter what the situation there is no single best responce, a mix of all of them are best to cover all ground.

cool..................