System uses a dry-wet sensing differential amplifier.

Amplifier already used as an optical differential amplifier:
http://www.stefanides.gr/Pdf/SENSORS_93_TECHNICAL_CHAMBER_OF_GREECE_PANAGIOTIS_STEFANIDES.pdf -------

http://contest.techbriefs.com/2012/entries/electronics/2796-a-light-differential-amplifier-as-a-core-of-a-speed-measuring-system
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http://contest.techbriefs.com/2012/entries/sustainable-technologies/2631-a-diferential-light-amplifier-applicable-to-a-solar-locator-and-tracker

Here the photoconductors sensing light difference are removed and left with only their leads (two pairs corresponding to the two photosensors). One pair of leads left to sense dry air, whereas the other pair is left on the ground in the area expected to flood with leakage of water, in a situation met of a faulty water tube (water heaters, located over bathrooms -- at least in Greece -- frequently present water leakage floodings).

An alarm siren may be used, or possibly the main water supply cutoff using electrovalves. A timer may also be employed for recording the fault period.

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