What's inside this liquid sensor?

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#1
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Hi,

I've recently been playing around with one of these sensors for a home project, and just wondering how it works internally. (The linked pages mention a reed switch and capacitor, but I'm no clearer to understanding the internal circuit).

http://www.rapidonline.com/Electronic-Components/Sensors/Float-Switches/Liquid-sensor/66470/kw/water+sensor
http://www.tanealarm.com/products/accessories/tane_ws_1.asp
http://www.reuk.co.uk/Understanding-Liquid-Sensors.htm

Providing the contacts are immersed (or shorted) it appears to conduct at any input voltage from around 0.6V upwards. Output is roughly equal to 90% of the input less that initial 0.6V (diode drop?). One terminal is electrically connected to the positive wire, but the other terminal appears isolated whatever the condition.

Cheers,
Jon

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A PNP transistor, protection diode in series, and capacitor at base (according to the last link) with sensor terminals - the one that behaves as insulated, plus positive common. Probably some protection diodes alike on input terminals, too.
At least such a design shall work as described imho.
Probably such a circuitry is well suited to alarm or warning system.

P.S. personally I would crash it. Just to be sure.

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I suppose it measures the resistance between its sampling leads. When the resistance is lower than threshold, it triggers the alarm. The water we are afraid of (flood, leakage) has lower resistance than moist air. So a pure H20 (or demineralized) should not trigger it.
Try that and condense a spoon of pure H20.

No RSTDISBL, no fun!

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Thanks for the suggestions. It is dependant on the resistance between the terminals, as adding salt to the tap water gives a slightly higher output voltage.

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If it is using resistance between the probes, then it would not trigger when submerged in a non conductive liquid like mineral oil.