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I recently cannibalized an old thermostat for parts, and found a strange looking component with no markings at all.

Here's a picture (since I can't figure out how to describe it):

photo of part

My first guess was that this was some sort of resistor, but I can't seem to measure any resistance across it with my multimeter...

I'm very much an electronics newbie so please excuse me if the answer is obvious :)

JohnB
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hairyhenderson
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2 Answers2

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It's a resistive hygrometer (or simply put: a humidity sensor). The resistance across the contacts varies depending on the relative humidity of the air it is suspended in.

Example from TDK

Mels
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    Can you please share the part number for this component and any manufacturer? – AKR Jul 23 '13 at 16:19
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    Thank you very much! I had no idea such a thing existed. I can confirm that this indeed works. To test it, I hooked it up to an LED and breathed on it... As I breathed, the LED lit up brighter. I'm sure I will find an excuse to use this in a project ;) – hairyhenderson Jul 23 '13 at 16:50
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    @dhenderson This could be used to automatically adjust the EQ to compensate for humidity's influence on guitar speakers. – Kaz Jul 23 '13 at 20:59
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    @AnandKumarRai: I found the image by doing a Google Images search for "resistive humidity sensor" when I was trying to confirm my answer. The original page: http://www.tdk.co.jp/tfl_e/sensor_actuator/CHS/ – Mels Jul 24 '13 at 09:57
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    Does it break if you get it wet? –  Aug 01 '13 at 17:41
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    Maybe, if by "wet" you mean "wet with anything other than pure H2O". For example, dipping it in tap water will probably leave you with harmful mineral residue in the substrate, which you may or may not be able to get rid of again. The sensor was not made for detecting liquids, it was explicitly designed to detect relative humidity of air. – Mels Aug 02 '13 at 10:32
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    Besides, if you dip it in water while using it (i.e. sending a current through it to measure the resistance), you will likely have major corrosion issues due to electrolysis. – Mels Aug 02 '13 at 10:35
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It's a Dew sensor, I've also found one in a Camcorder, This sensor was faulty and it generates an error code because the thin polymer on it is erased.

Voltage Spike
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user113486
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    Removing the question that this poster was trying to ask in the answer form doesn't leave a useful answer, but simply a weaker repetition of what was said seven years ago. This should simply be deleted. – Chris Stratton Sep 11 '20 at 13:42