This came out of a box of junk that had accumulated for a long time in my garage. So long a time that I’ve no recollection of what it is or where it came from.
I looked at it and thought that it was a temperature sensor because it was mounted on a bracket that could act as a heatsink. It looked similar to a CdS photocell, but light doesn’t affect it. The black part is about a half inch or 12mm on each side.
I connected it to my DMM and it initially measures over 800 ohms, so I held my soldering iron under it until the heatsink became too hot to touch. The resistance went down to 500 ohms, so I let it cool off, and it remained at a bit over 500 ohms. What’s going on here? If it was a temperature sensor, it would have gone back to about 800 ohms.
Puzzled, I pulled a strong magnet off the table and put it on the sensor, but it had no effect whatsoever. However when I was handling it, the resistance went up somewhat. I opened my mouth and breathed humid air on it, and the resistance shot up to 1200 ohms! What is puzzling to me is that normally when something gets more moisture on it, the resistance goes down, not up. In this case it was behaving just the opposite.
It puzzles me what kind of equipment would have a humidity sensor mounted on a heavy aluminum bracket that acts like a heatsink, It looks like the sensor and bracket were made to be mounted inside of something; if they were mounted so they were exposed, they would have more protection against being harmed by their surroundings.
I welcome comments on what this is and where it came from. Click on the picture to leave a comment.