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I live in a rented apartment, and there is this little plastic box on the wall that I had been ignoring. Here is what it looks like:

enter image description here

There are no markings or holes on the casing. I got curious and opened it up, and was surprised to find a small PCB inside:

the circuit board

Since it's about halfway up the wall, I was thinking it could be a carbon monoxide detector, but those are usually pretty identifiable or at least have some holes on the side. I was also thinking it could be a wireless doorbell, but we don't have one of those, nor do I see any evidence that there was one here previously. Any ideas?

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    Just as a note about your "surprise"... Almost every small box like that will have a PCB inside it, even if the components are ones which could be hand-soldered with wires such as switches, batteries and connectors. A PCB lends itself to automated assembly, which significantly reduces cost. Even if some components have to be hand-soldered to the PCB, this still reduces the assembly time and defect rate compared to soldering wires between components, which again makes it more cost-effective.
    – Graham
    Nov 5 '19 at 11:26
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    Oh, and it can help with identification if a photo of a PCB actually leaves the writing on the components readable ... Nov 5 '19 at 16:22
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    If you don't like how cold/hot your apartment is, try turning (just a little!) one of the screws on the upper-left! Nov 5 '19 at 18:36
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    @MarkStewart For me, they look to me like terminals where you can attach two wires?
    – glglgl
    Nov 6 '19 at 10:51
  • Yeah, closer look that seems right, and given their location on the edge. Nov 6 '19 at 14:03
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It's a temperature sensor w/ wireless transmitter:

https://www.inovonics.com/products/dual-input-temperature-transmitter-en1723/

I found that by googling the various numbers printed on the stickers -- EN1723 turned the device up as the top hit.

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    As context, these are common where there's a common heat source. (Boiler in the basement or something.) This is basically one part of a thermostat system that needs to cover an entire building. Nov 5 '19 at 1:41
  • What happens if it runs out of battery? Or it's moved into a storage box under the sink? Is there some explanation on the back?
    – Edd
    Nov 7 '19 at 17:33
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    @Edd, I assume that if it runs out of battery, whatever its transmitting to will notice and either display an alert or simply stop working. If it's in a box under the sink, probably the temperature readings won't be very accurate anymore.
    – Nate S.
    Nov 7 '19 at 17:35
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    @Edd That battery probably has a lifetime measured in years. (The same battery technology is used in those 10-year sealed smoke detectors.) Nov 7 '19 at 22:06
  • Great points from both - thank you
    – Edd
    Nov 8 '19 at 9:49
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It's basically a remote thermometer which sends the room temperature to a 'heat computer', which in turn turns the heat (or a/c) on or off based on settings comparing it with the other units in the building. Turning the screws will do nothing. They are used to attach another temperature sensor for transmission. The receiver/'heat computer' will detect and display a low battery message and the sensor will show as missing if the battery is dead or the unit is removed from the building (out of range). The units run about $75 or so, be prepared to pay for replacement if you remove it and/or toss it out. If you wish to try to 'fool' the sensor readings, put an incandescent lamp or other heat source under it to show a higher temperature, it's a little harder to show a lower temperature, maybe a small ziplock bag with ice? hanging above it? Be careful not to get it wet though!

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