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 ... – rackandboneman 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! – Mark Stewart 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. – Mark Stewart Nov 6 '19 at 14:03

It's a temperature sensor w/ wireless transmitter:


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. – user3757614 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.) – user3757614 Nov 7 '19 at 22:06
  • Great points from both - thank you – Edd Nov 8 '19 at 9:49

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