In late 2022 I noticed a weird behavior in one of the temperature sensors in a HomeAssistant home automation system I run where I live. The sensor is a AHT20 temperature and humidity sensor (this Adafruit one in particular) hooked up to an ESP32 microcontroller board running Tasmota - a pretty standard setup a little home automation board.

The strange behavior I noticed was an occasional large spike in the temperature readings. When the room temperature was around 65°F (this was in a drafty old room in the US in December), it would suddenly increase up to about 90°F, stay there for a little while, and then return to the same regular temperatures:

Very strange! It wasn't a huge problem because I don't have any systems that rely on that temperature measurement, but it was a weird thing to see on my temperature plots and also made it a little harder to see the regular daily temperature variations I'd expect to see for that room.

Given that I mostly wired up this sensor board by self, it was easy to assume I had a shoddy connection somewhere or an unreliable power supply. There was a corresponding jump (down) in humidity at the same time as these temperature spikes, so some kind shared failure mode (like a loose wire or a faulty voltage source) seemed likely.

I poked around a little bit, but never found a satisfying explanation of this temperature spike, despite it happening a handful of times per week. At the end of the day, it was more of an inconvenience than a problem so I mostly just left it alone.

Finally, on December 18th, I figured out what the issue was. While working from home that day, I happened to look over at the sensor and saw it as bathed in the low December light!

Immediately the temperature spike issue became clear:

• The temperature reading increases when sunshine hits the temperature sensor board and warms it it
• This appears as as spike because the sensor is only in direct sun for a short period dictated by the position of the sun, my windows, and any books that might also block the sun
• I didn't initially see this issue at all because when I was first using this temperature sensor in the summer and fall, the sun was high enough in the sky that the sensor could never be directly illuminated by this south-facing window!

Easy fix: I moved the sensor so it is now behind the TV and permanently shaded and the temperature spikes were going for good!