I have a humidity sensor. I would like to measure humidity in some boxes I store the filament in. However, it takes about 10 hours for the humidity to settle after resealing the box, and the battery drain of this particular instrument is pretty high. So I needed a way to switch it on and off inside the box.

A quick hack followed: I removed the button, and soldered a photodiode instead. No extra circuitry was added. It just happens to work.