I have a collection of old books and a basement to store them in. But in fact, I don't do that yet, because I had a suspicion that storing them there would not be healthy to the books. Old books and too much humidity don't go well together, especially if you want to keep the books in a pristine condition.
I could, of course, just go to the nearest builders merchant and get a cheap hygrometer for something like €7-15. But I wanted to track the humidity level over a period of at least a week, to see if factors like day/night conditions and changing weather (not to mention the tumble dryer in the next room) have any measurable effect on the level of humidity in the basement room.
Therefore I decided to build a humidity and temperature meter with logging capability from a Raspberry Pi and a few bits of electronics.