A3144 Hall-Effect Tachometer

Glue a magnet to a fan, point a Hall sensor at it, and read RPM straight off the serial monitor.

Difficulty: Breadboard. Estimated build time: about 30 minutes. Estimated parts cost: about US$2.00. 6-part bill of materials with live supplier pricing. The A3144 is a digital Hall-effect switch — a tiny TO-92 chip whose output goes from high to low when one specific pole of a magnet gets close enough.