If you manage to get a small lathe in your home shop, it will likely come with a dial to adjust the speed, but it may not have a tachometer to tell you if it’s actually spinning at your desired setting. Rather than accept this imprecision on his model, hacker Tony Scarpelli designed his own non-contact tachometer using an Arduino Nano.

The build is ingeniously simple, and mounts an infrared proximity sensor near gearing in the back of the lathe’s headstock. White paper is placed on this rotating surface, allowing the sensor to tell between this marker and the otherwise dark surface as it spins. Sensor pulses are recorded by the Arduino, which outputs RPM values on a small 16×2 LCD display.