“This is an easy-to-build timer to use while washing your hands which doesn’t involve trying to sing a song in your head. Just wave your hand in front of it and wait for the LEDs to go off.
Here are the modules I used to build it, but honestly, you could replace any of them with whatever you have in your parts box. The display could be a 7-segment display, or even a handful of LEDs instead. The proximity sensor could be replaced with an ultrasonic rangefinder, or a light-break setup but it’s best to avoid a physical switch that people would have to press before washing!
Use any Arduino or Arduino-compatible board. I used an Arduino Micro because it was small and I had one handy but anything will work.
A bit of perfboard is also useful to hold everything together. I 3D-printed a case for it but it could also be placed in a translucent food container (with a cutout for the proximity sensor).”