“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!
Display module:
https://www.banggood.com/MAX7219-Dot-Matrix-Module-4-in-1-LED-Display-Module-p-1072083.html
Proximity sensor:
https://www.pololu.com/product/1134
Arduino:
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).”