Let's be real here: everyone should be washing their hands after they use the bathroom. To help remind the forgetful types, this setup lights a sign if the sink wasn't used after the toilet was flushed.
The build comes from Make and uses an Arduino Microcontroller and some piezo sensors, which are capable of turning pressure and vibrations into a small electrical signal. Running some basic code, the system senses the water running through the toilet's pipes when it's flushed, then waits to detect water running through the sink's pipes. If the system doesn't detect the sink running water, the sign outside blinks and makes noise when the door opens.
If nothing is detected five minutes after a flush, the system resets for the next use. For instructions and a list of materials, check the link below. And wash your hands!