We all know the problem when you've been to the toilet for some big business. While washing your hands you think: 'just air-freshner is not enough! There should be a way to warn innocent visitors to stay away from this place for a little while.'
This biohazard warning is the solution for this global problem.
Just set the timer for the hazard you left and in time the warning scale will turn to 'safe' again.
It is an easy and cheap build. The only thing I bought for this project is a cooking timer. I mostly used trash materials and scraps.