We've all walked into a bathroom only to be greeted by unpleasant aromas. Instructables user kenyer built a bathroom timer that can warn unknowing souls before they enter.
Photo by kenyer
Using a basic kitchen timer and a few plastic containers, kenyer constructed the threat level indicator. The further you turn dial, the higher the threat level. As the timer sets back, the threat level slowly goes back to a normal "Safe to Enter" state.
After you've put it all together, mount it somewhere visible outside the bathroom and revel in the knowledge that you've saved many noses. Remember you can also use some air freshener, light a match, or turn your toilet paper into an automatic freshener to battle the worst of smells. Check the link below for the complete guide.
Biohazard toilet warning [Instructables]