In our house, we have two teenagers and 1.5 bathrooms. Since they both like to spend a very long time taking showers and getting ready, this means that much of the time my wife and I are left with only the half-bath to use. This is a problem.
We've tried several approaches in the past, including:
Asking them to set a countdown timer on their smartphone to 30min.
Placing an actual clock in the bathroom.
Turning down the hot water.
Starting the dishwasher and/or the washing machine.
Yelling, begging, etc.
Nothing has worked.
I decided that what we really needed was a door alarm, but in reverse - an alarm that sounds when the door is closed, instead of opened. To fair, the alarm should give the occupant ~30-40 minutes to do their business before sounding, and give plenty of warning that the clock is running.
Enter the Arduino!