After a discussion about keeping the toilet seat up or down, I decided to create a device that automatically puts down the toilet seat after use. The device determines presence

using a infrared motion detector and an ultrasound distance sensor (sonar). I programmed a rudimentary sensor fusion function to eliminate false positives from echoes in the bathroom or people showering or brushing their teeth. After detecting that the toilet has been used, the Arduino waits 10 seconds (although the timer resets if it detects motion) before pushing the seat down using a rotating arm. The sensors, motor, and Arduino are held in place using Legos from my childhood Mindstorms kit (my inspiration). Although a flush detector attached to the toilet handle would be a simpler, and perhaps more accurate method of use detection, such a system would require modifying the toilet, something I am not comfortable doing as a renter.