A door sensor, for when my kid was sleep walking. There are various door sensors out there on the market, but they all set off a siren. I just wanted something that would alert my phone and wake me up, in case she did it in the middle of the night. Your not supposed to wake up a sleep walker, and I sure as hell didn't want a siren going off in the middle of the night.

Kinda sorta worked ok...just in time for her to stop sleep walking.