Dangerous Prototypes forum participant pppd described his door lock reminder that buzzes to remind him to lock his apartment door. It consists of an ATtiny13 with two reed switches and a buzzer.

I don’t know about you guys but I constantly catch myself wondering whether I have locked my apartment door or not. I never actually forgotten to turn the key but I did come back just to make sure a bunch of times. Usually it is when I am in a rush trying to catch a bus knowing the next one won’t be soon. Anyway, I wanted to do it for a long time, but I finally got some time this evening and I came up with a simple device that will do the job of remembering for me.

[Via Dangerous Prototypes]