CNet’s Crave gadget blog has a cool story about a DIY secret door lock.
DIY’er Steve Hoefer used a piezo speaker, a tiny gear reduction motor, an Arduino, and a PVC pipe to make his secret-knock door lock.
It opens a door when it detects a secret knock. Here it is in action:
Now if only I had this when I was 10 years old and had a clubhouse in my backyard. No girls allowed!