Open your door with a special knock

I've always wanted to make a secret room in my house. Unfortunately I've had my house long enough that people would probably noticed if I somehow shifted a wall or two to accommodate the extra hidden room. But if I were to accomplish that somehow, I'd want a really cool way to get into said room.

What better way than by having a secret knock? You won't even need someone on the other side to listen for the special series of taps, you can just make your own gadget to listen for you.

Someone decided to create just such a gadget for getting into rooms that were locked. Granted, their motives might not have been the most ethical, but they did make a pretty cool device.

knock response automatic door opener [via hackaday]