Sprite Mods sent in a fantastic Knock to Open Door hack that they have just come up with. It is a door that will unlock when the correct knock pattern is performed. Funny thing is I had a similar idea a few years ago except my idea was to use the door bell as an input device and use bi-color LEDs mounted inside the door bell as indicators to allow the user to enter in a code.

Video after the jump.

“After some brainstorming, we came up with a James Bond-worthy concept: knock on the door in a certain fashion, and the door’ll open automatically. The idea was perfect: no need to drill holes, sneaky enough so the landlord wouldn’t accidentally trigger it, easy enough to remember.

The complete device gets its power from a laptop PSU. The power-supply is riddled with capacitors: we’re switching coils here, so there will be lots of EMF introduced on the power-supply-lines and we don’t want that messing with our CPU. In the middle, the brains of the thing: an ATTiny2313. The ATTiny drives four BUZ11 MOSFETs, who in turn switch the bipolar stepper motor which turns the knob. While not drawn, the BUZ11s have integrated ESD-protection diodes to protect form EMF backlash by the motors coils.

To convince the device to turn its motor, two ways are provided: first of all, a set of buttons for testing purposes. Secondly, there’s the analogue stuff right from the microcontroller, which basically is an elektret microphone plus a really crude and hacked pre-amp. It took some fiddling to get the resistor values right, but when I drew the schematic of what I had built, that turned out to be no surprise: as you can see in the schematic, T2 is upside-down, with its collector and emitter switched. ”