Texas Instruments aren't perhaps a name you'd associate with watches, but the TI MSP430 is no regular timepiece. The hack-friendly wristwatch has a 3-axis accelerometer, pressure sensor, temperature sensor and RF wireless, and in this latest DIY door security system it's used to remotely unlock a door using a sequence of taps.
Video demo after the cut
The taps are recognized by the accelerometer, and transmitted using 128-bit AES encrypted wireless to a battery powered receiver on the inside of the door. That controls a servo that physically unlocks the door. After each unlock/lock sequence, the encryption codes are junked and new ones used, so replaying the radio transmission won't be enough to bypass the security.
In fact, it's actually more secure than a regular electronic door locking system, since you not only have to know the code but have the appropriately-paired TI watch, too. The whole thing was thrown together for the TI MSP430 Ultra-low Power Challenge; if you're interested in tinkering with the watch yourself you can order one here ($49; currently out of stock however).
[via Hack a Day]