Knock Lock - Unlock a Door with a Secret Knock
Interesting little idea, Home-brew domestic security at it's best.
The concept should work on most single-action domestic locks, without modification to the lock.
Source: http://hackedgadgets.com/2009/11/04/kno ... ret-knock/
More Info: http://grathio.com/2009/11/secret-knock ... -lock.html