The secret knock detector workshop door
Check out this great knock detector build by BFarn. It was inspired by the great secret knock detecting door lock ible by Grathio
I'm extremely impressed by the design and technique's he used to make his own knock detector. I mean Grathio's detector was already awesome but this one just makes it a beautifull piece of art and also limits the area where can be knocked (which is a plus on security for me). So even when somebody accidently overhears you knock they still won't know where they should knock to get the door open!
Check out the video to see it in action and for more info on the build you can visit his website here