A designer who goes by the name BFarn has created a knock detector lock in the form of a mechanical iris for authenticating visitors. As he documents on his site, BFarn used a combination of projects by other DIY  inventors to create his secret knock detector  for his workshop door.

    I combined Chris Schaie’s beautiful Mechanical Iris, Steve Hoefer’s Secret Knock detecting Arduino code, and an engraving made with my scratch-built CNC-Router to make the entrance to my garage/lab a bit more special.  These people all inspire the hell out of me, and I just want to feed a little bit of that back to the source.

http://www.psfk.com/2010/08/mechanical-iris-hides-a-secret-knock-detector-lock.html