The Arduino board has been the engine of many wild and wacky projects over the years, from bakery tweeting to various musical instruments. For its next trick, our man Steve Hoefer has taken his microcontroller and, along with a piezoelectric speaker, a gear reduction motor, and some PVC pipe, programmed it to listen for a preset sequence of knocks -- a secret knock, if you will -- and unlock the deadbolt upon hearing the right combination. Although this might not be the security system you want to put in place for your home, this definitely looks like a fun weekend project. Check out the read link for instructions and schematics, but not before you see the video of the thing in action. It's after the break.