Knock Response Automatic Door Opener

I’m at the beach this week – finally on vacation. Thanks to DD-WRT, I’ve got a handy wireless repeater to keep the beach house covered with network access. [sprite_tm]’s doing his best to make my vacation easier. He built this knock activated automatic door opener for a friend who wanted easy access to some locked, but empty flat (think british apartment) rooms. He used an ATTINY with some fets to drive the stepper (you can find those in old dot matrix printers for free) and a small transistor amped mic circuit to pick up the knocks. Knock the right code, and you’ll get let right in.