RFID card readers becoming passÃ©? Maybe what you need to guard the door to your high-tech lair is a secret knock detecting door lock. Using an Arduino and a bunch of parts found around the lab, Steve Hoefer built a device that unlocks your door when it receives a certain knock pattern. It works by counting the time between successive knocks, and can be re-programmed at the touch of a button.
Of course, this system is susceptible to a replay attack, because anyone can listen to the knock pattern and then know how to get in. If you are planning to use something like this, I would recommend either incorporating a timestamp into the message, or using a series of one time knocks, in order to make it harder to break into. Actually, that might make it more secure than a regular lock.