Automatic Secret Knock-Detecting Door Opener
A few secret knock detecting Arduino projects exist around the web, meaning the basic micro-controller logic for the knock sensing function has been fleshed out. With confidence that the software end would work, I set out to build a secret knock detecting door opener that would open my door if provided a secret knock.
My original plan was to interface with the electric deadbolt in my apartment door, but upon inspection the unit is upwards of $900... and I'd prefer not to take any chances messing with the internals. A bit less elegant is a mechanical system that would open the door by the door handle itself. Since the project requires no finesse or accuracy in motor control, I figured a cheap high torque servo motor flipped through a pulley would likely do the trick. An MG995 13Kg-cm high torque servo costs about $6-7 and can be modified for full rotation. I designed a custom winch system to pull the door knob open.
Because the Arduino handles all the analysis and control, the circuit is relatively simple. I've sketched it out on the right. A variable potentiometer allows the sensitivity to be adjusted and the LEDs indicate success or failure of the knock for debugging purposes.