I believe that kids (and not only kids) that have their hands on electronics, wanted at least once in their lives to make an electronic keylock system for their house or room. When i was a kid, i had bought a Smart Kit Keycode lock system, and i had adapted it on the external door of my house. But this door had already the automatic-release mechanism. I only had to find which wires actuate this mechanism and hook the relay of the kit in parallel to this.
Then, i discovered that making a key-code circuit, is simple, very simple. As a matter of fact, so simple, that i designed this very simple key-code circuit, similar to the one from smart-kit but much better (mine had automatic reset operation when wrong code was entered), which took me only a few hours to design. Not to mention the microcontrollers… But why don’t i have a keycode-lock for my apartment’s door? Simple because it is expensive and needs a great mod on the door or frame.
Servo-Actuated Door Keylock Hack with capacitance touch-pad – [Link]