[Fileark] built an RFID entry system that uses a pretty ingenious alternative to an electronic strike plate. An electronic strike is a rather expensive hinged plate that mounts in the door frame and catches the door latch. But this system opens a set of double doors. The door without the handle is fixed in place and has a normal strike plate. But it also has a deadbolt mounted in line with that plate. When the deadbolt is extended it is flush with the strike plate, pushing the latch from the door knob back and freeing the door to swing open. This is a bit hard to put into words so watch the video after the break to clear things up.
The system uses a cheap RFID package that provides a single signal line. This line connects to an old VCR motor which turns the deadbolt. Timing is provided by a 555 chip, and the deadbolt movement is limited by a couple of switches mounted along with the motor.
Now that the unlocking mechanism has been built it would be simple to use other authentication methods for unlocking the door, like a wristwatch-based proximity system.