I didn't want to carry my keys around, so I built an RFID lock into my dorm room door.
This project contains the lock's source code and some pictures. I tried to design the project to be as easily removable and minimally invasive as possible.
The lock consists of an Arduino Uno connected to a high-torque servo motor fitted with a four bar mechanism. The mechanism fits over the lock piece of my dorm room door, so that when the servo rotates, the lock piece does as well. I 3D printed the four bar mechanism and fitting over the door lock, and used small 1/4-20 nuts and bolts as the pivots. The RFID chip is an $8 component available off of Amazon.
A video of the lock in operation:
Here are a couple of pictures:
The system as a whole:
Close-up of the RFID module:
And the four bar mechanism: