Sick of using those old keys to get into your house? Redditor charmonkie shows off a homemade, keypad-powered deadbolt without actually modifying the door on his apartment.
The setup includes a stepper motor that turns the deadbolt from the inside, which is connected to a motor driver board and an arduino. The keypad sits on the outside of the door, with the wires travelling through the peephole on the door -- which is a genius way of making sure you don't lose your bond -- and everything's mounted in a tin on the other side. It isn't the prettiest (though you could probably make it prettier), but it's a pretty clever build, and handy if you wanted to let people in that don't have a key. Hit the link to see more.