There are all kinds of locks and alarms that you can make. You can make alarms that are controlled with keys, keypads, remote controls, scanners, or anything else that you can think of. Just for fun, I designed an alarm that is controlled by resistors.

This is a basic motion-sensing alarm that detects when someone enters the area. When an intruder is detected, it activates a siren. But what makes this system unique is that it is activated and deactivated by touching a resistor to a pair of metal contacts on the housing. This makes for an interesting kind of security system.