Throughout technology's glorious run, the basic burglar alarm has undergone very few substantive changes. Modern-day alarms are little more than souped-up versions of their 20-year-old predecessors, which functioned on the same essential circuit-breaking principle. Namely, an open circuit is closed by an intruder, thereby triggering a signal. What happens after that, of course, can vary widely.

Nowadays, when a circuit is interrupted, the signal is sent from that circuit to some kind of control box that either triggers a piercing siren, wildly flashing lights, or a phone call to the authorities. With more sophisticated systems, all three may be done simultaneously for maximum protection. Usually, the lights and alarm alone are enough to drive away would-be burglars. Nevertheless, most alarm companies will still come out to the site of the break-in.

How Burglar Alarms Work

One of the reasons the design of the burglar alarm has endured as long as it has is its simplicity. A switch detects an interruption in the continuous flow of energy, just as it does in any basic electrical device. Throwing the switch up or down or side to side can either open or close that circuit depending on your goals. For most home security devices, it's much easier to use a closed circuit, so that break-ins (and not door or window closings) trigger the signal.

Most burglar alarms only cover the perimeter of a property, however, so once an intruder is inside, some ancillary form of protection is needed. Here, most families and businesses opt for motion detectors, which operate on a similar principle of signal or current interruption. Taken together, burglar alarms and motion detectors make for a powerful home protection tandem.