In the Good Old Days, how burglar alarms work was simple -- the dog barked. But now, how burglar alarms work has become a multi-million dollar science in an industry that is growing rapidly and advancing every day.

We asked Home Security Store's founder and owner Stacey Winn to help us explain how burglar alarms work.

“You'll find that there's a huge variety in alarm systems, from do-it-yourself kits to whole-house wired systems. But most alarms work on the same basic principles, and even the most complex system is based on a pretty basic concept of how burglar alarms work,” says Winn.

How Burglar Alarms Work: Circuit Alarms

The most common first line of defense when it comes to how burglar alarms work is to use the circuit principle. You can have two kinds of circuit alarm: closed circuit or open circuit.

The basic concept is the same: you run an electrical circuit through a door or window. In an open circuit, the electrical current is not completed until the door or window is open, which then triggers the alarm. The only drawback to this system is that all the criminal has to do is cut the wires to prevent the circuit from being completed. In a closed circuit, the current is broken when the door or window is open, triggering the alarm. The all-important control box monitors all the doors and windows and once the circuit is broken, the control box will keep sounding the alarm till someone resets it with a predetermined code.

The control box is often placed in an out of the way place so a criminal can't easily find it and break it.

The closed-circuit method of how burglar alarms work can be used all around the perimeter of the home. Just create a circuit that will be broken when a criminal tries to enter and you have an alarm system. To make sure the alarm goes off if the window is broken rather than simply slid open; make the glass itself a circuit by running a current through it with a thin foil wire.

Even floor mats can be made into an open circuit alarm. When someone steps on the mat, the circuit is completed and an alarm sounds.

You can surround your home with a closed circuit alarm system that will sound the alarm when an intruder breaks the circuit. But once the criminal is inside, you need a whole different approach to how burglar alarms work.

How Burglar Alarms Work: Basic Motion Detectors

Circuit alarms are highly effective around the outside of your home and business because criminals have to get past the perimeter to get inside. But once they are inside, their movements are too unpredictable to have circuits everywhere inside the house. So then you have to rely on motion detectors.

You run into motion detectors all the time when you are approaching an automatically opening door that uses a radar-based motion detector. A control box above the door sends out radar waves (either microwave or ultrasonic) and then waits for those waves to be reflected back in an irregular pattern, indicating a presence is in the space. For automatic doors, that reflection detection opens the doors, while in an alarm system, it triggers the alarm.

So a motion detector works by constantly monitoring a room for irregular wave patterns. When a room is empty, the radar waves behave one way. But when motion is presented in the room, the pattern changes and the sensor detects the change to trigger the alarm.

How Burglar Alarms Work: More Advanced Motion Sensors

Other than radar-based motion detectors, there are also photo-sensor motion detectors. How burglar alarms work in this case involves beams of light. You aim the beam of light at a light sensor across a passageway in your house. When someone walks through it, the light beam is broken and the sensor triggers the alarm.

More advanced are passive infrared (PIR) motion sensors. These motion detectors see the heat given off by a body. Though the infrared energy tends to fluctuate in any given space, a PIR motion sensor only triggers the alarm when the energy rises rapidly, i.e. when a human, whose average body temperature is 98.6 degrees, enters an otherwise approximately 80-degree room.

If you have a motion sensor alarm system, there is usually delay of a few seconds on the control box to give you a chance to disengage the alarm when you enter the room.

How burglar alarms work has advanced beyond barking dogs and strung-up pots and pans. The science of how burglar alarms work applies measurements of time and space, incorporating the whole room into its security. A good alarm system would combine both circuit and motion sensor alarms, giving you two lines of defense when it comes to how burglar alarms work.