The most basic design: a simple electric circuit. Electricity only flows when you give it a path between two points of opposite charge. When you open or close a part of the circuit, you will turn the electricity off. To open or close a flashlight circuit, you use a switch. In a burglar alarm the switch will detect an act of intrusion.

Alarms are divided into two categories.

Closed Circuit System: The electric circuit is closed when the door is shut, the electricity can flow uninterrupted when the door is shut. When the door is opened, the electricity cannot flow and the alarm is triggered.

Open Circuit System: When the door is closed, the electricity begins to flow and the alarm is triggered when the circuit is completed.

Closed circuit is the best choice because an intruder can deactivate the open circuit circuit by cutting the wire.

A magnetic sensor in a closed circuit consists of a few simple components; for the most basic design, you need:

       *A battery powering a circuit.

       *A spring-driven metal switch built into a door frame.

       *A magnet embedded in the door, lined up with the switch.

       *A separately powered buzzer with a relay-driven switch.

When the door is closed, the magnet pulls the metal switch closed so the circuit is complete. The current powers the relay's electromagnet so the buzzer circuit stays open. When you move the magnet by opening the door, the spring snaps the switch back into the open position. This cuts off the current and closes the relay, sounding the alarm.

If you have this system in a window, an intruder pushes the window open, the magnet slides out of line with the switch and the buzzer is activated.

Another simple burglar alarm uses a small button as the switch. The button is embedded in the door frame, so closing the door pushes it in. When somebody opens the door, the button is released, changing the circuit and sounding the alarm. This system in not too great-as the intruder can just shut the door again to turn off the alarm. That is why most modern burglar alarms incorporate the the control box.

The control box is hooked up to one or more alarm circuits, but it also has its own power supply. It monitors the circuits and sounds the alarm when they are closed or opened (depending on the design). but once the alarm is triggered, the control box cut it off until someone enters a security code at a connected keypad. For added security, the control box is usually positioned in an out-of-the-way spot.

Using this basic concept, you can create all sorts fo alarm systems. Just imagine what a burglar might do to break into a house, and then turn that action into a circuit switch. For example id you think an intruder might come in through the window-you could make that glass a circuit by running a current through a thin line of foil wire affixed to the surface of the glass. If someone breaks the glass, the alarm is triggered.

You can also use floor mats with an open circuit design with two metal strips spaced apart. When someone steps on the mat, the pressure pushes the two metal strip together, completing the circuit.

These circuit systems are best for guarding the perimeter of a house or business.