A.  Burglar alarms have become standard equipment in stores and other businesses, and they're becoming very common in private homes also.  If you've ever shopped for a home security system, you know there is a wide variety of options available.  These systems range from do-it-yourself kits you can pick up for $10 to sophisticated whole-house security networks that have to be installed by professionals.  Most alarm systems are actually built around the same basic design concepts.  Some of the most effective alarm systems are also the simplest.

 B.  The most basic burglar alarm is a simple electric circuit built into an entry way.  In any circuit, whether it's powering a flashlight or a computer, electricity only flows when you give it a path between two points of opposite charge.  To turn the electricity on or off, you open or close part of the circuit.  In a burglar alarm, the switch detects the act of intrusion - for example, opening a door or window.  These types of alarms are divided into two categories:   --- In a closed-circuit system, the electric circuit is closed when the door is shut.  This  means that as long as the door is closed, electricity can flow from one end of the circuit to the other.  But if someone opens the door, the circuit is opened, and electricity can't flow.  This triggers an alarm. 
--- In an open-circuit system, opening the door closes the circuit, so electricity begins to flow.  In this system, the alarm is triggered when the circuit is complete.  Closed circuits are usually better than open because an intruder can deactivate the open circuit just by cutting the connected wires.

C.  Circuit alarms are very effective for guarding the perimeter of a house, but they don't work so well inside a building.  The intruder's actions are very unpredictable - you don't know where they'll go or what they'll touch.  A specific "trigger" isn't very effective.  To detect an intruder who's already in the house, you need a motion detector.  Basic motion detectors are pretty common theses days.  The automatic door opener is an example of a radar-based motion detector.  The box above the door sends out bursts of microwave radio energy, or ultrasonic sound waves, and then waits for the reflected energy to bounce back.  If someone disturbs the reflection pattern, the motion detector sends an alarm signal to the control box.  More advanced security systems include passive infrared motion detectors.  These sensors "see" the infrared energy emitted by an intruder's body heat.  When an intruder walks into the field of view of the detector, the sensor detects a sharp increase in infrared energy. 

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