Intrusion Detection System Types

Motion Sensor Systems

• Motion detection systems use photo-electric, infrared or microwave sensors to catch intruders. A photo-electric sensor projects a beam of light across an open space to a light-sensitive cell on the opposite side. If anything crosses the beam an alarm is triggered. Mirrors can also be used to cover larger areas with multiple beams of light. Infrared sensors compare body heat to the temperature of the space they're protecting and any difference sets off an alarm. Microwave sensors bounce ultrasonic frequencies off the surfaces in a room and analyze them as they return. Any change in the frequencies caused by movement in the room will sound an alarm.

Shock/Acoustic Sensor Systems

• These sensors are used to detect intruders who break windows to gain entry, but they can also be activated by pneumatic tubes when a vehicle passes over them. In window applications, shock/acoustic sensors detect the wavefront of noises or patterns of sound produced by breaking glass. An adhesive or suction cup is used to mount them on windows. In pneumatic tube applications, a rubber hose is laid across a driveway and the increased pressure of vehicles passing over it causes a diaphragm to emit a frequency that's detected by the sensor. In both cases an alarm goes off when the target frequencies are detected.

Magnetic Sensor Systems

• These intrusion detectors can be used to protect doors and windows. They have a reed switch with electrical contacts that are held open by a magnet. The magnet is attached to a door or window and the switch is embedded in the frame. When the door or window is opened, the magnet moves away from the reed switch and the electrical contacts close. This completes a circuit or sends a radio frequency to the system's control panel, which identifies the type and location of the sensor and sounds an alarm. The radio frequency version of this system is easier to install since you don't need to run wires to the control panel.