Some alarm systems serve a single purpose of burglary protection; combination systems provide both fire and intrusion protection. Intrusion alarm systems may also be combined with closed-circuit television surveillance (CCTV) systems to automatically record the activities of intruders, and may interface to access control systems for electrically locked doors. Systems range from small, self-contained noisemakers, to complicated, multirally systems with computer monitoring and control. It may even include two-way voice which allows communication between the panel and Monitoring station.
Devices which detect intrusions. Sensors may be placed at the perimeter of the protected area, within it, or both. Sensors can detect intruders by a variety of methods, such as monitoring doors and windows for opening, or by monitoring unoccupied interiors for motions, sound, vibration, or other disturbances.
These indicate an alarm condition. Most commonly, these are bells, sirens, and/or flashing lights. Alerting devices serve the dual purposes of warning occupants of intrusion, and potentially scaring off burglars. These devices may also be used to warn occupants of a fire or smoke condition.
In addition to the system itself, security alarms are often coupled with a monitoring service. In the event of an alarm, the premises control unit contacts a central monitoring station. Operators at the station see the signal and take appropriate action, such as contacting property owners, notifying police, or dispatching private security forces. Such signals may be transmitted via dedicated alarm circuits, telephone lines, or the internet.
The hermetically sealed reed switch is a very common type of two piece sensor that operates with an electrically conductive reed switch that is either normally open or normally closed when under the influence of a magnetic field as in the case of proximity to the second piece which contains a magnet. When the magnet is moved away from the reed switch, the reed switch either closes or opens, again based on whether or not the design is normally open or normally closed.