A security alarm is a system designed to detect intrusion, such as unauthorized entry, into a building or other areas such as a home or school. Security alarms used in residential, commercial, industrial, and military properties protect against burglary (theft) or property damage, as well as personal protection against intruders. Security alerts in neighborhoods show a connection with diminished robbery. Car alarms likewise help protect vehicles and their contents. Prisons also use security systems for the control of inmates. Some alarm systems serve a single purpose of burglary protection; combination systems provide fire and intrusion protection. Intrusion-alarm systems are combined with closed-circuit television surveillance (CCTV) systems to record intruders' activities and interface to access control systems for electrically locked doors. There are many types of security systems. Homeowners typically have small, self-contained noisemakers. These devices can also be complicated, multirole systems with computer monitoring and control. It may even include a two-way voice which allows communication between the panel and monitoring station. Design The most basic alarm consists of at least one sensor to detect trespassers and an alerting device to indicate the intrusion. However, a typical premises security alarm employs the following components: Premises control unit (PCU), alarm control panel (ACP), or simply panel: The "brain" of the system; reads sensor inputs, tracks arm/disarm status, and signals intrusions. In a modern device, there are typically one or more computer circuit boards inside a metal enclosure. Many newer networks often use sealed plastic boxes out in the open. Some also have their control units built-in to the keypad or other human-machine interface. Sensors: Devices that detect intrusions. Sensors' locations are at the perimeter of the protected area, within it, or both. Sensors can detect intruders by different methods. For example, sensors can monitor doors and windows for openings or monitor unoccupied interiors for motions, sound, vibration, or other disturbances Alerting devices: These indicate an alarm condition. Most commonly, these are bells, sirens, also and or flashing lights. Alerting devices serve the dual purposes of warning occupants of intrusion and potentially scaring off burglars. These devices can also warn occupants of a fire or smoke condition Keypads: Small devices, typically wall-mounted, function as the human-machine interface to the system. In addition to buttons, keypads typically feature indicator lights, a small multi-character display, or both. Interconnections between components. Interconnections may consist of direct wiring to the control unit or wireless links with