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| A '''burglar alarm control panel''' is a wall-mounted unit where the detection devices and wiring of the alarm are ultimately connected and managed. These includes devices such as bells, ]s, door contacts, ]s, etc. Typical panels are located in utility closets or access rooms. | |||
| ==Overview== | |||
| The simplest type of burglar alarm control consists of a single ]. In this type, the sensor circuit (called the '']'' in industrial terminology) holds the relay energized. Since the path for the loop goes through a set of contacts which are normally open (when the relay is restored they are open, when the relay is energized they are closed), when the loop opens, even momentarily, the relay will drop out and stay that way. A second set of contacts on the relay, normally closed (when the relay is restored they are closed, when the relay is energized they are open) is used to operate the annunciator, usually a bell. The system is disarmed by a key-operated shunt which forces the relay to energize, and is armed by closing all traps and then by opening the shunt. While burglar alarm controls are now very elaborate, the single-relay control incorporates all the functionality of any control. These controls and a closely related dual-relay design are still widely used in stand-alone applications, powered by ]. | |||
| ===Modern Panels=== | |||
| Modern alarm controls are solid-state devices and do not use relays. The front end for the loop is usually a double op-amp comparator sensing a current flow through the protection. Because the loop resistance can be up to a couple of hundred ohms at installation, and because resistance transients can occur which are not the result of a loop violation, there is usually a fair bit of tolerance as to the limits of the current flow which are considered normal operation. The current is set by an end-of-line resistor located at the contact for the point being protected. If the loop is opened, the current will decrease to zero and the control will go into alarm. If the contact is strapped across, the current will increase (the control incorporates current limiting to prevent damage) and the control will go into alarm. | |||
| ==Summary== | |||
| Early (c.a. 1980) solid-state alarm controls used shunt switches or momentary closures on the key circuit to arm or disarm the control. Modern controls can use these arming techniques, but more frequently use a ] which sends operating information to the control. Thus, there is no point in attacking the keypad, as there is no intelligence in the keypad, it is all located in the control. Also, many controls feature integrated transmitters, using wired telephony or optionally, cellular telephony. These controls also monitor the status of the ], and can be programmed to trip if the telephone line fails (or is cut). The controls which utilize cellular telephony report either periodically or at a pseudo-random interval to the central station, and a failure to report will result in a dispatch. | |||
| High-security alarm controls use current and impedance monitoring on the premises, and may report to the central station via dedicated voice-grade or DC (obsolescent) circuit, or by means of multiple-drop AC grade transmitter (multiplex). | |||
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