Burglar alarm control panel
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 include devices such as bells, sirens, door contacts, motion detectors, smoke detectors, etc. Typical panels are located in utility closets or access rooms.
The simplest type of burglar alarm control consists of a single relay. In this type, the sensor circuit (called the loop 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 lead-acid batteries.
Modern alarm controls are solid-state devices, and do not use the relays that the older alarm panels used to go into alarm. They make use of relays to modulate the alarm notification device as needed. And they use a relay to seize the telephone line to communicate to the monitoring station. Most switching devices are N.C. (normally closed) circuits, so when the device is not in an alarm condition, the circuit is closed. Most alarm circuits (zones) are also set up to open or close on reading a certain resistance, usually between 1K and 5K ohms when inactive and double the value when active. This wiring system is called dual loop and allows for both alarm and anti-tamper detections to be incorporated into one circuit (anti-tamper occurs when the resistance level moves outside normal open/close values). This is the standard circuit in most modern systems.
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). 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 keypad 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 telephone line, 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.