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A burglar alarm control panel is a panel that is used to control a burglar alarm.

    The simplest type of burglar alarm control consists of one relay.  In this type, the

sensor circuit (called the loop in industrial terminology) holds a 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 key operated 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 carbon cells.

    In a modern full-functional control there are two, possibly three classifications of

loop, and each classification of loop has two types. The classifications of loop are instant and delay, and day loops. The types of loops are single-circuit and double-circuit. In modern controls there may be several instant and/or day loops, but there is usually only one delay loop. Depending on the control, any loop may be single-circuit or double-circuit.

    The loop types will be discussed first.  If we refer to the prototypical single-relay

control, the current comes from the battery on the positive or high side, goes through the relay and the latching circuit, and then back to the battery on the negative or low side. Since the loop is a series circuit (all contacts are closed when the points they are protecting are secured and they are therefore wired into a series circuit) there are two places in the circuit where the loop can be inserted. The loop can be inserted on the high side, between the battery and the relay, or it may be inserted on the low side, between the relay and the return. If the loop is taken out to the protection only once, then the system is a single-loop system and the other side of the loop is simply strapped at the control. If both the high side and the low side of the loop are taken out to the protection, then the system is a double-loop system.

    The advantage of the single loop system is lower cost and lower complexity.  However,

the disadvantage of the single loop system used in our example is that by strapping across the contact, an intruder could open a hole in the protection and thereby defeat the system. The double loop system, while it is more expensive, does not have this disadvantage, as an attempt to strap across the contact can also strap across the relay, dropping it out and tripping the system.

    The three classifications of loops as mentioned above are instant, delay, and day

circuits. In our example there is only one loop, the instant loop. Not all controls have all classifications of loops.

    The instant loop is armed as soon as the control is armed, and any loop violation,

no matter how transient, will cause an immediate trip.

    The delay loop, found primarily in controls designed for use with keystations

located within the protected area, does not arm as soon as the control is armed. Instead, there is a delay of nominally 45 seconds before this loop arms, to allow the person arming the control time to clear the protected area. Also, when the protected point covered by the delay loop is opened, the control will not go into alarm for nominally 45 seconds, to allow the person entering time to go to the keystation and disarm the system.

    The day loop is a supervisory circuit for protection such as window bugs, tampers,

and window foil. If this loop is violated when the control is armed the result will be that the system will trip and the control goes into alarm. If this loop is violated when the control is not armed the result will be that a supervisory alarm, or trouble alarm will be signaled.

    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.

    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.

    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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