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, etc. Typical panels are located in utility closets or access rooms.
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.