System Diagram
Figure 1 illustrates the sensors and alerts idea. Sensors detect a condition and either close or open a switch; the switch closure triggers the processor and, depending on decisions software might make, can generate any of several alerts.
FIGURE 1: Basic security system
Basic security requires monitoring for unauthorized entry, so every potential entrance to the structure — windows and doors in most buildings — requires a sensor to detect an opening. That’s potentially a lot of switches in even a relatively small building, because buildings often have many doors and windows, so to save money it’s common to connect sensors into related groups called zones. You connect normally open switches in parallel (as in Figure 1), or normally closed switches in series. If any switch changes state, the interface detects it and notifies the processor. There’s no way to tell which switch in a zone tripped, but you have localization to the level of the zone.