All electronic alarm systems include the master control panel, at least one keypad for arming and disarming the system, a selection of sensors (entry sensors at perimeter doors and windows plus motion detectors for areas inside the home), and a warning signal such as a siren and/or strobe lights.

• The master control panel is home to the CPU (Central Processing Unit), the brains of the system. Based on information it receives from the keypad and alarm sensors, it decides when to sound the sirens or report to the central monitoring station. For hard-wired systems, the master control panel usually is a wall-mounted metal cabinet installed in a closet or the basement.
• The keypad is the system's command center. It’s where you arm and disarm the system and also where you silence a sounding alarm (usually by entering your system's code number on a keypad). Most keypads also have a panic button that you can press at any time to sound the alarm and, in monitored systems, to notify the central monitoring station of an emergency. Because you'll need to use the keypad each time you enter and exit your house, locate it just inside your most frequently used door (often the one that leads to the garage). Additional keypads in the front entry and master bedroom are conveniences you may find worth the added cost.

Sensors are essentially magnetic switches that trigger the alarm whenever a door or window is opened. One part of the switch is installed on the door or window frame; the mating part attaches to the door or the window sashes. When the unit is opened, separating the parts, the interruption in current triggers the alarm.

• Door sensors are programmed with an automatic alarm delay to give you time to enter without setting off the alarm; a buzzer on the keypad sounds to remind you to disarm the system before the siren is activated.
• Window sensors can be mounted in more than one location on the lower sash of a double-hung window, letting you keep the system armed with the window partially open. For added protection, you can install special screens that trip the alarm if the screen is cut or removed from the window frame. And to thwart intruders who attempt to circumvent the system by breaking the window glass rather than opening the window, acoustic glass-break sensors will activate the alarm when they detect the unique sound frequencies of breaking glass.
• Passive infrared motion sensors (PIRs) are electronic devices that detect body heat. A single PIR unit mounted in the corner of a room near the ceiling usually will be able to monitor the entire room. To avoid false alarms, just be sure to locate it well away from objects that rapidly change temperature, such as furnace vents, wood stoves, and sun-heated windows. And if your pets have free rein of the house, choose a "pet-immune" model.
• If you don't have a good location to mount a motion sensor -- or you don't want to install a complete security system -- consider a portable alarm.