A home alarm system depends on where its sensors sit. The control panel and the monitoring connection get most of the attention, but it is the sensors that notice when something changes. Each type is built for a different job, and putting the right one in the right place is the difference between a system that catches real problems and one that either cries wolf or misses an entry entirely.
Door contacts are usually two-part devices: a sensor mounted on the frame and a magnet on the door. When the door opens and the magnet moves away, the sensor reports it. They belong on every exterior door, including the ones you rarely use. The front door is obvious. The back door, the door from the garage into the house and any side or patio doors matter just as much, and they are often less visible from the street.
Recessed contacts are installed inside the door and frame so they are hidden, while surface-mounted contacts sit on the face of the frame and are simpler to add to an existing home. Either works if it is installed so the door closes fully and the magnet lines up with the sensor.
Think about the garage too. An overhead garage door can be protected with a contact designed for that type of door, and the house-to-garage door deserves its own contact, since a garage can be a quiet way in.
Windows that open can be protected with contacts much like doors. Ground-floor windows, and windows that can be reached from a porch roof, deck or fence, are the priority. Windows facing a side yard or backyard, where someone would be out of sight, deserve special attention.
Not every window needs a contact. Upper-story windows with no way to reach them, or fixed windows that do not open, are usually better handled by interior detection. A walk around the outside of the house, asking whether someone could reach each window and open it quietly, is a useful way to decide.
A contact only reports when a window opens. If someone breaks the glass and climbs through without opening the sash, the contact may never notice. Glass-break detectors listen for the sound of breaking glass and can cover several windows in a room. They suit rooms with large windows, sliding glass doors or a lot of glass in general.
They need to be placed with the room's acoustics in mind, and heavy curtains can affect them, so placement is a job for whoever designs the system.
Motion detectors watch interior spaces rather than entry points. They are the backup layer: if someone gets in without tripping a contact, a motion detector placed along the natural path through the house should notice. Sensible locations include the main hallway, the stairs and the room you would have to cross to reach the bedrooms or the place where valuables are kept.
Motion detectors tend to work well when someone walks across their field of view rather than straight toward them, so a corner mount looking across a room or down a hallway is common. They do not belong pointed at heat sources such as heating vents, fireplaces or a sunny window, or anywhere a pet can wander through unless the detector and its placement account for pets. If you have animals, tell the installer about their size and habits before anything goes on the wall.
Where sensors go also determines how you can use the system. Door and window contacts form the perimeter, which can stay armed while you are home at night. Motion detectors are usually armed only when the house is empty, because people moving inside would trip them. A layout with solid perimeter coverage lets you arm the system at night and sleep with protection in place, while interior detection adds a second layer when you are out.
A pet door large enough for a person, a door into an attached workshop, an attic access reached from the garage, and sliding doors onto a back patio are all easy to overlook. Sliding doors in particular deserve a contact of their own, and a window that looks painted shut may still be forced, so do not leave it off the plan without a reason.
The right number and placement of sensors depends on the house. A careful installer walks the property, asks how you live and explains each choice. Kingwood Alarm, for example, describes its residential work as tailored to each home's needs.
Whoever you work with, ask them to show you on a floor plan where each sensor will go and what it protects. If a door or window is left out, ask why. A clear answer, such as the window being unreachable or covered by a motion detector, is a good sign that the layout was thought through rather than copied from the last job.