Sensors are what an alarm actually sees. The panel and siren are useless if the sensors are the wrong type, in the wrong place or set up to cry wolf. This guide explains the main sensor types, what each can and cannot do and where to put them. It is part of our complete guide to home security alarm systems.
The sensor types at a glance
| Sensor | What it detects | Watch out for |
|---|---|---|
| Door and window contact | The door or window opening. Two parts, a magnet and a contact; the alarm triggers when they separate | Misalignment; it does not detect broken glass |
| Motion (usually infrared) | Movement in a room or hallway | Pets, foliage and balloons can cause false alarms |
| Glass-break | Breaking glass, by sound, vibration or both | Acoustic types can miss tempered or laminated glass |
| Smoke, heat and CO | Fire and carbon monoxide; see fire safety at home | Must be replaced on schedule |
| Flood and water | Water near water heaters, pipes and washers; Guardian Protection notes it alerts you but does not stop a leak | Needs a spot near the source |
Door and window contacts: the foundation
Contact sensors are the most common alarm device, according to Bob Vila. They are easy to install and can be surface-mounted or recessed. Guardian Alarm calls them the bedrock of perimeter security and says they protect the frame, while glass-break sensors protect the pane. Put one on every ground-floor door and every accessible window. Our guide on windows and sliding doors adds locks and film.
Glass-break sensors: a second layer
Bob Vila describes three types: acoustic, shock and dual-technology. ConsumerAffairs calls them a second line of defense, not a replacement for contacts, cameras or motion sensors. Because one acoustic sensor can hear several windows, it is cost-effective for rooms with large glass areas such as a sliding door wall. It may not trigger on specialty glass, so check the specifications.
Motion sensors: cover the paths
- Place one in the main hallway or room an intruder must cross, not facing a window or heating vent.
- If you have pets, use pet-immune settings and keep the sensor off the stairs they climb; see pet-friendly home security.
- Outdoor motion sensors are often linked to lights; see security lighting.
Wired or wireless sensors?
Wireless sensors are the norm for DIY systems and run on small long-life batteries, according to Bob Vila. Frontpoint uses 100 feet as a comfortable range guideline between a sensor and the panel. Replace batteries when the app warns you. For a deeper look, see wireless alarm systems.
Where to start
- Contacts on the front, back and garage doors.
- Contacts on ground-floor windows and sliders.
- A motion sensor covering the main path.
- Smoke and CO detectors on every level.
- Glass-break and flood sensors as extras.
Then install them following our DIY setup guide and decide whether you want professional monitoring.
Sensor checklist
- Every ground-floor door and accessible window has a contact.
- Large glass areas have glass-break coverage.
- Motion sensors are pet-aware and away from vents.
- Smoke and CO detectors are on every level and in sleeping areas.
- I check sensor batteries and alignment monthly.
Frequently asked questions
Do I need both contact and glass-break sensors?
Contacts are the foundation. Add glass-break sensors for rooms with lots of glass or where windows are not opened.
How many sensors do I need?
One per ground-floor entry, plus motion coverage for main paths. Count your doors and windows first.
Why does my motion sensor keep false alarming?
Pets, heating vents, sunlight and moving plants are common causes. Move it or adjust sensitivity.
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