Motion Sensors, Contact Sensors, and Alarms

The Sensor Layer of Your Security System

Cameras and smart locks get most of the attention, but sensors are the backbone of a complete security system. They are the devices that detect when something happens and trigger everything else. A camera records, but a sensor tells your system to start recording, sound an alarm, send you an alert, or turn on the lights. Without sensors, your cameras are just passive observers.

There are two main categories of sensors used in home security: motion sensors and contact sensors. Each serves a different purpose, and a well-designed system uses both.

Motion Sensors: How They Work

Motion sensors detect movement in an area. The most common type for home security is the passive infrared (PIR) sensor, which detects changes in heat. When a person walks through the sensor's field of view, their body heat creates a noticeable change against the background temperature, and the sensor triggers.

Other types include:

• Microwave sensors emit microwave pulses and measure the reflection. They cover larger areas but are more expensive and can be triggered by movement behind walls or glass.
• Dual-technology sensors combine PIR and microwave. Both must trigger simultaneously for an alert, which dramatically reduces false alarms.
• Camera-based motion detection uses software to analyze video frames for changes. Most security cameras have this built in, but it is less reliable than dedicated hardware sensors.

Placing Motion Sensors Effectively

Where you put a motion sensor matters as much as which one you buy. Follow these placement guidelines:

Indoors:

• Place sensors in hallways and chokepoints that someone must pass through to move between rooms. A hallway sensor covers more ground than a room sensor because there is no way to avoid it.
• Mount them in corners at about 6 to 8 feet high, angled downward. This gives the widest coverage and prevents pets from triggering them (most PIR sensors have pet-immune settings up to 40 to 80 pounds when mounted at the correct height).
• Avoid placing them near heat sources like radiators, sunny windows, or heating vents. Rapid temperature changes near the sensor cause false alerts.

Outdoors:

• Use outdoor-rated motion sensors near entry points, garage doors, and gates.
• Point them across the path of expected approach, not directly at it. A sensor detects motion best when something moves across its field of view rather than straight toward it.
• Keep landscaping trimmed around outdoor sensors. Bushes swaying in the wind are a top cause of false alarms.

Contact Sensors: Simple and Reliable

Contact sensors are the simplest devices in your security system, and they are among the most important. A contact sensor is a two-piece device: one piece mounts on the door or window frame, and the other mounts on the door or window itself. When the two pieces separate, the sensor triggers.

There is no motion detection, no heat sensing, no software analysis. It is purely mechanical. When the door opens, you know about it. This simplicity makes contact sensors extremely reliable with virtually zero false alarms.

Where to place contact sensors:

1. Every exterior door including the garage entry door that people often forget.
2. Ground-floor windows that could be used as entry points, especially those hidden from street view.
3. Interior doors to sensitive areas like a home office, gun safe room, or medicine cabinet.
4. Garage doors using a tilt sensor variant that detects when the door raises.

Alarms and Sirens

When a sensor triggers, something needs to happen. Alarms and sirens are the loud, impossible-to-ignore response that serves two purposes: alerting you and scaring off intruders.

Most smart security hubs include a built-in siren, but you can also add standalone smart sirens for louder coverage. Here is what to consider:

• Volume: Look for sirens rated at 85 decibels or higher. For reference, 85 dB is about as loud as a food blender, and 100+ dB is painfully loud. An interior siren at 105 dB will make an intruder very uncomfortable very quickly.
• Indoor vs outdoor: An indoor siren drives intruders out. An outdoor siren alerts neighbors and draws attention. Ideally, you want both.
• Delay settings: Configure an entry delay of 30 to 60 seconds on your main door so you have time to disarm when you come home. All other sensors should trigger instantly.

Reducing False Alarms

False alarms are more than annoying. If your system cries wolf too often, you start ignoring alerts, which defeats the entire purpose. Here are practical ways to minimize them:

• Use pet-immune motion sensors if you have animals. Set the weight threshold appropriately.
• Enable dual-technology sensors in large open areas where single-technology sensors are unreliable.
• Test your sensor placement during different times of day. A sensor that works fine at noon might get false triggers at sunset when shadows shift rapidly.
• Keep sensor batteries fresh. Low batteries cause erratic behavior and phantom triggers.
• Use your system's home and away modes. Motion sensors should only arm when nobody is home. Contact sensors on doors can stay armed all the time.

With cameras on the outside, locks on the doors, and sensors filling in the gaps, your system is nearly complete. The final piece is deciding how your system should respond when something triggers, which brings us to monitoring.