How Motion Sensors Work in Burglar Alarm Systems | Queen

The motion sensor is the most important element in a burglar alarm. We explain how it works and how it tells natural movement from intrusion.

How it works

A PIR sensor detects the infrared radiation from a human body; when a person moves within its range it detects the change and triggers the alarm.

• Detects heat and movement
• Defined range and coverage angle
• Adjustable sensitivity

Reducing false alarms

Modern systems ignore small pets and adjust sensitivity to reduce false alarms, along with correct angle installation.

• Pet-immunity feature
• Installed at the right angle
• Sensitivity tuned to the place

FAQ

Does it false-alarm with pets?

Good systems have a pet-immunity feature to reduce false alarms.

How a motion sensor actually works

A PIR motion sensor does not detect movement — it detects a change in heat across its field of view. That single fact explains everything the device does, including its false alarms. It catches a warm body moving (a person, an animal) and ignores movement of anything at room temperature.

Why false alarms happen

Air-conditioning airflow across the sensor is a sudden heat change — so it alarms. Direct sun through a facing window does the same. Cats and dogs trip it unless a pet-immune sensor is used and aimed away from the floor. Insects entering the sensor body pass directly across the lens. Every one of these is prevented by mounting position, not by a more expensive device.

Coverage direction

A PIR catches movement crossing in front of it far better than movement coming straight toward it. So mounting it so an intruder cuts across its field, rather than walking directly at it, gives a faster, more reliable trigger.

The main types, and where each fits

Choosing burglar alarm systems starts with knowing the types rather than the brands. Each type was designed for a different situation, and matching the type to the situation matters more than the price. The wrong type in the wrong place is behind most systems that disappoint — not weak hardware.

Before you decide

Ask for a survey before a price — a number given over the phone is a guess that changes on site. For this system in particular, ask about partitions so you can arm part of the property and not the rest, and about how false alarms will be prevented at installation. And ask what happens the day something fails: who responds, how fast, and whether the handover left you the documentation to let anyone service the system.