Short answer
Most home motion sensors are passive infrared detectors: they read heat moving across zones in their field of view and alert the panel when the pattern looks like a person. Pet-immune models are designed to ignore animals below a stated weight. Aim sensors across hallways, away from vents and sunlit windows, and arm them only when nobody is moving inside.
In this article
How do motion sensors detect movement?
A passive infrared, or PIR, motion sensor does not send anything out. It reads the infrared energy that warm objects give off, through a lens that divides the room into separate detection zones. When a warm body crosses from one zone to the next, the infrared level changes in sequence, and the sensor signals the alarm panel.
The lens is what makes this work. Its segments split the field of view into fan-shaped zones with gaps between them, so a person walking across the room passes in and out of several zones in a few steps. The sensor looks for that rhythm, not for heat alone, which is why a warm radiator that stays warm usually does not trigger it.
The same design explains the weak points. Someone walking straight toward the sensor stays in roughly the same zone and produces a smaller change, so detection can come later. Anything that changes temperature quickly, such as a heating vent cycling on, sunlight moving across a floor, or a space heater, can create a pattern that looks like movement. When room temperature gets close to body temperature, the contrast shrinks and detection range can drop.
What is the difference between PIR, microwave, and dual-technology sensors?
A PIR sensor reads heat. A microwave sensor sends out a low-power radio signal and measures the reflection, so it detects movement straight toward it and can sense through some materials. A dual-technology sensor combines both and alarms only when both agree, which reduces false alarms in rooms with heat sources or drafts.
Microwave detection on its own can be too sensitive for a house, because it may pick up movement through a thin wall, a window, or a door into the next room. Pairing it with PIR keeps that reach in check. Dual-technology sensors usually cost more than PIR alone and suit difficult spaces, such as an attached garage, a sunroom, or a basement with a furnace. For most living spaces, a well-placed PIR sensor is the standard choice.
How do pet-immune motion sensors work?
A pet-immune motion sensor is designed to ignore a body below a certain size, usually stated by the manufacturer as a weight limit. It does this through lens design that limits coverage near the floor and through processing that discounts small heat signatures. It works for pets on the floor, not for pets that climb.
The limits matter in practice. A cat on a bookcase or a dog bounding up the stairs puts its body in the zones meant for people, and the sensor cannot tell the difference. Pet-immune sensors also need to be mounted at the height the manufacturer specifies, since mounting higher or lower shifts the protected band. In homes with a large dog that is free at night, interior motion sensors are sometimes left out of the night arming mode, with door and window sensors carrying the load.
Where should motion sensors be placed?
Motion sensors work best where an intruder has to pass: main hallways, the foot of the stairs, and large rooms between entry points and bedrooms. Mount them at the height the manufacturer specifies, usually in a corner, aimed across the path of travel rather than down it, and away from heating vents and windows that take direct sun.
A corner mount gives the widest view of a room and sends a person across the zones rather than toward them. Avoid aiming at fireplaces, ceiling fans moving warm air, curtains near a vent, or stairs that pets use. One sensor covering a central hallway often protects more than several in rooms nobody would cross.
How do motion sensors fit into arming the system?
Motion sensors are interior sensors, so they are usually armed only when nobody should be moving inside. In away mode, perimeter contacts and motion sensors are both active. In stay mode, motion sensors are bypassed and the perimeter protects the people at home. That choice shapes where motion sensors are worth installing.
In practice, that means motion sensors rarely work alone. Door and window contacts cover the house when people are home; motion sensors add a second layer when it is empty, helping detect someone who came through an opening without a sensor. Some panels allow a night mode that keeps a downstairs motion sensor armed while the household sleeps upstairs. Our overview of how home security sensors work together shows that layering, and deciding where each sensor goes is part of planning a security system installation.