How do motion sensing lights and burglar alarms work?

Two solar motion sensor lights
Motion sensor lights, like the solar light in this example, detect infrared energy. annick vanderschelden photograph / Getty Images

­Motion detection stands as a cornerstone of modern security, both for businesses and homes. From simple sensors illuminating a backyard to complex home security systems and high-security areas, motion detection has changed the way we interact with environments. They've fortified boundaries in a way that fences and walls cannot.

Motion sensors can operate in a number of ways, using microwave radio energy or passive infrared radiation, to name two common types. In this article, we'll provide simple examples of motion sensor technology you see in daily life. We'll also discuss the principles, types, and applications of motion detection, providing a comprehensive understanding of this fascinating and ever-evolving technology.


Everyday Motion Detectors

Although we often think of motion detection in covert terms, where security cameras detect objects with microwave or infrared sensors, we interact with them in much more pedestrian ways virtually everyday. For example, it is common for retail outlets to have a beam of light crossing a room near the door, and a photosensor on the other side of the room. When a customer breaks the beam, the photosensor detects the change in the amount of light and rings a bell.

Many grocery stores have automatic door openers that use a very simple form of radar to detect when someone passes near the door. The box above the door sends out a burst of microwave radio energy and waits for the reflected energy to bounce back. When a person moves into the field of microwave energy, it changes the amount of reflected energy (or the time it takes for the reflection to arrive) and the box opens the door.


Since these devices use radar, they often set off radar detectors. The same thing can be done with ultrasonic sound waves, bouncing them off a target and waiting for the echo.

Motion Sensors With Infrared Radiation

All of these are active sensors. They inject energy (light, microwaves or sound) into the environment in order to detect a change of some sort. The "motion sensing" feature on most lights (and security systems) is a passive system that detects infrared energy.

These sensors are therefore known as PIR (passive infrared) detectors or pyroelectric sensors. In order to make a sensor that can detect a human being, you need to make the sensor sensitive to the temperature of a human body. Humans, having a skin temperature of about 93 degrees F, radiate infrared energy with a wavelength between 9 and 10 micrometers. Therefore, the sensors are typically sensitive in the range of 8 to 12 micrometers.


The devices themselves are simple electronic components not unlike a photosensor. The infrared light bumps electrons off a substrate, and these electrons can be detected and amplified into a signal.

Motion Detector Limitations

You have probably noticed that a motion sensor camera is sensitive to motion, but not to a person who is standing still. That's because the electronics package attached to the sensor is looking for a fairly rapid change in the amount of infrared energy it is seeing. When a person walks by, the amount of infrared energy in the field of view changes rapidly and is easily detected. You do not want the sensor detecting slower changes, like the sidewalk cooling off at night.

Your motion sensing light has a wide field of view because of the lens covering the sensor. Infrared energy is a form of light, so you can focus and bend it with plastic lenses. But it's not like there is a 2-D array of sensors in there. There is a single sensor (or sometimes two) inside looking for changes in infrared energy.


­ If you have a burglar alarm with motion sensors, you may have noticed that the motion sensors cannot "see" you when you are outside looking through a window. That is because glass is not very transparent to infrared energy. This, by the way, is the basis of a greenhouse. Light passes through the glass into the greenhouse and heats things up inside the greenhouse.

The glass is then opaque to the infrared energy these heated things are emitting, so the heat is trapped inside the greenhouse. It makes sense that a motion detector sensitive to infrared energy cannot see through glass windows.


Common Types of Motion Detection

There are a variety of motion detection sensors, each with unique capabilities and applications. Motion sensor lighting is a popular choice for home security, activating lights when movement is detected, thereby deterring intruders and illuminating paths. On the other hand, dual technology motion sensors combine two detection methods, typically including a PIR motion sensor and a microwave sensor, to reduce false alarms and enhance accuracy.

The PIR motion sensor is particularly efficient in detecting body heat, while the microwave sensor sends out waves that reflect off moving objects. Additionally, pet immune motion sensors are engineered to disregard movements from pets, preventing unnecessary alerts. Each sensor sends specific signals based on the detected motion, which are then processed to trigger an appropriate response, whether it's turning on lights, sounding an alarm, or sending a notification.


This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.

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