Question

I have a "motion sensing" light outside my house. When I walk past it after dark it turns on automatically, and it has a very wide field of view. Do you know how it works?

Answer

There are many different ways to create a motion sensor. For example:

• It is common for stores to have a beam of light crossing the 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.

The "motion sensing" feature on most lights (and security systems) is instead a passive system that detects infrared energy. These sensors are therefore known as PIR detectors (Passive InfraRed) 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 being. Humans, having a skin temperature of 93 or so 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.

You have probably noticed that your light 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 you can see covering the sensor. Infrared energy is a form of light, so you can focus and bend it with plastic lenses. But it is not like there is a 2-D array of sensors in there. There is a single (or sometimes 2) sensors 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.

Here are several interesting links:

• How Infrared motion detector components work
• Patent #3,958,118: Intrusion detection devices employing multiple scan zones
• Patent #4,982,176: Solar powered lighting and alarm systems activated by motion detection
• Motion Detectors
• Black Body Radiation

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