There are many different ways to create a motion sensor. As an example:
It is typical for shops to have a beam of light crossing the space near the doorway, and a photosensor on the other side of the room. The photosensor detects the change in the amount of light and rings a bell, when the beam breaks.
Many grocery stores have automatic door openers using an extremely simple form of radar to detect when someone passes near the doorway. The box above the doorway waits for the energy to bounce back and sends out a burst of microwave radio energy. It changes the amount of reflected energy and the time it takes for the reflection to arrive when someone moves to the field of microwave energy, and the doorway opens.
The same thing could be achieved with ultrasonic sound waves, bouncing them off a target and waiting for the echo.
All of these are what are known as active sensors. They exude energy (light, microwaves or noise ) to the environment in order to discover any sort of change that may occur.
The"movement sensing" attribute on most lights (and security systems) is a passive system that detects infrared energy. These detectors are therefore known as PIR (passive infrared) detectors or pyroelectric sensors. In order to make a sensor that could detect a human being, you want to earn the sensor sensitive to a human body's temperature. Humans radiate energy. The sensors are sensitive in the range of 8 to 12 micrometers.
The devices themselves are electronic parts not unlike a photosensor. The infrared light bumps electrons off a parasite, and these electrons can be detected and amplified to a sign.
You've probably noticed that your light is sensitive to movement, but not to a person who is standing . That is because the electronics bundle connected to the sensor is looking for a reasonably quick change in the amount of infrared energy it is seeing. When someone walks by, the amount of energy within the field of view changes rapidly and is readily detected. You don't need the sensor detecting changes that are slower, like the sidewalk.
Your movement sensing light has a wide field of view due to the lens covering the detector. Energy is a type of light, so you can focus and bend it. Like there is a 2-D array of sensors in there but it's not.
If you've got a burglar alarm with motion detectors, then you might have noticed that the movement sensors cannot"see" you once you're outdoors looking through a window. Because glass isn't very clear to infrared energy, that is. This is the cornerstone of a greenhouse. Light moves through the glass and heats up things inside the greenhouse. The glass is then opaque to the infrared energy these heated things are currently emitting, so the heat is trapped within the greenhouse. It makes sense that a motion detector sensitive to infrared energy cannot see through glass windows.
Rather than being professional thieves, the normal profile of a burglar would be that of a 17-year-old kid who's looking to quickly catch something precious and bounce, says Marcus Felson, a professor of criminal justice in Texas State University and a pioneering expert in understanding the nature of criminal actions.
"Burglaries are quickly, generally over within five minutes and often within a minute," states Felson. "A lot of the time the door isn’t even locked. If they come in and rummage, they rummage quickly and get out of there."
By better understanding when, why and how residential burglaries happen, homeowners can come up with the best strategies for deterring them. Which brings us to the issue of the day: do house or porch lights do anything to stop burglars?
Felson claims that lighting won't stop someone, although that lights have their place in a home security plan. For instance outside lights at your house's front and back are an effective way to spook a burglar, Felson says, but only if the lighting is positioned to light the intruder's face. All they will see is a blinding light, but maybe not the burglar, if it's pointed toward the neighbor's house.
