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.
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.
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
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 (or sometimes two) 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.
3 comments:
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