We had a security light installed for my mother at her back door which
worked fine for a couple months. Now when someone walks up to the door it
no longer comes on but when you walk back to your car then it comes on. How
do we fix this? TIA
You sure about that, Mr. Science???
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
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.
The actual detection is simple. Look at the lens on the sensor. See
the sectioning in the lens? That is a grid of different thicknesses of
plastic that pass differing amounts of IR energy. Imagine pointing a
flashlight through a picket fence. If you stand still there is a
steady state, either light or dark. As you walk along the level
switches as the light moves across the pickets. That is the same
effect as the ridges on the lens. The sensor itself only responds to
levels that change. If it sees enough changes in X miliseconds it
I usually have to crank down the sensitivity in winter (florida) and
turn it up in summer to get similar performance. I have over a dozen
motion detectors around the house. I have bought more than I can count
and Home Depot has replaced a bunch for free. Direct sunlight will
Have you ever seen IR photography? Heat doesn't have to burn you to be
there. The background heat is interupted by your flag or tree. The
same principle applies as the target moving across the grid.
If there is enough delta between air and water temperature, the heat
coming off my pool will turn on the light with the slightest breeze
just stirring up the heat. The water is still slick calm. The only
thing "moiving" is the heat.
The dog, (raccoons, possums, cats) is a warm body animal moving
across the lens field
Tell all your friends to walk backwards when they come to the door...
It may have moved a little from winds. Adjust the angle, there is
usually a sensitivity control too.
Of course if you got a tree branch growing near it, or some other
change that can affect it too.
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.