SEM Security Systems
a division of Southeast Mobility
Motion detectors are the most widely used sensors in the
industry, they are also the sensor that is most responsible for
false alarms. For this reason it is very important that care be
taken in the location and mounting of motion detectors.
There are basically two technologies that are used in motion
detectors, Passive Infrared (PIR) and Microwave. The PIR should
be thought of as a camera of sorts. Most people have seen
commercials on television where someone shows you a picture of
a home and in that picture you can see where the heat is
escaping. Of course they have a solution for that and for a small
fee they can solve that problem for you. A PIR basically sees that
same type of picture. It sees heat signatures. You may have a
light bulb burning in the middle of a room, that is obviously a heat
source and the motion detector will detect it but as long as it's not
moving, there is no motion. When you walk across a room, your
body temperature is detected by the PIR and that heat signature
is moving so it trips the PIR.
For this reason it is very important that PIR's be positioned away
from heating and air conditioning vents as well as return vents.
Refrigerators, fire places, ice makers, and other appliances that
may generate hot or cold air currents are also possible false
alarm causes for PIR's.
Microwave motion detectors work exactly like a police radar.
They emit a signal which is bounced off of objects back to the
motion detector. A second signal is sent and if the time that it
takes the second signal to bounce back is different than the time
that it took the first signal, then something is moving. It's that
simple. Microwave detectors have their place but they also have
some draw backs. For one, microwave motion detectors can see
though walls, so if you have one pointed at the front of the house,
it may pick up a dog one the porch or a deer in the yard.
Microwave motion detectors are also more expensive than PIR's.
Then there are duel tech motions. These motions use both
microwave and PIR technologies. The big advantage here is
almost no false alarms since both the microwave and the PIR
technology must detect motion before the sensor will trip. Of
course, these are more expensive still.
One last lesson on motion detectors. Since the PIR sees a
"picture" and must see something moving in that picture, it is
much easier to see something that is moving from side to side
than to see movement from something that is moving straight
toward the motion. For that reason, PIR's need to be mounted so
that the likely intruder will be crossing it's path, not walking
straight toward it. Unless the customer insists, we don't normally
put a motion detector at the end of a hall since the intruder would
be walking toward the motion detector instead of across it. On the
other hand the microwave motion detector works just the
opposite. It will pick up something more readily that is moving
directly toward or away from it, than something that is crossing
Each technology has it's advantages and applications, give us a
call and we'll match your needs with the components that fit best.