Infrared Sensor Buying AdviceInfrared motion sensors can be an important
component in home security. We now all knew that the body of
the any human being or the animal could emit the infrared
rays. Infrared radiation exists in the electromagnetic
spectrum at a wavelength that is longer than visible light. It
can not be seen but it can be detected. Objects that generate
heat also generate infrared radiation and those objects
include animals and the human body whose radiation is
strongest at a wavelength of 9.4um. Infrared in this range
will not pass through many types of material that pass visible
light such as ordinary window glass and plastic. However it
will pass through, with some attenuation, material that is
opaque to visible light such as germanium and silicon. An
unprocessed silicon wafer makes a good IR window in a
weatherproof enclosure for outdoor use. It also provides
additional filtering for light in the visible range.
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It is important to survey your home and determine where you
intend to put an infrared sensor. You can choose areas where an
intruder is likely to go. Common locations are in a hallway or
stairway. Please remember that a passive infrared motion sensor
detects heat. Eliminate spots where you must aim at heat vents,
radiators, open windows where sunlight shines through, or windows
where car headlights often shine through at night. It is suggestible
that buyers select either a hard-wired or wireless motion sensor,
which will depend on whether you have a hard-wired or wireless
security system. If you have a hard-wired system, you need to drill
holes in walls and route wires in and out of walls to the control
panel. There will be a power source for a wireless motion sensor.
Some infrared and dual technology motion sensors, however, come with
a lithium battery that supplies power.
When using a passive
infrared sensor and if you have to aim it where a heat source is
located; you can mask the sensor so that it doesn't "see" that heat
source. An infrared sensor includes a concave mirror that looks like
a fly's eye - a lot of notches on it. Each notch is pointed toward a
different direction, and each notch watches a particular area of the
room. If a heat source is within sight of the sensor, you need to
determine which notch is watching that area and mask it with
electrical tape so that the sensor will no longer see the heat
source.
The pyroelectric sensor is made of a crystalline
material that generates a surface electric charge when exposed to
heat in the form of infrared radiation. When the amount of radiation
striking the crystal changes, the amount of charge also changes and
can then be measured with a sensitive FET device built into the
sensor. The sensor elements are sensitive to radiation over a wide
range so a filter window is added to the package to limit incoming
radiation to the appropriate range which is most sensitive to human
body radiation.
Typically, the FET source terminal pin 2
connects through a pull down resistor of about 100 K to ground and
feeds into a two stage amplifier having signal conditioning
circuits. The amplifier is typically bandwidth limited to below 10Hz
to reject high frequency noise and is followed by a window
comparator that responds to both the positive and negative
transitions of the sensor output signal. A well filtered power
source of from 3 to 15 volts should be connected to the FET drain
terminal pin 1.
Buying Tips:
Infrared motion
sensors are important to every home when thinking about home
security. When buying infrared sensor, pay special attention to the
lens used for you selection cause it affects the response speed of
your sensor. Also, backup battery is crucial as you don't want your
infrared motion sensor to cease functioning if there is a blackout.
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