Instruction manual

INFRARED DETECTOR
Infrared light was first discovered back in 1801 by
W. Herschel. Infrared is a form of radiated energy
in which the wavelength is longer than the
wavelength of visible light. A wavelength can best
be understood by the physical analogy shown in
Figure 2.
If you were standing at the beach watching the
waves come in to shore, you would be able to see
the peaks of each wave as they approached. If you
could measure the distance from one peak to the
next, you would know the “Wavelength” of those
waves. We will use the eleventh letter of the Greek
alphabet “λ” (lambda) to represent the distance
between valleys to determine the length of the
wave (see Figure 2). A wavelength can be defined
as the distance between any two exactly equal
points on identically repeating waves.
What would happen if we reduced the distance between the peaks to 1/2 the original distance. Would it not be
true, the peaks would strike the shore twice as often as before? The frequency of the peaks reaching the shore
would be twice that of the longer wave. For people who like big words, we would say “Frequency is inversely
proportional to the wavelength”. In simple words, “If the wavelength goes up, the frequency goes down and if the
wavelength goes down, the frequency goes up”. The mathematics of waves applies also to the radiation of light.
It is common practice, therefore, to talk about light as lightwaves. The wavelength of infrared light ranges from
.78 micrometers (µm) to 100 (µm). A micrometer is one millionth of a meter.
Infrared can be thought of as heat radiation because the radiant energy is transformed into heat when it strikes
a solid surface. All solid bodies at a temperature above absolute zero emit thermal radiation. As a body’s
temperature rises, the shorter the resulting wavelengths become. The human body’s maximum thermal
radiation is between 9µm and 10µm in the infrared stage. Motion can be detected by special elements which
are highly sensitive in the infrared range. Such devices are called Pyroelectric Infrared Detectors.
PYROELECTRIC EFFECT
When certain materials change temperature, they produce electricity. A Pyroelectric crystal is an example of
such a material. If a Pyroelectric crystal has been at the same temperature for a period of time, there will be
no voltage across it’s electrodes. When the crystal temperature changes, a voltage is produced at the
electrodes of the crystal element. This type of crystal is used in this motion detector kit inside the infrared (IR)
detector.
INTERNAL DESIGN
The IR detector contains two crystals connected with each other in
opposite polarity and with a 1 millimeter (mm) optical spacing.
These two crystals are located behind an optical filter or lens (see
Figure 3). The output power of the crystals is very low. A special
device called the Field Effect Transistor (FET) is used to increase
the power output. The FET can be compared to water pipes as
shown in Figure 4. The center of a small section of pipe is made
of thin, flexible rubber surrounded by water from a third pipe called
the gate. When pressure (voltage) is applied to the gate, the
rubber tube closes and pinches off the flow of water (current) from
source to drain. In a similar manner, as infrared radiation is
detected, the crystals produce a voltage at the gate
Figure 2
Figure 3
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