Elenco Motion Detector Kit - Manual (Page 6)

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 0.78 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 of the FET.
-5-
Figure 3
Figure 2
Infrared Rays Optical Filter
Gate
Drain
Source
Resistor
Ground
Crystals
Dual Element Detector Scheme