pointed at your camera (even a cellphone camera will
show this) and press the power button. With your eyes
you see nothing but as shown in Figure 3, the camera
shows a light.
Some Common Uses For IR Sensors
One of our most common electronic IR emitters is,
of course, on that blessing to couch potatoes everywhere:
the TV remote control. It broadcasts a coded IR signal to
the IR detector on the TV console that decodes the signal
and saves you the excruciating drudgery of waddling over
to the TV to change the channel.
And where would the military be without IR for target
acquisition? Fire a Sidewinder missile (named after a pit
viper) at the exhaust pipe of your enemy’s jet and say
good-bye to your little friend. Another cool way to
improve your kill skill is with night vision goggles. This way
you can be like Mr. Snake.
There are many other uses, such as on a factory
production line that senses tomato soup cans passing
on a conveyer belt. This is much more boring than blasting
enemies (or eating bunnies), but that’s the practical
example we will construct by the end of this Workshop.
IR Reflective Object Sensor:
The Arduino Projects Kit has an IR Reflective Object
Sensor (the QRD1114) that we will use for several
projects. The QRD1114 is made from an IR LED emitter,
an IR phototransistor detector, and an IR opaque shield
that holds them together. You can remove the emitter and
detector by holding the legs and pushing toward the front
of the device. If you do this, remember which way the
legs were so that you can get it reassembled properly.
Also remember to watch for protruding beyond the shield,
as this will radically affect the way the device works. The
functional concept is simple: The shielded emitter radiates
IR from the open end which can only be seen by the
shielded detector if the IR bounces off something. This is
shown in a cut-away view in Figure 4.
Making IR Visible
The QRD1114 must be calibrated to a specific
application to be used to its greatest effect. Calibration is
necessary for two main reasons: 1. The sensor detects IR
not just from the reflections of the associated emitter, but
from ambient (environmental) IR; and 2. The amount of IR
reflected back from the emitter varies with the IR reflectivity
of the reflecting object and the distance to that object.
IR Doesn’t Know Black From White
White reflects all visible light and black absorbs it. You
might think that a white area would reflect more IR back
SMILEY’S WORKSHOP ☺
■ FIGURE 2. Dog IR thermoscan.
to the sensor than a black area. I did, but it turns out that
the operative part of the definition of black and white is
‘visible.’ Some things that appear black and white to me
reflect the same amount of IR and look identical to the
IR sensor. I printed my first motor wheel encoder on
photographic paper and my detector couldn’t tell the
difference in the white and black bars.
Naturally, I assumed that the problem was something
in the software or hardware and spent a lot of time
looking for a bug. It turns out that printing the same image
on plain paper works pretty well. Apparently, the shiny
surface of the photo paper is transparent to the human
eye but reflects the incidental IR. I found that I got an
even better sensor reading if I went over the plain paper
printed black areas with a black Sharpie© ink pen. Again, I
couldn’t see much difference but the detector sure could.
Breadboard: Show Invisible IR
Intensity With Visible Red LED
You can see a drawing of the QRD1114 along with its
schematic symbol in Figure 5.
■ FIGURE 3.
■ FIGURE 4. QRD1114 cross-section.
October 2009 19