Let’s Get Technical
by James Antonakos
Electronic Theories and Applications From A to Z
Let’s Get Technical
Optical Illusion and the Light Emitting Diode
My three-year-old daughter
recently became fascinated
by a small hand-held electronic toy that spins a disk round and
round very quickly (see Figure 1).
Light emitting diodes (LEDs)mounted
on the edge of the disk turn on and off
in all sorts of interesting patterns.
Those familiar with my background in
electronics will recognize that I cannot
stand idly by, watching this $10.00 toy
flash its lights at me without beginning
to think about how to do it myself.
So, I began studying it when my
daughter was not looking. The child
in me enjoys watching it anyways, so
staring at it for long periods of time
was not a problem. Well, I began to
see patterns. This was not difficult,
since the toy was designed to create
lots of different patterns. But when I
say “I began to see patterns,” I am
saying I began to see how the
patterns were being created.
Sometimes an LED would stay
on, forming a solid colored line as it
spun around. The LEDs would take
Figure 1. Hand-held electronic
toy that creates many different
optical patterns on five colored
LEDs as they spin around.
turns switching from off, to solidly
on, to pulsating, seemingly at random. The pulsating patterns looked
like slowly moving dashed lines.
The lengths of the dashes (and
thus the number of dashes present in
one rotation of the disk) varied as
well, from as few as two to more than
20. Even the rotation of the patterns
seemed to change from clockwise to
counterclockwise and back (when
viewed from above), with each LED
doing its own thing.
Okay, how are they doing that, I
wondered? After thinking about it for
a while, I decided they might be pulse-width modulating the LEDs, playing
with the duty cycle of the waveforms
turning the LEDs on and off. The
direction changes could be due to the
out-of-phase timing relationship
between the motor spinning the disk
holding the LEDs and the sequencing
waveform for the LED. At least this is
what I think — based on experience,
endless hours of physics labs as a student, and an internal hunch.
So, there will
be a little mathematics involved during the design
process, as the
rotational speed of
the DC motor is
Spinning related to the timing
Disk sequences used to
control the LEDs.
Figure 2 shows
a diagram of the
device. This drawing is based on a
Power visual inspection of
Switch the toy. Without
taking it completely apart, I can only
wonder where the
Figure 2. Diagram of the
electronic toy.
Rotation
LEDs
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NUTS & VOLTS
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Batteries
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24
electronic controller chip is located,
but my guess it that it is inside the
spinning disk. This is not a Sherlock
Holmes type of deduction. If you
look closely at the bottom of the disk
where it meets the motor shaft, you
will see a small metal wiper that
makes contact with a round metal
band on the disk shaft.
Obviously, you cannot have
wires connecting the disk to the
base of the unit; they would quickly
wrap themselves silly around the
shaft. Is the motor shaft itself being
used as a conductor to the disk?
With only one — or possibly two —
visible conductors connecting the
disk to the base, it seems unlikely
that the controller chip is located in
the base of the unit.
I imagine the controller chip
itself having at least seven pins: two
for power and ground, and five for
the five LEDs on the disk. Maybe the
controller chip is a microcontroller,
maybe it is an ASIC (Application
Specific Integrated Circuit). Does it
have intelligence just because it
enables lots of patterns?
I am awaiting the arrival of a
newly minted microcontroller development board. The light disk is
going to be the first project I develop
for it. This sure seems like a lot of
trouble to go through just to create a
pleasing optical illusion. Especially
when you can just go buy the toy for
$10.00. But when you do that, you
rob yourself of the joy of discovery,
the pride of creating something new,
and the lessons learned designing,
troubleshooting, and testing your
creation.
Next time, we will walk through
the design process and bring this
device into existence. NV
JULY 2005