Nuts & Volts - July 2005

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

^Disksequences used to

control the LEDs.

Figure 2 shows a diagram of the device. This drawing is based on a

Power visual inspection of ^Switchthe toy. Without

taking it completely apart, I can only wonder where the

Figure 2. Diagram of the electronic toy.

Rotation

LEDs

F o r

E l e c t r o n i c s

NUTS & VOLTS E v e r y t h i n g

Batteries

DC Motor

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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.