Nuts & Volts - September 2005

Let’s Get Technical

by James Antonakos

Electronic Theories and Applications From A to Z

Let’s Get Technical

Making the Illusion Real

Listing 1

Figure 1. Hand-held electronic toy that creates many different optical patterns on five colored LEDs as they spin around.

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

‘ Blink five LEDs in different patterns

tick    var
rot     var
led     var
num     var
patt    var
poff    var
counts  var
pon     var
state   var

byte
word
word
word
byte[5]
byte[5]
byte[5]
byte[5]
byte[5]

Start:
‘ Initialize off/on tick counts
       for Led = 0 to 4
           counts[led] = 0
           random num
           if (Num // 10) > 5 then
               patt[Led] = 1
           else
               Patt[Led] = 0
           endif
           if patt[led] = 1 then
               random num
               poff[led] = num // 16 + 1
               random num
               pon[led] = num // 16 + 1
               low led
           else
               high led
           endif
        Next Led

‘ Begin one second of rotation
       for rot = 1 to 20

‘ Perform 128 ticks of activity
           for tick = 1 to 128

‘ Examine / adjust state of each LED
               for led = 0 to 4
                    if patt[led] = 1 then
                       counts[led] = counts[led] + 1
                       if state[led] = 0 then
                           if counts[led] = poff[led] then
                               high led
                               state[led] = 1
                               counts[led] = 0
                           endif
                       else
                           if counts[led] = pon[led] then
                               low led
                               state[led] = 0
                               counts[led] = 0
                           endif
                       endif
                    endif
               next led
           next tick
       next rot

Goto Start       ‘ Repeat forever
End

22

In my last article, I challenged myself to duplicate the interesting patterns created by five small LEDs mounted on a spinning disk (Figure 1). My three-year-old daughter does not know that I have taken her $10 toy apart to assist in my investigation, so I may end up in a miniature doghouse (or possibly even Clifford’s) if I cannot get the original device, or my creation, working.

I decided to use the PIC 16F628 microcontroller in my design. This was a challenge, since I had never before worked with a PIC, written code for the device, or tried to program one. After obtaining an EPIC programming board from Jameco Electronics, along with a batch of 16F628s, it took me about 30 minutes to get an LED flashing on a protoboard. Then I had to think about how to flash five LEDs at different rates, with the rates changing randomly.

First, I watched the original toy again, trying to get a handle on several properties: how fast the disk was spinning; how many times a single LED flashed during a single rotation; and how often the patterns changed. If I could have videotaped the toy and watched the LEDs flash in slow motion, I could learn what I needed to learn. But, instead, I simply made an estimate of what I thought was happening. These estimates were as follows: the disk looked like it was spinning at the rate of 20 revolutions per second; it looked like there were around 16 small dots in a quarter revolution when the LEDs were flashing quickly; and the patterns changed once each second.