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Daisy chaining 2 Adafruit 8x8 LED displays for the Raspberry Pi

One our first Raspberry Pi projects was to use an Adafruit 8x8 LED display to show time and temperature.  This was a fun project...we actually learned how to generate numbers on the 8x8 by hand.  



Yesterday we had lunch with Dave and Bill from Cyntech, and Dave suggested it would be neat to enclose one or more 8x8's inside a clear Cyntech case.

Well this sounded like fun so we figured why not!

This time around I didn't want to hand generate text, and lo and behold there are a lot of great projects documented out there.  Raspihub has a great blog post documenting how to chain 3 of these together using C.  

Unfortunately we could only fit 2 of the displays in the case, and even that was a bit of a problem.  In order to get them to fit, we ended up removing the Composite Video and Headphone Jack from the a Model B.  We tend to use HDMI anyway, and we have 100's of Pi's in our shop at any given time.  The Pi did not seem to mind having these pieces removed, and it was easy to do with a pair of side cutters.  

 Solder together the 8x8's per the Adafruit tutorial, and pay attention to the part about chaining 8x8's together.  It is super easy to do, but you have to make sure to bridge together the address pins  with some solder.  When you hook it all up, you can run "i2cdetect -y 1" to see which addresses you should use.


Since we are not very good at C, we decided to modify the source code as little as possible.  It turns out that even though the code is designed for 3 matrices, you can run it with 1 or 2.  We achieve the best results when we did the following:

  • Address 1: fake address for non existent matrix, eg, 0x74
  • Address 2: address for the matrix placed to the right, eg 0x70
  • Address 3: address for the matrix placed to the left, eg 0x71

And here is the result!  We did not go into a ton of detail since others have done so in the past, but please contact us if you have any questions!

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Average Man - May 5, 2014

Nice work! I tried to play with some generic 8×8 matrix units but no matter what I tried I got a degree of light bleeding. Would love to see more detail on this – both hardware used and an idea of the code.

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