[Noisebridge-discuss] Sanity check on LED matrix design.

John Adams jna at retina.net
Sun May 20 09:40:02 PDT 2012


On Sun, May 20, 2012 at 9:18 AM, Daniel Pitts <Daniel at coloraura.com> wrote:

>  I'm going to try to make a simple LED matrix display powered by an
> Arduino, and wanted to validate my approach with the more experienced
> people here.
>

Lag time on that chip is proportional with temperature. On a good day, at
room temp (+25C), it'll pass an input to an output at 15nS or so. Get the
chip hot and it'll get really bad.

Given the Arduino's speed and the small size of your display you could
probably just clock the entire display as a gigantic shift register. This
is what commercial LED displays like the Translux data wall do, and do
well.

Use a 74HS164, tie one pin to clock from the Arduino, and another for data.
Tie all of the reset lines together and tie that to another arduino pin.
The operation at that point is reset -> clock data through -> rinse ->
repeat.  Vary the on-durations in the bitstream to change display
brightness.

As your display gets larger and larger you'll have to increase the update
frequency of the clock. Eventually you'll exceed the clock speed you can
output from the arduino's pins and you'll have to take other approaches
like outboard memory, etc.

The beauty of the shift-register approach is that you can scale for larger
displays just by adding more LEDs and changing the clock frequency. Adding
color is stupidly easy too. Just add more shift registers.


-john
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