LED Matrix Failure And Vindication

If necessity is the mother of invention, what’s failure the mother of? Improvement? Anyway, [prpplague]’s second version of his roll-up 70×30 RGB LED display looks a lot better and more reliable than the first, and that’s precisely due to “failing”.

Sometimes you design the hardware around the software, and sometimes vice-versa. It’s all about the balance of pain. [prpplague] initially wired the strips together in a consistently left-to-right raster arrangement to make the coding easier, but this means long wires on the backside of the fabric returning from the right side back to the start again at the left. These long wires snagged on stuff, and pulled the solder connections apart.

600px-Dotstar-adapter-solder3The fix? Alternate rows of left-to-right with right-to-left to minimize wiring and make nice, robust connectors for the ends, and a much more elegant implementation at the expense of more complicated software to drive the device. (Alternating rows have to be flipped horizontally, so this means custom driver routines.)

The second gremlin was that the interfacing board that [prpplague] was using didn’t have enough current sourcing capability on the SPI lines, and he discovered that he couldn’t communicate reliably with the strings if the first pixel was more than 24″ of wire away from the board. Once the signal got to the first pixel, though, everything was fine. [prpplague] figured out that the RGB LEDs themselves had more drive capability than the SPI source.

The solution? Add a single pixel at the front of the chain to buffer the SPI lines and serve as a bonus status indicator. Cute.

We’d hardly call these “fails”, but rather “learning experiences”. Anyway, here’s two design “mistakes” that we won’t make when making a roll-up flexible pixel display. Thanks [prpplague].

12 thoughts on “LED Matrix Failure And Vindication

    1. then i look forward to your lengthy “101 stuff that everyone should obviously know, because i already know” article.


      don’t be ‘that guy’ that just assumes because you know, everyone knows. it give the hacking community a bad name. if you want to encourage people to learn, do things to help them, not cut them down because they just found out something that you already knew..

  1. Q.
    If necessity is the mother of invention, what’s failure the mother of? Improvement?

    Indeed. The failure is the mother of learning (like in learning from mistakes) and the mother of evolution (like in replication mistakes that leads to mutations and diversity, which combined with the selection of the best fitted leads to evolution).

    1. Well really success is the driver of evolution. The less-suitable creatures only help out by failing to reproduce and then dying.

      Also I like to use “evolution by natural selection”. It’s more proper, and it takes away the Pokemon-style evolution that people tend to think of, where there’s always “advancement” and things tend towards “higher” life forms, which isn’t the case at all. Hence why cockroaches outnumber people by millions to one.

      It’s kind of sad that most of the daft Yanks who’ve gone all anti-Darwinist in the last decade or two, more than a century after Charles wrote the theory, don’t actually understand it at all. Or even the meaning of the word “theory” in the scientific sense. If I were Dawkins et al I wouldn’t dignify them with a response. Having actual educated people involved gives the idea that an argument actually exists, and that both sides have equal authority. Which really really isn’t the case.

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