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LED Matrix — Parallax Forums

LED Matrix

FredBlaisFredBlais Posts: 370
edited 2014-09-26 19:42 in General Discussion
I built a custom LED matrix with 56 red 56 green and 56 blue LEDs for a total of 168 (12x14). I have a total of 12 cathodes and 14 anodes.
For the LEDs I have, I made some tests for optimal brightness and it's between 15-20mA. At 25mA they slowly fade and die at about 40-50mA.

My questions are :
1. what refresh time should I use if I have to alternate between 12 column continually?
2. I'm putting a current limiting resistor at each anode to protect the LEDs. This resistor is chosen so that when I am steady on one column, the current in each LEDs is about 20mA. Because I have to allocate some time between 12 columns, should I choose a lower resistor so that the current is a bit higher on each refresh?

Fred

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Comments

  • tomcrawfordtomcrawford Posts: 1,126
    edited 2014-09-26 09:50
    0. I wonder that you have observed your LEDs slowly fade and die as you increase current. One would expect (and I have observed) that LEDs get brighter as you increase current (until you burn them out and they become DEDs (dark emitting diodes)).

    1. You will have to refresh your array at about 24-30 Hz to eliminate flicker and fuse motion. Say 28 Hz. That's 36 ms per frame or 3 ms per column. Doesn't sound like much until you think 3,000 uSec or 60,000 Propellor cog instructions. Might even be able to do it in spin.

    2. Since you have the anodes of the various colors tied together, you cannot have separate resistors for each color. You will have to control the relative brightness by making the "on" times different. I think you will find the blue will have to be on longer than green which in turn will have to on longer than red to match brightness.

    -1. I assume you know about store-bought RGB arrays. They only come in 8 x 8 which may be a show-stopper for your application.
  • FredBlaisFredBlais Posts: 370
    edited 2014-09-26 14:56
    tomcrawford wrote: »
    2. Since you have the anodes of the various colors tied together, you cannot have separate resistors for each color. You will have to control the relative brightness by making the "on" times different. I think you will find the blue will have to be on longer than green which in turn will have to on longer than red to match brightness.

    Thanks for the info Tom. I use 12V on the LEDs, so 470ohm is about 20mA on the 3 colors. Rather inefficient but solve the brightness matching problem.
  • kwinnkwinn Posts: 8,697
    edited 2014-09-26 19:42
    FredBlais wrote: »
    Thanks for the info Tom. I use 12V on the LEDs, so 470ohm is about 20mA on the 3 colors. Rather inefficient but solve the brightness matching problem.

    It's not just about the forward voltage across and current through the three colors of leds, it's also about the efficiency of the individual leds and and the response of our eyes to the various wavelengths emitted, so Tom has a valid point.
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