How do I design the circuit for a led screen made up of leds?

Discussion in 'Homework Help' started by Ernesto Monge, Sep 5, 2017.

  1. Ernesto Monge

    Thread Starter New Member

    May 17, 2017
    14
    0
    Im trying to make a controlable 310x488mm led screen made up of many 8x8 led matrices. The controller for this project willbe a rasberry pi. I need to be able to put simple geometric shapes on the screen since it will be used to teach kids with down syndrome. It will also have a touch panel on top of screen for interactive learning but for now i need to know what is the best way to go about the circuit design. I was considering using a whole lot of demultiplexers ,arranging them on x and y to select the desired led that will turn on. The rasberry pi doesnt have enough pins to control all the multiplexers .So will i have to use another set demuxes to control the demuxes controlling the leds? Or is there a simpler way?
     
  2. ScottWang

    Moderator

    Aug 23, 2012
    6,293
    973
  3. WBahn

    Moderator

    Mar 31, 2012
    22,990
    6,884
    They physical dimensions are very important at this point. What are the dimensions in terms of pixels?

    What kind of refresh rate do you need?

    Is this a school project?
     
  4. BobTPH

    Senior Member

    Jun 5, 2013
    1,190
    239
    I would use LED strips with the adressable WS2812 chips. You can then control all of them woth one pin.

    How many pixels and how quickly must they update?

    Bob
     
  5. philba

    Active Member

    Aug 17, 2017
    959
    302
    You sure about those dimensions? 310 isn't divisible by 8.

    ws2812 - that would be about 2 FPS if a string that long could be supported. I shudder to think how much current that would draw.

    HC595 could do a lot better but would still need the screen to be subdivided into say 10 groups of 31 lines of 488. Each would be driven by a micro controller. A master controller would hand each group controller the line data. For a monochrome screen, you would only need 18,910 HC595s and 2363.75 8x8 LED matrices. Triple that if you want RGB. The good news is you can get those parts for really low prices at that volume. The bad news, that's about $17,000 for mono.
     
    Last edited: Sep 5, 2017
  6. WBahn

    Moderator

    Mar 31, 2012
    22,990
    6,884
    He didn't say that it was 310 LEDs in that dimension, he said that it was 310 mm x 488 mm.

    Without knowing the pixel pitch in each dimension, we can't tell what the display format is. That's why Bob and I both asked the TS to provide more information.
     
    philba likes this.
  7. philba

    Active Member

    Aug 17, 2017
    959
    302
    hehe, you're right, I just saw 310x488.
     
  8. Ernesto Monge

    Thread Starter New Member

    May 17, 2017
    14
    0
    Its a project for a learning center for kids diagnosed with down syndrome.
     
  9. WBahn

    Moderator

    Mar 31, 2012
    22,990
    6,884
    But is it a project for which you are receiving any kind of credit for school?
     
  10. Ernesto Monge

    Thread Starter New Member

    May 17, 2017
    14
    0
    Yes if it is completed i will recieve credit for it later on.
     
  11. WBahn

    Moderator

    Mar 31, 2012
    22,990
    6,884
    Thank you. Since it is a project for credit, the expectation is that you will do the bulk of the work since the primary objective is for you to maximum what you learn from it. Our role is to look over your work and help you get past specific hurdles by giving hints and suggestions or pointing out things you might be overlooking.

    Now, a couple of people have asked how large your array is in terms of how many LEDs by how many LEDs. The physical dimensions of the display really don't tell us anything.

    We've also asked how frequently you need to be able to update the display.

    These are not the only two factors that drive major decisions regarding your project, but they are two of the biggest.
     
  12. Ernesto Monge

    Thread Starter New Member

    May 17, 2017
    14
    0
    112x72 is what i will use in leds ,and as far refresh goes i guess 2 or 3 fps
     
    Last edited: Sep 6, 2017
  13. WBahn

    Moderator

    Mar 31, 2012
    22,990
    6,884
    Since you are using 8x8 modules, you are looking at 14 x 9 modules.

    Do you have enough data lines to provide 14 bits of data plus a few control signals?

    What is the range of values for each LED? Just on/off? Just one color?
     
  14. Ernesto Monge

    Thread Starter New Member

    May 17, 2017
    14
    0
    Its just gonna be on/off ,RGB is gonna be too expensive ,I do have enough data lines for 14 bits + control signals
     
  15. Ernesto Monge

    Thread Starter New Member

    May 17, 2017
    14
    0
    I have 17 input/output pins
     
  16. WBahn

    Moderator

    Mar 31, 2012
    22,990
    6,884
    Thanks for answering 1 of the 4 questions I asked. How about the others. Please don't make this like pulling teeth.
     
  17. Ernesto Monge

    Thread Starter New Member

    May 17, 2017
    14
    0
    0-3.3v ,one color, just on and off ,i have 17 data lines, I answered your questions up there
     
  18. WBahn

    Moderator

    Mar 31, 2012
    22,990
    6,884
    Ah, my apologies. The link took me to your reply to my post and I didn't realize there was one further up with the information.

    How many of the 17 data lines can you devote to driving the display output? All 17, or do you need to do other things, as well?

    You know how many LEDs you have in the display. You know how many bits of information are needed per LED. You know how frequently you need to update the display. Given all of that, what does your data throughput need to be (bits/second) in order to keep up with the display goals?
     
  19. Ernesto Monge

    Thread Starter New Member

    May 17, 2017
    14
    0
    I can devote 12 of those lines ,I dont know how to answer your last question,I need to keep a geometric shape on the display until the child traces over it ,not much activity is going to be going on the display ,it will also have a simple game selection menu.
     
  20. WBahn

    Moderator

    Mar 31, 2012
    22,990
    6,884
    How many LEDs are in your display? ___________

    How many bits of data need to be sent to each LED per frame? __________

    From these two numbers, how many bits of data need to be sent to the display per frame? __________

    How many frames are there per second? ______________

    From these last two numbers, how many bits do you need to send each second? ___________

    Answer it as best you can.
     
Loading...