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Multiplexing

Discussion in 'Electronic Design' started by roxlu, Jun 1, 2006.

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  1. roxlu

    roxlu Guest

    Hi!

    I'm working on a LED display and I want to see how much LEDs I
    can ultimately drive using a 5v, pic16f628. Now, I'm using 2 shift
    registers and multiplexing to drive 64 LEDs. Now I want to extend
    the amount of LEDs I'm using.

    I'm "scanning" through the columns and only one column is on
    at a certain moment. Per column I put on the rows for which I want
    the LED to be on.

    I heard that when I want to use more than 10 columns, I must push
    more current through the LEDs. I can imagine that this must be done
    because the time when the LED is on decreases when the number of
    columns increases. And pushing more current trhough them will
    let the shine brighter, so our eyes won't see a difference....
    Is this why they need more current? And how many columns or rows
    can I use?


    Greetings
     
  2. The more leds you drive, the more area you want to illuminate.
    From a certain area up, you need to dim the lights to see it.

    Rene
     
  3. Luhan

    Luhan Guest

    You can just increase the current as you decrease the time each LED is
    on - until you reach the peak current spec of the LED. Then you fry
    them.

    Luhan
     
  4. roxlu

    roxlu Guest


    Hi Luhan,

    That technique sounds promising but I've also heard that just using
    multiplexing can
    be used to drive hundreds of leds. As I'm working to a display with at
    least 512 LEDs,
    I want to start with that.

    Gr. Roxlu
     
  5. John Fields

    John Fields Guest

    ---
    If you use more than _one_ column you must push more current through
    the LEDs, the reason being that since all of the LEDs in column 1
    will be off when column 2 is active, then to keep column 1 as bright
    as if the LEDs were being driven CW you'll need to increase the
    current into the column 1 LEDS by a factor of 2 during the time
    they're lit. The same goes for column 2. If you increase the
    number of colums to 10, then you'll need to increase the current
    through the LEDs by an order of magnitude. For 20mA LEDs that means
    you'll need to push 200mA through them during the time they're lit,
    so you can see that it won't take too many columns before you'll
    exceed the peak current allowed through the LEDs.

    I noticed from another post that you want to expand your array to
    512 LEDs. How do you plan to dimension the array?
     
  6. roxlu

    roxlu Guest

    Hi Jonh,

    Actually I'm not sure how many LEDs I want to drive exactly because I
    don't know how much I can drive with one pic16f628. But I'm going to
    try to drive 512 as my first step.. Than I want to try even more.

    Greetings Roxlu
     
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