Want to check my 7 seg. LED schematic?

Discussion in 'The Projects Forum' started by tracecom, Jul 25, 2012.

  1. tracecom

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  2. takao21203

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    1. Connect /MR + /OE directly. No need for "pullup resistors.

    2. 74HC595 don't need any resistors for LED 7 SEG. They have equivalent of ~150 Ohms built in.

    3. Why not use multiplex? If you use 6-phase, you only need 2x shift registers. 1x for the 7SEG data, 1X for the sink drive. For the sink drive also digital MOSFETs are appreciate.

    4. You don't need a ceramic capacitor, or multiple one's. However one electrolytic cap., about 10uF etc. can make sense. Depends on the data rate= cable length.

    I have used these chips many times for multiplexed displays, with absolutely no resistors. I have never seen ANY failure, burnout, or abnormal heat dev., not even at full 5 volts.
     
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  3. tracecom

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    I have started laying out the PCB, and I am trying to keep the overall size to 3.8" wide and 3.0" high. Even double sided, it is challenging because of the large number of traces. Will .01" widths be sufficient for the signal tracks and .02" for the main power and ground tracks be sufficient?

    Thanks again for looking and helping me spot any errors.
     
    Last edited: Jul 26, 2012
  4. Rbeckett

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    Sep 3, 2010
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    Takeo,
    Do you have a schematic for the multiplex circuit? Tracecoms circuit looks ok, but I would like to see your alternative.
    Thanks!!!
    Wheelchair Bob
     
  5. takao21203

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    Yes I have some variants on my computer.

    Here a 4-digit LED display is used which is already multiplexed.
    12-pin. It is a NFD-3641

    The DIL switch simply is meant to provide PCB space for a LED bargraph.

    The LCX244 is used for sink drive!

    I have run such a circuit from 4 volts for quite a long while (months).

    On the PCB, 74hc595 + LCX244 = SOIC (1.27mm SMD). Capacitor I added manually 1x 1uF just to feel more safe.
     
  6. Markd77

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    Here's another multiplexed one:
    [​IMG]
     
  7. MMcLaren

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    Hi tracecom,

    Can you provide a link to the datasheet for the displays you're using, please?

    Cheerful regards, Mike
     
  8. tracecom

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    See attachment.
     
  9. MMcLaren

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    How do plan to layout the displays, please? All six side-by-side, or, two by two by two, like a clock display?
     
  10. tracecom

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  11. MMcLaren

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    That's a pretty nice layout.

    Check out a couple designs using a MAX7219 by Raj Bhatt on his Embedded Lab blog. I never really considered this SPI 8-digit driver chip because it's something like $12 (USD) in single quantities at Mouser but you can find them for about 55 cents a piece on eBay. Anyway, Raj designed a four digit board and an eight digit board that look very nice.

    Cheerful regards, Mike

    [​IMG]
     
  12. tracecom

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    Last edited: Aug 2, 2012
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