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Question about 7 segment displays ?

Discussion in 'Electronic Basics' started by capitano, Jul 24, 2003.

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  1. capitano

    capitano Guest

    I have noticed from datasheets of 7 seg displays that there is
    no pin for enable/disable. How then do I implement a panel
    with lets say three 7-segment displays where all the individual
    displays share the same "segments bus" but only one will be lit at
    any one time ? (please note, I am not referring to the driver/
    decoder/encoder because I implement the function of the
    driver with a PIC using the trick of lighting each led
    separately in quick succession)
  2. Switch the common anode or cathode.

  3. Kevin Kilzer

    Kevin Kilzer Guest

    To expand on that a bit...

    1. Use a transistor or FET switches for the individual commons, and
    drive the transistors from the PIC; the outputs from the PIC are
    probably not strong enough to drive all 7 segments (the number "8"),

    2. Put current limiting resistors in the 7 segment wires, not the
    common wires, otherwise the "8" will be dimmer than the "1".

    3. Make sure each digit is lit at least 30 times per second or the
    display will flicker. Four digits implies 120 digits/second, or one
    digit every 8 milliseconds.

    4. Don't worry about overdriving the LEDs; since they are only on
    briefly, they can handle more current than if they are on
    continuously, and higher current makes a brighter display.

    5. You can make the brightness adjustable by turning the digits off
    prematurely, but that complicates the software somewhat.

  4. capitano

    capitano Guest

    Thanks a lot for your reply, it has several interesting points, which I
    must admit I have not thought about. The only think I was not quite
    sure was when you said not to worry about too much current in the
    leds because in 2. you said that I should limit the current with resistors.
    I suppose what you mean is not to have too much current that will
    cause a visible difference between different numbers, but not to worry
    about the leds burning out ?
  5. There's an excellent book to look for, which covers ideas like
    this in some detail (and the not-infrequent times this comes up
    and is debated in comp.arch.embedded), is HP's Optoelectronics
    Fiber-Optics Applications Manual (2nd ed). But this is such a
    common need that you can find something on it just about
    Yes. His comment in item 2 is saying that if you use resistors
    to limit the current (and that's just one way) then putting that
    resistor in the common anode/cathode will mean that the
    brightness of the segments will be related to the number of
    active segments. And that's usually NOT desired. So, an
    alternative to that is to use separate resistors "on the other
    side," so to speak. In this way, the current though each LED
    doesn't flow through a common resistor, but its own and thus the
    brightness can be arranged to be roughly independent of the
    number of active LEDs.

  6. John Fields

    John Fields Guest

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