7 Segment LEDs

Discussion in 'The Projects Forum' started by simf14, Dec 20, 2009.

  1. simf14

    Thread Starter Active Member

    Jul 27, 2007
    43
    0
    Happy Holidays and Thank you for looking.

    I have a project that is driving a 7-segment led block just fine with a maxim 7219 display chip. I need to replace the 7-segment LED block with 7 individual LEDs and thought it would be an easy swap. LED segments are just individual LED's right? Well when I swap the 7 individual leds for the one segment block all the individual led's light up even if I display a blank with the 7219. What basic concept am I missing?

    curtis
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    What is the part number of the 7-segment display you are using?

    hgmjr
     
  3. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Can you post a schematic so that we can see how you have you circuit hooked up.

    hgmjr
     
  4. simf14

    Thread Starter Active Member

    Jul 27, 2007
    43
    0
    Hgmjr, I would post the schematic but I figured it out the problem although the solution doesn't make sense.

    With one polarity the LED would stay lite, reversing the polarity of the LED made it turn on and off as expected. How could this happen? I thought a LED could only light in one direction thus the term Light Emitting Diode. !!??
     
  5. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    That makes sense in that the device you are using is intended for common-cathode type 7-segment displays. I suspected that you had put the LEDs in backwards.

    hgmjr
     
  6. simf14

    Thread Starter Active Member

    Jul 27, 2007
    43
    0
    hgmjr, exactly right, just curious how the led's could stay lite when put in backwards... shouldn't they just not light? Anyhow Merry Xmas!
    curtis
     
  7. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    The most likely explanation is that the drive for the common cathodes is pulled high when not active and the separate drives to each of the anodes is pulled low when not active. This assumption, if true, would explain the "always on" condition when the LEDs are installed backwards.

    hgmjr
     
  8. simf14

    Thread Starter Active Member

    Jul 27, 2007
    43
    0
    Thats exactly right! The project is working like a champ now. THank you so much hgmjr and have a great week!!!
     
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