Ghost image in POV display

Discussion in 'General Electronics Chat' started by mayank rob, Feb 11, 2014.

  1. mayank rob

    Thread Starter New Member

    Sep 18, 2012
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    I am using an integrated 8 digit 7 segment display and using controller to drive the 7 segment. using only one 32 bit port.
    7 bits- to drive the segments (a,b...g)
    8 bits- to keep switching the common cathode pin for individual digit
    and create the POV effect.
    i am refreshing the each digit at the rate of 8milisecond.
    1 millisecond for each digit.
    but the issue is i can observe a mild ghost image in the digits which i keep blank in case not used.
    (for eg:-when m displaying 2001 m not using the 4 digits before it and keeping it blank.)
    I wanted to remove this ghost image problem.
    Probable reason i can think of:-
    transient Effect of the components used,since BJT is used to drive the LED's. So even after i have switched to different digit changing the common cathode (CC),still it keeps the LED on for few microseconds for ghost image to appear (decay voltage observed in CRO_decay voltage exists for around 300usec) (because i can see the ghost digit very prominently on the display)

    I hope i was clear with my question
     
  2. Austin Clark

    Member

    Dec 28, 2011
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    Adjust your code so that you only update the output port once with each loop. Alter an internal variable as you go on instead of altering the port as you go. Hopefully that makes sense, the port shouldn't pass through unwanted states while changing different pins, it should all change immediately, like a latch.
     
  3. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    It's common with LED multiplexing. Thar hardware cannot turn the LEDs off fast enough.

    All you need to do is add 10 to 20uS delay between multiplexing where all LEDs are off.
     
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  4. Austin Clark

    Member

    Dec 28, 2011
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    I never knew that! Thanks for the info!
    I'd be surprised if the micro-controller can't switch it's pins ON/OFF fast enough, so I assume it's the LEDs themselves that can't dim fast enough. Is this correct?
     
  5. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    What does your circuit look like. LEDs should turn on/off much faster than that. You have likely some capacitance in your circuit or a floating pin that slowly drains charge to your transistor base.

    A schematic is worth a thousand theoretical scenarios.
     
  6. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    The last time I encountered this issue was on a 4 digit 7-seg display, where there were 4 NPN transistors to drive the 4 digits common cathodes.

    I believe it was the transistors being slow to turn off. Adding a small pause between digits where all the LEDs were off stopped the ghosting. Probably only needs a few uS.
     
  7. mayank rob

    Thread Starter New Member

    Sep 18, 2012
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    Thank you all
    the problem got solved when i added delay of few counts (by trial n error)
    since my controller was very fast (STM32) hence ghost images were observed
     
  8. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Glad to hear you got it fixed. Those few extra uS of turnoff are all it takes.

    And thanks for letting us know. :)
     
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