Inverting a Common-Cathode Common-Anode Led Display

Discussion in 'General Electronics Chat' started by tbfunk, Mar 5, 2016.

  1. tbfunk

    Thread Starter Member

    Dec 9, 2010
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    I have a common-cathode common anode 4 digit LED 7-segment display and a circuit that would power a similar LED display, however my display is the wrong polarity from my circuit. How hard would it be to invert all the signals to power said display with said circuit? I know it would be easier to buy new parts, I am simply interested in finding out if this can easily be done with transistors or something.
     
  2. #12

    Expert

    Nov 30, 2010
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    One resistor and one transistor per segment.
     
  3. Dodgydave

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    Jun 22, 2012
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    You need to swap the Pnps for Npns that drive the Common Anode,

    Usually common Anode have pnps to drive them, common Cathode uses npns, you will also have to invert the signal to the segment drives, this can be done in software.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    So which is it--common-cathode or common-anode? :confused:
    It can't be both.
     
  5. dl324

    Distinguished Member

    Mar 30, 2015
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    Could be as simple as inverting all of the signals with something like CD4069 or CD4049. Post the circuit.
     
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    You got four different responses because you left out a few critical details from your question - like, all of them. What is the device that is driving the display now? Does it have LED current limiting built in? Does it require external driver transistors for the segments? Does it require external driver transistors for the digits?

    Schematic, schematic, schematic...

    ak
     
    panic mode likes this.
  7. hp1729

    Well-Known Member

    Nov 23, 2015
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    Is this what he has?
    Converting it depends on what the rest of his circuit looks like. I would tend to say he can't EASILY. Create a new board that piggy-backs against the old board?
     
  8. ian field

    Distinguished Member

    Oct 27, 2012
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    Years ago I acquired an incomplete frequency counter. It was intended to use Beckman 7-segment neon displays which can only be common anode. All I had was common cathode LED digits and the decoder/driver chips only had a couple of mA drive capability.

    The completed instrument ended up with an extra board carrying rows of inverter/buffer transistors.
     
  9. tbfunk

    Thread Starter Member

    Dec 9, 2010
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    hp1729, that is exactly what I have for an LED display except all the diodes on my diagram would be reversed. So I guess I would say I have 4 common Anodes and 7 common cathodes to light the display.

    I was figuring I could change the polarity of all the signals with PNP and NPN transistors like #12 and DodgyDave suggested however I tried and it and I dont think I quite got it right. I thought it would be as simple as this rough schematic here.

    PS, I quickly drew this in paint lol...
     
  10. tbfunk

    Thread Starter Member

    Dec 9, 2010
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    You wouldnt happen to have the schematic for the extra board would you?
     
  11. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    "How hard" is a relative question that depends on your experience and available tools.

    So, I am guessing the answer to first question is, fairly hard.
     
  12. tbfunk

    Thread Starter Member

    Dec 9, 2010
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    Fair enough, what is the minimum amount of non-IC parts I can add to this circuit to acheive my goal of driving a simple display?

    I have a board that does many things that I am not going to take the time to provide a circuit for, 4 wires come off that and are a positive signal, 7 other wires come off this same board and are a negative signal, combined they drive a 4 digit 7 segment display. Now I tried what I thought would invert the polarities and I think I ended up inverting the signal via not gate instead of inverting the polarity of each signal which is all I want to do.
     
  13. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    @tbfunk

    Post the part number of your display. I will then tell you which of these two circuits go on the digit common (1 - 4) and which go to the segments common (a - g)


    image.jpg
     
  14. tbfunk

    Thread Starter Member

    Dec 9, 2010
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    @GopherT

    Those diagrams look like what I already tried to do... Maybe I was right, just didnt have it hooked up properly...
    Ill try them out. I already know the NPN is for the anode, though correct me if I am wrong.
    The diagram I posted above was me guessing after initially failing

    Anyway the display I intend to use I cannot find an exact datasheet for however here is all the markings:
    KINGBRIGHT
    2007-47 CC56-21SEKWAMOT V2

    Closest I could find: http://pdf1.alldatasheet.com/datasheet-pdf/view/233423/KINGBRIGHT/CC56-21SRWA.html
     
    Last edited: Mar 10, 2016
  15. Dodgydave

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    Jun 22, 2012
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    @tbfunk, do you have the circuit that drives the original led display, yes or no?
     
  16. ian field

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    Oct 27, 2012
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    #12 told you how in post 2.
     
  17. tbfunk

    Thread Starter Member

    Dec 9, 2010
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    @Dodgydave I do have the circuit that drives the original led display, I dont have a schematic for it and dont plan on taking the time to create one.
    Im going to try out what GopherT suggested as I suspect I didnt hook it up carefully the first time and thats why it failed. Anyways thanks!
     
  18. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    The "CC" tells me it is a common cathode.

    It is a small, low power display that likely needs the cathode multiplexer NPN circuit I showed but no resistor. Just connect the COLLECTOR to the common cathode pin for each of the four digits.

    For the anode pins, just use a 330 to 1k ohm resistor in series and connect directly. There is rarely a transistor used there unless you have multiple displays of the same information.
     
  19. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    I would have put the 10k as the pull-up/down and the 1k as the base resistor.
     
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