7 Segment Display

Discussion in 'Homework Help' started by Celros2, Oct 17, 2012.

  1. Celros2

    Thread Starter New Member

    Oct 11, 2012
    5
    0
    Hey there!

    This is my first post so please bare with me.sing

    I'm currently working on building a 7 segment display circuit using 2 6 position DIP switches to a 4 bit adder (7483), to a 7 segment decoder (7447), then through an inverter chip (7404) then finally to my 7 Segment Display (common cathode---hence the inverters).

    My problem is, when I add up different binary nibbles I can only get full displays for the numbers 7 8 and 9

    Looking at the attached thumbnail my issues are:

    0 is missing "f"
    1 is missing "b,c" (it doesn't show up at all)
    2 is missing "g,b"
    3 is missing "g"
    4 is missing "b,c"
    5 is missing "c,a"
    6 is missing "a,c"

    As I said 7,8 and 9 are fine.

    My issue is, I have checked my wires like a thousand times, they are fairly neat and I can't understand how a number like 8 can show up (all the LED's are lit) and 1 can't. Even if there was a wire out of place, wouldn't it effect both numbers? Same thing with bent pins etc etc.


    I'm at a loss for ideas, I would appreciate your help

    Thank you
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,634
    2,342
    Hello,

    Can you post a schematic?
    Also the details of the used display.

    Bertus
     
  3. Celros2

    Thread Starter New Member

    Oct 11, 2012
    5
    0
    I can't give you my specific schematic as I don't have the software or hardware to get it to you, however I can give you the schematic I was working from if that is any assistance.
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,634
    2,342
    Hello,

    I always draw my schematics in paint, so I do not use special software.

    Have a look at this blog from Bill_Marsden:
    http://forum.allaboutcircuits.com/blog.php?b=49

    There you will find some zip files with electronics symbols.

    Bertus
     
  5. elec_mech

    Senior Member

    Nov 12, 2008
    1,513
    193
    I agree with Bertus, we can't help much without a schematic. You can do it in MS Paint, but if nothing else, you can draw it by hand and take a picture.

    Is there any voltage to the other segments? Use a meter to check. Also see if the input to the 7404 is correct - low when you expect the output to the segment to be high (ON). This will help you determine where the problem lies.

    Based on your descriptions alone, are you connecting the 7-segment display directly to the 7404? If yes, the 7404 may not be able to provide the needed current to drive the display depending on the current requirements of the display. You can use transistors, MOSFETs, or a source driver similar to a UDN2981/2 (no longer made, but alternates exist) between the 7404 and the display. Doesn't sound like this is the cause of the issue though since 8 displays okay.

    Be sure to use current-limiting resistors on the anode pins of the display.

    Since the 7447 is an open collector, you may need to use pull-up resistors on the inputs to the 7404 to prevent false triggering. This could cause the problem you are seeing.

    Can you use a 7448 in place of the 7447? This would eliminate the need for the 7404 for a CC display.

    According to the 7447 sheet I glanced at, segment "a" is off when 6 is displayed. Same is true for segment "d" for digit 9. Is this correct?
     
  6. Celros2

    Thread Starter New Member

    Oct 11, 2012
    5
    0
    I appreciate the response. I'm sorry for not responding earlier as I was in class trying to solve the problem as well as create the decoder out of basic logic gates (fun?). However, it turns out my issue was less technical and more organizational. the resistors I had running from the 7404 were shorting out against each other. A few little snips and the problem was solved. I'll try to have schematics ready next time. This is my first semester in the program so the information I need to provide for help escapes me sometimes, but I'm getting the hang of it.

    Thank you,

    Sam
     
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