7448 BCD to seven segment display

Discussion in 'Homework Help' started by Yami, Mar 30, 2016.

  1. Yami

    Thread Starter Member

    Jan 18, 2016
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    I've connected a 7448 to a common cathode seven segment display. Using switches for the input data. It didn't work only when I input 0V to the input of the IC does the segment lights up.
    What am I doing wrong any help would be greatly appreciated
    Thanks
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    With the 7448 you do not need resistors for the leds.
    The common cathode goes to ground, the segments go to the outputs of the 7448.
    See the datasheet for more info.

    Bertus
     
  3. Yami

    Thread Starter Member

    Jan 18, 2016
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    Thanks Bertus,
    I've removed all the resistors going from the IC to the LEDS. The segment does not still work. When I connect any input pin of the IC to the ground it works - so the switches cannot be used to work how it should. Shouldn't it be 5V at the input of the IC to make it drive.
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Can you provide a schematic how you connected everything?

    Bertus
     
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  5. Yami

    Thread Starter Member

    Jan 18, 2016
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  6. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    RBI pin turns off all output if the BCD value is zero all low (used if the segment is a leading zero for hours, you don't want 03:00, you want 3:00.

    Anyhow, either make the RBI pin low, or set the input switches to any other BCD value.
     
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  7. Yami

    Thread Starter Member

    Jan 18, 2016
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    Thanks
    The segment lights up when all the inputs are low (connected directly from the common to all the pins). When all the inputs are connected I get a "0" as expected and other numbers as expected too(depending on which pin is connected to the common and which pin is not connected). Then what is the use of the 5V connections through the switches?? and also how can I set the BCD values if I cannot use the switches. If I connected the switches to the common I can make it work but then I have to press the switches the opposite way to get the values. So to get "0" I have to push all the switches...
     
  8. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Post a photo of your circuit. The real circuit - a few photos with different angles so we can see all the wires.

    Also, make sure your switches are the normally open verses normally closed type.
     
  9. Yami

    Thread Starter Member

    Jan 18, 2016
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    The LEDS are on when the LT is connected to common so that works.
     
  10. MrChips

    Moderator

    Oct 2, 2009
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    1kΩ pull-down resistors are too high to sink the input currents of 7400 TTL gates.
    Try 100Ω instead.
     
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  11. Yami

    Thread Starter Member

    Jan 18, 2016
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    Great it has done the job!! its working perfectly. I still can't get my head around the pins LT, BI/RBO and RBI, and also how would I make a counter using J-K flip flops.
    Thanks so much everyone for the help :)
     
  12. MrChips

    Moderator

    Oct 2, 2009
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    /LT = Lamp Test is used to check that all the LED segments are working. Connect /LT to GND and the display should show 8.

    BI/RBO and RBI are used when you have multiple 7-segment displays and you want to blank leading zeros.

    BTW - connect with our good member @R!f@@ and he might be able to give you personal assistance.
     
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