Chip does not provide enough voltage for LED display

Discussion in 'The Projects Forum' started by Loonie, Jul 10, 2012.

  1. Loonie

    Thread Starter New Member

    Aug 30, 2011
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    Hi, my friend recently asked me to make a clock like LED display. Essentially, it will consist of 4 LED DIGITS like:

    [00:00]
    and you can set it to say
    [12:23] (Note that this is NOT A CLOCK, merely a display)

    I am a budding electrical engineer, and haven't really done anything like this (Year 1 College), but decided to try to figure out a way to do so.

    My idea is to use a 74HC191 counter with a button to manage the numbers for the digits. The output of the 74HC191 counter will be connected to a BCD to 7 Segment decoder, 74HC48. Then I can just connect the output from the 74HC48 chip to the LED display, and all will be solved.

    However, based on my research, the display set that he is planning to get, seems to work on higher voltages than 5V.

    • Large (58mm) 7-segment LED display
      Digits color can be either red or green.
      Type: 12101BEG
      If max: 30mA
      If peak 1/10 dutycycle, 0.1ms: 100mA
      Vf red: 7.2-8.8V
      Vf green: 8.8-10.0V

      Each segment consists of four LEDs connected in series.
    So, my problem is, the 74HC48 chip will only supply a voltage of around ~5V. Is there any way to boost the voltage. Or can I reverse the voltage using a 74HC47 chip instead (9V into the display and then rely on the 5V of the chip to counter it???). Should I just get a different LED display or is there some way I have not considered?
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    Use a transistor buffer to switch the LED segments from a higher voltage supply.
     
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  3. KJ6EAD

    Senior Member

    Apr 30, 2011
    1,425
    363
    If you don't need to be able to change the display, you can simply hard wire the segments you want on with an appropriate current limiting resistor for each to a 12V supply. You wouldn't need any additional circuitry.

    If you need to be able to preset and/or manually increment/decrement the display, consider using 4000 series CMOS components that can operate at a higher voltage.

    http://www.kpsec.freeuk.com/components/cmos.html

    For higher drive current, a darlington array such as ULN2803A might be more convenient than a lot of separate transistors.
     
    Last edited: Jul 10, 2012
  4. Loonie

    Thread Starter New Member

    Aug 30, 2011
    17
    0
    Thanks for your replies!

    @crutschow: Umm, I haven't worked with transistor buffers before, and a google search turned up complicated diagrams. Since I am working with 4 displays with 10 inputs each, doesn't it mean I need to set up 40 buffers? Or is there a chip where I can just link the output(s) of the 74HC48/47 chip and it will step up the voltages?

    @KJ6EAD: Ah, didn't know these chips exist! All the ones in my lab have been the 74 series. But I think I might be able to find the 4000 series outside.
     
  5. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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  6. KJ6EAD

    Senior Member

    Apr 30, 2011
    1,425
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    The number of drivers you need will depend on what needs to be controlled, not simply the number of leads available on the displays. For example, you may decide to hard wire the colon (not on your displays), select the color anode by a manual switch and leave the decimal points unconnected. You've shown us what display you're using but haven't defined how you're going to use it in much detail...yet.
     
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  7. Loonie

    Thread Starter New Member

    Aug 30, 2011
    17
    0
    Ah, thank you, Thumbwheel switches seem like a very simple way to implement this but they are a bit expensive.

    @KJ6EAD: I made a rough sketch of what I am thinking below. So I'll just substitute the 4000 series in place of the 74 series?. Imma go and source for parts today
     
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