Driving multiple led from cmos output

Discussion in 'The Projects Forum' started by thunderace7, May 7, 2016.

  1. thunderace7

    Thread Starter New Member

    May 7, 2016
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    I'm building an led sequencer from 13 cascaded cd4017s driven by a 555 timer. The outputs will go to a bank of pins that will take flying sockets from the led display boards. In this way I can change the sequencing of the displays by moving the socket connections along the pin bank. As I will be using multiple displays of rgb leds each output could be driving up to 12 uln2803 inputs. I am proposing to use bc548 transistors as emitter followers to interface between the cd4017s and the uln2804s. Does this sound feasible? I was not intending to have Base resistors and rely on the uln28034s to provide the load.
    I thought of using tc4427 line drivers but they would be too expensive as I would need more than 50 of them.
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Schematic please.

    Just the driver section will cover it. Otherwise we have to guess your specifics.
     
  3. dannyf

    Well-Known Member

    Sep 13, 2015
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    Take a look at the datasheets and see how much cd4017 can deliver and how much uln2804 takes in order to drive your leds.
     
  4. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Should work.
     
  5. thunderace7

    Thread Starter New Member

    May 7, 2016
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    This is where I get confused. The datasheet for the CD4017 states that output current is 1mA but the general consensus from the web seems to be that it will limit the current and 10-15mA is an acceptable value.
    The ULN2804 takes 1-1.5mA at 12 volts.
    I was going to run it at 12 volts but if I reduce this to 9 volts and the above is correct it would seem that up to 12 ULN2804s could be driven directly without the emitter follower.
     
  6. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Ahh, I see your problem.
    You need to start with the led current and voltage you want to use to get that current.
    Then you can go to the ULN2804 data sheet to get the input current needed to supply that output current. (there is a graph for that)
    Keep in mind the loss of voltage from the darlington.
    Then you can check the data sheet for the 4000 series ICs to see what will happen to their output voltage at that current.
    Here is a datasheet that has curves for that.
    http://www.futurlec.com/4000Series/CD4001.shtml
    So the things to watch out for is that the more current you want from the CD logic the more voltage it drops.
    The more current you want from the ULN output the more current you need in.
     
  7. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    A 4017 might make 10 mA, but the output voltage will sag down several volts. I vote for adding the emitter followers, no base resistors, collectors tied directly to 12 V.

    ak
     
  8. dannyf

    Well-Known Member

    Sep 13, 2015
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    Sounds like fun time for experiments.
     
  9. thunderace7

    Thread Starter New Member

    May 7, 2016
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    Thanks to all for the replies. I think the best thing I can do is to knock up a quick output stage and take some measurements. I suspect that I'll be going with the emitter follower.
     
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