Problem with 74HC595 and ULN2004

Discussion in 'The Projects Forum' started by ephpeter, Sep 9, 2013.

  1. ephpeter

    Thread Starter New Member

    Jun 6, 2013
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    0
    Hi Guys i got a nice kit from velleman the K8089 and want to drive a big 12VDC driven LED display. Can somebody give me a hint, i used now the ULN2004 and the 7 segment display works reversed. :confused:
    schematic http://www.onderdelenspecialist.be/schematic_k8089_rev1.pdf

    The segment that have to light up goes out and the segments that dont have to light up lightning up.
    On the builded kit with the original led display's its working like hell!!!
    can somebody help me, i dont see it anymore grrrrrrrr:mad:
     
  2. ScottWang

    Moderator

    Aug 23, 2012
    4,850
    767
    I can't see the ULN2004, but only 74HC595.
    Or you want to using ULN2004 to replace the 74HC595?
     
  3. ephpeter

    Thread Starter New Member

    Jun 6, 2013
    3
    0
    I made a extern PCB and used the outputs from The 595 To set on The inputs For The uln
     
  4. ScottWang

    Moderator

    Aug 23, 2012
    4,850
    767
    Could you attach the circuit, maybe it just the high and low logical problem.
     
  5. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    The ULN2004 will invert the signal as ScottWang thought it might. To use it between the '595's and the displays, you would need to add inverters between the '595's and the ULN2004 inputs. This assumes your larger displays are common anode like the kit displays.

    EDIT: Please clarify: Was the original build with original displays working OK ?
     
    Last edited: Sep 9, 2013
  6. ephpeter

    Thread Starter New Member

    Jun 6, 2013
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    0
  7. t06afre

    AAC Fanatic!

    May 11, 2009
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    What kind of display unit do you have. Any links/product number
     
  8. ScottWang

    Moderator

    Aug 23, 2012
    4,850
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    That was what I thought, you have to reverse(as inverter) the logical of software from 74HC595.

    It means that when you didn't add the ULN2003, you send a low to drive the 7-seg from 74HC595, but when the ULN2003 was cascaded then you have to send a high to the ULN2003, and ULN2003 output a low to drive the 7-seg.

    Original:
    74HC595 (Vout=0) → (0)7-seg, 7-seg(+) → +5V

    ULN2003 added:
    74HC595 (Vout=1) → (Vin=1) ULN2003,ULN2003 (Vout=0) → (0)7-seg, 7-seg(+) → +5V

    When you using ULN2003 to drive the 7-seg, you have to adding seven resistor to limilting the current, and the pin9(com) of ULN2003 connected to GND.
     
    Last edited: Sep 10, 2013
  9. elec_mech

    Senior Member

    Nov 12, 2008
    1,513
    193
    As others have stated, the ULN2003 sinks current when it sees a high signal on its input which means you need an inverter between the 595 and the ULN2003. I made a quick drawing showing how to connect a 74HC14. Unfortunately, these only have 6 inverters, so you'll need 5x 74HC14s total for your four digits.
     
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