ULN2803 driven by 7HC595 problem

Discussion in 'The Projects Forum' started by iimagine, Jul 15, 2016.

  1. iimagine

    Thread Starter Active Member

    Dec 20, 2010
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    Hi all,
    I'm using ULN2803 to control some LED, driven by a 7HC595 and encountered a problem i cant figure out yet. Theoretically, when 7HC595 outputs are low (0v), LED at the output of ULN2803 should be off; but in my case they are on, and when high they are also on but brighter! They are direct connected with no pull down/up resistors. I'm using Arduino SPI to control the 7HC595. I'm thinking that maybe the 7HC595 low outputs are not true at 0v but floating; I did measure with an oscilloscope and got 0v though. I'm not sure whats going on here. Please help.
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    What is pin 10 (COM) of the ULN2803 connected to?
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Sounds like you are connecting the LEDs backwards, when the output is low the LED should be on, the outputs sink to GND, the COM does not need connecting for LEDS.
    Max.
     
  4. Papabravo

    Expert

    Feb 24, 2006
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    Schematic, Schematic, did I mention schematic. Speculation on a verbal description is worse than useless.
     
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  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Did you try actually taking the ULN input to GND and see if the LED turns off?
    Max.
     
  6. iimagine

    Thread Starter Active Member

    Dec 20, 2010
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    9
    SOLVED!

    I used the 12v to 5v to power the 74HC595 and ULN2803 while the 12V for the LED. COM (pin10) of the ULN2803 was connected to 5v. As soon as i changed LED power over to the same 5v, it worked. So i'm guessing i cant use 2 diff power source to the ULN2803. I would like to know it is even possible?

    Thanks everyone for replying and help.
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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    That is done all the time for 12v 24v relays etc.
    You only need the COM connection for inductive devices.
    Max.
     
  8. iimagine

    Thread Starter Active Member

    Dec 20, 2010
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    So it should work if I disconnect the 5v power to the COM, and use the 12V for the LED? <-- Dont even bother this stupid question lol. Ofcourse it should work
     
    Last edited: Jul 15, 2016
  9. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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  10. MaxHeadRoom

    Expert

    Jul 18, 2013
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  11. iimagine

    Thread Starter Active Member

    Dec 20, 2010
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    The reason i did not post the schematic because i thought it was so straight forward, outputs from one IC to inputs of the other, the only part that got me into trouble was powering the COM with 5v
     
  12. Papabravo

    Expert

    Feb 24, 2006
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    That is hardly a justifiable reason for not posting a schematic. Had you done so the problem would have been immediately apparent.
     
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