Debugging LED Circuit?

Discussion in 'General Electronics Chat' started by DarkNova, Dec 18, 2011.

  1. DarkNova

    Thread Starter New Member

    Dec 18, 2011
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    Hello,

    Our video baby monitor recently stopped producing light from its infrared LEDs and so I took it apart to see if I could figure out what was going wrong, but am having trouble debugging it.

    Everything else works with it, just the IR LEDs aren't working.

    I've attached 2 pictures of the board, one of the front and one of the back. The photoresistor on back seemed to be working and responds OK to light changes.

    The visible LED (green) on the front lights up fine. I measured a 1.8V drop across that LED. There are 8 IR LEDs, and none light up.

    Its hard figuring out exactly what is going on without the schematic.

    I measured the voltage drops across the IR LEDs, starting in the lower left corner of the picture and going clockwise they are:

    0.1V
    10.1V
    0V
    0V

    0.1V
    9.1V
    0V
    0V

    Does anyone have any advise for figuring out what is going on here? I'm kind of a beginner at this and am not sure how to proceed. Thanks.
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    How can you tell that the infrared LEDs are not working?
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    I'm betting cause the camera can't see any light, but still works with visable light. Am I correct?
     
  4. DarkNova

    Thread Starter New Member

    Dec 18, 2011
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    Yes, the monitor works in the daytime, with natural or artificial light, but just stopped working in the dark one day, whereas it showed an infrared picture before.

    I also tested with a camera and don't see them light up (I verified with the same camera that I could see a remote control IR LED flash).
     
  5. gerty

    AAC Fanatic!

    Aug 30, 2007
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    Where you measured 9.1 and 10.1 volts , if in fact it was across an led(s), they are probably open. You can replace those with other IR leds.
    I would also check the operating current of the leds to see what it is.
    Since we don't know what the ratings of the existing leds are, I would try to keep the current > 20 ma.
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    The IR LEDs seem to have four connected in series. There are two sets of them.
    The ones measuring 9.1V and 10.1V are burned out and the others might still work but might also burn out when the burnt ones are replaced.
     
  7. gerty

    AAC Fanatic!

    Aug 30, 2007
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    Boy. hows that for timing!!
     
  8. MrChips

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    Oct 2, 2009
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    Good detective work guys! Maybe DarkNova should install a resistor in series (about 22-ohm) to try and reduce the current a bit?
     
  9. Wendy

    Moderator

    Mar 24, 2008
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    To test the theory put a resistor across the suspect LEDs and see if it lights up. Infrared LEDs are pretty common and cheap, if you replace one (and verify it works) you may want to replace them all.
     
  10. DarkNova

    Thread Starter New Member

    Dec 18, 2011
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    Thanks so much for the responses. What you are saying is making sense. I spent a little more time debugging today.

    First, I found that one of my multimeters had a diode test function. I used that on the LEDs and found that the forward voltage drop on the IR LEDs is around 1.0V, except for the two IR LEDs that measured the 10.1V and 9.1V drops in my previous test (which you all thought were the bad ones) -- those measured 0V so that seems to confirm that those indeed are the bad ones.

    Now I need to figure out which IR LEDs to buy, as there definitely are a lot out there. I came up with what I think is the circuit for each side:

    [​IMG]

    The resistor is 15 ohms. I used this LED series calculator and found that for a 6.3V source voltage, if each LED has a 1.5V forward voltage and a 20mA forward current, that would make sense with 4 LEDs. That seems to be a fairly common spec for an LED. Does that make sense? Does it make sense to buy one that can handle a larger current?

    That being said, the strange thing I found is that the actual voltage is not 6.3V. The voltage point on the PCB is labeled 6.3V, but when I measure it it reads 11V (I tested with 2 different multimeters). I thought that was strange so I measured the voltage from the AC/DC wall wart -- it is labeled 9V, and the port on the device itself is labeled 9V, but the actual power output is 16V! 16V / 9V also happens to equal 11V / 6.3V.

    So, now I'm faced with another question: is this power supply bad as well? Do they fail in this manor, by increasing their voltage? The rest of the unit (video & audio + transmitter) works just fine with this voltage. It would seem strange that all 3 voltage labels (wall wart, input power plug, & PCB label) would be wrong, it seems more likely the wall wart just went bad, but it is curious that everything else functions fine.

    I'm assuming that if the wall wart increased the voltage, that could explain why the two LEDs burnt out (increased current).

    Does my above speculation make sense? Thanks for the help!
     
  11. DarkNova

    Thread Starter New Member

    Dec 18, 2011
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    On second thought, could the wall wart read that much higher voltage just because its unloaded?
     
  12. MrChips

    Moderator

    Oct 2, 2009
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    Yes. Transformers are generally rated when on full load. When not loaded they will output a higher voltage than the rated voltage.
     
  13. Audioguru

    New Member

    Dec 20, 2007
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    IR LEDs are 1.0V to 1.2V, not 1.5V. Then the current in the IR LEDs is way too high when the current-limiting resistor is only 15 ohms and the loaded DC voltage is 6.3V.

    I don't know the max allowed continuous current for ordinary 5mm IR LEDs. It might be 40mA or 50mA.
     
  14. MrChips

    Moderator

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    Did I hear someone say something about adding a resistor in series to limit the current?
     
  15. Audioguru

    New Member

    Dec 20, 2007
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    The OP said there is a 15 ohm resistor in series with each set of 4 IR LEDs.
    Then the current in the LEDs will be way too high and will burn them out.
     
  16. KJ6EAD

    Senior Member

    Apr 30, 2011
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    It sounds like you have one of those low quality unregulated electromagnetic wall warts and the output is too high for the IREDs. I'd replace all of them with 880 nm (easier for the camera to see), not 940 nm IREDs and I'd build a LM317 current regulator for each string. Audioguru's right about IRED continuous current ratings being quite a bit higher than for LEDs.
     
    Last edited: Dec 20, 2011
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