Problem with RGB LED color combinations.

Discussion in 'The Projects Forum' started by lilrips1, Jan 12, 2011.

  1. lilrips1

    Thread Starter Member

    May 7, 2010
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    Hi,

    I purchased this RGB LED: http://www.pololu.com/catalog/product/1074

    I have a breadboard and resistors, and I tried hooking the LED up to try out the various color combinations.

    I can get red, green, blue, and cyan to show, but cannot combine red with anything. When I combine the red lead with any of the other two anode leads, all I get is solid red. So that's magenta, yellow, and white that are unable to work. It's not as if the other colors are just really dim; only the red led turns on.

    Does anybody have any experience with 5mm RGB LEDs that could shed some light on my problem?
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    Show us a schematic of how you are hooking it up.

    hgmjr
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    Of course only the red lights when the colours are all connected directly in parallel. The red LED has the lowest voltage so there is not enough voltage for any other colour.

    Use a series resistor for each colour and a high enough supply voltage then they will all light.

    Watch out! Many of those cheap ******* LEDs are shown common-cathode but are actually common-anode.
     
    Last edited: Jan 13, 2011
  4. CDRIVE

    Senior Member

    Jul 1, 2008
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    My gut tells me that your supply voltage is to low. What is it?
     
  5. Wendy

    Moderator

    Mar 24, 2008
    20,766
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    A good point.

    Different colors of LEDs have different voltage drops. ALL LEDs require a resistor, if you don't have a resistor the odds of blowing up an LED are pretty good.

    LEDs, 555s, Flashers, and Light Chasers
     
  6. CDRIVE

    Senior Member

    Jul 1, 2008
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    Damn, I hate duh moments! I was correct about the low voltage but for the wrong reason!! :eek:
     
  7. lilrips1

    Thread Starter Member

    May 7, 2010
    44
    0
    I attached my setup.

    Here are the voltages for each color at 30 mA:
    Red - 1.95 V
    Green - 3.2 V
    Blue - 3.5 V

    When I hook up both green and blue at the same time they make cyan, so I don't get why hooking one of them up with a lower voltage light wouldn't work. 3.2 + 3.5 > 3.2 + 1.95 ...

    I don't see how cyan even works.

    I'm not sure how this would work in series.

    By the way, my project (of which the attached schematic is the part which consumes almost all of the PS) only calls for TWO of the colors being lit at any point in time, so white is not necessary.
     
  8. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    Did you look at the datasheet?

    Longest lead (pin 2) is Cathode (-)

    Pin 1 - MDK - (~650nm) Red - Short Lead (same length as Green)
    Pin 2 - Negative/Cathode Longest lead
    Pin 3 - CB (~470nm) - Blue - 2nd longest lead
    Pin 4 - BG (~520nm) - Green - Short lead (same length as red)


    Pin 2 (longest) is the common cathode to all 3 colors.

    Just want to make sure you are looking at it correctly, the datasheet is a bit odd about defining the colors and lead names.
     
  9. lilrips1

    Thread Starter Member

    May 7, 2010
    44
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    Yes, I have the leads hooked up correctly. I'll try playing around with it a bit more tomorrow. I'm just befuddled as to why cyan works.
     
  10. CDRIVE

    Senior Member

    Jul 1, 2008
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    His schematic is using three limiting resistors and seems to eliminate the all three LEDs in parallel theory.... What was that famous Holmes quote? :)
     
  11. CDRIVE

    Senior Member

    Jul 1, 2008
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    Uh, we're running out of educated guesses here!:confused:
     
  12. lilrips1

    Thread Starter Member

    May 7, 2010
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    Hmm... Am I incorrect in thinking that blue and green should not light up with only 4.5 volts?
     
  13. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Do you have a DMM that will measure the Vf of each individual LED? (up to 3.5V?) If so, what are the diode readings with black lead on pin 2 (longest), and red lead on pin 1 (shortest, on flat side), then lead wire on pin 3 (mid-length lead), and red lead pin 4 (lead opposite flat side)? The black wire would be attached to pin 2 the entire time.
     
  14. Audioguru

    New Member

    Dec 20, 2007
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    Your circuit shows a resistor in series with each LED. Did you "hook up" the LEDs directly in parallel with only a single resistor?

    Did you measure the voltages of the LEDs? The voltage listed on the datasheet is only an approximation because the voltages are in a range of voltages. An LED clamps the voltage.
    So the blue and green voltages might be close enough for both to light if they are directly in parallel.
    But the voltage for the red LED is so low that it clamps the voltage to about 1.95V which is much too low for the blue or green to light.

    LEDs in series (or anything in series) allows the parts to have exactly the same current. Of course the voltages of the LEDs add when in series.
     
  15. lilrips1

    Thread Starter Member

    May 7, 2010
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    I do not own a multimeter, but the datasheet says that the indicated voltages have a tolerance of +/- 0.1 V.

    Perhaps I should explain in detail what I did:u

    I hooked up the LED with one resistor per LED lead, except the ground of course. The power ran to a row that included one end of a resistor. The other end of the resistor ran to a new row which included one of the LED leads. I did this for each of the anode leads. I ran a wire from the negative side of the battery to the cathode ground lead.

    Audiouru: That would explain why nothing works with red, but I'm pretty sure the way I described that hooked it up was in series, but maybe not.
     
  16. Audioguru

    New Member

    Dec 20, 2007
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    You need to learn the difference between connecting parts in series and connecting them in parallel.

    Your schematic shows each LED colour connected in series with its own resistor. Call them series strings. But your description talks about "rows" that makes no sense. Maybe you shorted all the anodes together by connecting them all to one "row"?

    Then you connected the green LED string or the blue LED string to the power supply or connected both strings to the power supply and they both lighted.
    But when you also connected the red LED string also to the power supply then only the red LED was lighted.
    It doesn't make sense that all the LEDs were not lighted.
     
  17. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Double-check that the resistor on the anode of the red LED is in fact 100 Ohms; brown-black-brown (toleranceband) If it's brown-black-black instead, that's 10 Ohms, and would cause lots of current flow through the red LED and possible loading of your supply, bringing it's voltage too low to light the other LEDs.
    The 47 Ohm resistor should be yellow-violet-black
    The 39 Ohm resistor should be orange-white-black.
     
  18. jason 77

    New Member

    Jul 27, 2009
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    I agree it is confusing the way it is worded... The OP did say he was using a bread board, maybe he is using "rows" to describe each row of holes that are connected together?

    Maybe the OP could take a picture of his circuit the way he has it hooked up, that would allow us to see how exactly he has wired up his circuit?
     
  19. SgtWookie

    Expert

    Jul 17, 2007
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    Just to throw together a quick graphic, I used PEBBLE. Link: http://www.4dsystems.com.au/prod.php?id=93

    Pebble doesn't have an RGB LED, so I used three discrete LEDs and connected them to be what I think is the electrical equivalent - and how they described that they hooked it up.

    [​IMG]
     
  20. CDRIVE

    Senior Member

    Jul 1, 2008
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    Wow, who wudda thunk you could have a thread run in excess of 19 posts regarding lighting LEDs from a DC supply, with absolutely no switches, relays, or any controlling components whatsoever? Gotta be a record! :D
     
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