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I have an RGB LED, which I belive to be the diffuse version of this one: https://www.sparkfun.com/datasheets/Components/YSL-R596CR3G4B5C-C10.pdf, though I am not completely sure, as I bought it some while ago.

I have a 5V power supply (USB), and based on the data sheet I would calculate the needed resistor values as 90 Ohm for green and blue and 150 Ohm for red. I'm using a 100 Ohm resistor for green and blue each and 100+47 Ohm for red, i.e. pin 2, the common ground, connected to ground and each of the other pins connected to 5V through the respective resistor(s).

I am doing my connections on a breadboard.

Now if I connect any of the pins separately, leaving the others disconnected, the LED lights up more or less blue. Except if, after having connected the red pin, I touch the blue pin with a wire which is touching my skin, then it actually lights up red. If I now touch the green pin with the wire it reverts to blue. By playing with touching and connecting the pins in different orders I manage to get a nice white color, and I have managed yellowish hues and something remsembling violet or pink. I haven't managed to get green and I've even had what I think are mixed colors from just one pin connected. The blue from the red pin is fainter (because of the larger resistor, would be my guess), but the red is quite bright.

What is happening? I have multiple LEDs of the same type and all the ones I tested show this or similar behaviour.

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  • \$\begingroup\$ "...which I belive to be..." - why do you believe this? Do your LEDs have a black object inside them, like this? ebay.co.uk/itm/… image.micros.com.pl/_dane_techniczne_auto/olrgb.5m600.pdf \$\endgroup\$ – Bruce Abbott Oct 9 '18 at 0:03
  • \$\begingroup\$ Double check that you're using the LEDS in a common cathode arrangement. Common is pin 2. \$\endgroup\$ – K H Oct 9 '18 at 1:07
  • \$\begingroup\$ @BruceAbbott: Oh my, yes, thank you, that was the crucial hint. I'm going to test this theory but right now it seems very likely to me that what I have is some kind of "smart" RBG LED. The reason I believed that I have the ones in the link is that I only remembered the shop where I had bought them and searching their website I only found the one I had linked to. Seems I didn't look closely enough. Now I also think I vaguely recall buying NeoPixel-compatible LEDs, and the shop in question also sells those. I had completely forgotten such a thing even existed... \$\endgroup\$ – manu Oct 10 '18 at 20:11
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Sounds like you are touching the wrong pins or you have wiring issues. These are very common LEDs and work as expected.

Your resistor values are incorrect. There is no way the red should be the brightest, green should be the brightest with the least resistance and highest luminous intensity. You most certainly have wiring issues.

Yellow would be a combination of red and green.
Pink, magenta, and violet would be a combination of red and blue.

enter image description here



Resistor Values

You need to balance the red and blue by luminous intensity and forward voltage .

You then must reduce the green to the luminous intensity of the red and blue.

Notice the red has a Vf of 2.2V and blue 3.4V

enter image description here


Red has the least luminous intensity so run it at the max current of 20 mA.

At 20 mA the red resistor should be about 150 Ω where intensity ≈ 800 mcd

Blue has 900 mcd and should be reduced by about 11% from red.
At 18 mA the blue should have a resistor of about 90 Ω where intensity ≈ 800 mcd

Then green should be about 5 mA to drop the luminous intensity from 4000 mcd at 20 mA to 1000 mcd at 4-5 mA with a resistor of about 300 - 400 Ω intensity ≈ 800 mcd

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  • \$\begingroup\$ Thank you for taking the time to answer in such detail. I think in this case Bruce solved the mystery in his comment. I was wrong about what kind of LED I have. I'm pretty sure now that I have a "smart" LED and by touching the contacts to skin apparently I managed to get signals on the pins that were interpreted as commands for what color to switch to. That seems like the most plausible theory to me right now. I can't see any way in which the behaviour I was seeing would be consistent with a simple LED with separate pins for each color. \$\endgroup\$ – manu Oct 10 '18 at 20:19
  • \$\begingroup\$ For future projects with "dumb" LEDs: is luminous intensity typically linearly related to input current? \$\endgroup\$ – manu Oct 10 '18 at 20:25

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