Getting Yellow from an RGB LED

[Govmnt_Lacky]

Folks....

I have been trying to get a good mix between Green and Red to make Yellow out of a diffused RGB LED. I am 'assuming' that the magic is in the CLRs that I am using. In the past, I have used 10K resistors for both colors but, recently I have thought that the Red CLR needs to be reduced to get more red in there. It always seems with the same value CLRs that it is too much Green.

Thoughts? Recommendations for CLR values?



BTW... this is a common Anode LED running with 5VDC

[davent]

Have you tried trimmers to dial in the colour?

[nocentelli]

Anode to +5v, then use a 10-20k pot with the two cathodes connected to lug 1 and 3 respectively, wiper connected to a 1k then to ground. Dial in the desired colour, then remove the pot and measure to find required CLR values.

[stallik]

Went through this myself a while ago. Theoretically, red and green make yellow but this presupposes that the red and green are pure, of equal brightness and that the led physically allows both colours to mix evenly. My experience is that at least one on the above criteria is not met

Using pots as suggested is the way to go to get even brightness. Set red to a comfortable level and raise the level of green till it looks ok. If you're unhappy with the resulting yellow, move on to a different led. I found that the larger the LED, the more chance there was of it looking more like red and green next to each other rather than a fusion of the two

[Govmnt_Lacky]

Well...

I tried the trimmer method (used a 10K pot) with a 1K to GND. I was able to determine that, with this particular diffused RGB LED, that 3.3K on the Red and 6.8K on the Green gets me about as close to Yellow as I can. This is a 5mm LED and no matter where I had the pot... I could never get rid of the individual colors. 

Thanks to all who made recommendations. I feel stupid now for not thinking about this on my own 

[digi2t]

The closest I got with the Ludwig red/green LED was an orangy yellow.

[davent]

Some reading material i've only skimmed...

http://www.ledsmagazine.com/articles/print/volume-10/issue-6/features/understand-rgb-led-mixing-ratios-to-realize-optimal-color-in-signs-and-displays-magazine.html

[greaser_au]

Part of my electronics study from way back when (over 25 years ago) was the part-time TV servicing course (6 hours a week for 4 semesters). A lot was included on phosphor wavelengths and colour mixing ratios using the standard full-saturation colour bar test pattern as a starting point - the additive mix ratios and luminance order for the pattern were examinable!  This article points out why the ratios for LEDs are different. It includes a little physics and physiology.

Due to the physically separated layout of the LED chips inside the (even diffused) encapsulation it will be difficult to get a nice blend/mix of the light at normal distances. From across the room it will be yellow, at bench distance it will be red/green blotches.

Just for entertainment,  you could play with the 'saturation' by mixing in a little bit of blue, to see if you like the yellow better that way!

david

edit: Dave beat me to it!!!

[GibsonGM]

PWM?

[duck_arse]

yeah, what gibson said.