RGB LED woes

I bought a batch of these, based on past satisfactory transactions, but the RGB LEDs didn't turn out to be as nice as the single-color ones I got from that vendor.

I've used 2-color LEDs on past projects, and they seemed to do a pretty good job of blending the colors so, e.g., turning on a combination of red and green looked more-or-less like a "yellow" or "amber" LED.

I was hoping to get the same effect from these, enabling me to get something that looked like a "custom colored" LED, but the different emitters are clearly visible, and I see something that looks more like an amorphous multi-colored "energy being" from an episode of Star Trek. :( Is this typical of RGB LEDs?

Another problem is that I forgot about the varying sensitivity of the eye to different colors: it looks like the R, G, and B emitters are all about the same intensity, so the red looks a lot dimmer. I can compensate for that by doing a table or formula to adjust the adjust the duty cycles, but it means keeping the green and blue way below maximum output. It would have been better to buy LEDs with colors at the same perceived intensity. Something to keep in mind when you're shopping.

Adding more diffusion (like tissue paper) helps the color mixing problem, but the project I had in mind for these is based on having something that looks like an ordinary diffused LED, but lights up in different colors. Are there other 5mm RGBs out there that do that?

Thanks,

Ran

I don't recall who posted it, but there was another thread in here where someone posted resistor values to balance the intensities of the different colors.

I chicken-scratched the numbers down: R: 220 G: 910 B: 512

I ended up using 220, 1k, and 470+100 as those were what I had available. It seemed to give a reasonable balance.

As for the emitters being distinct, I've seen that on the LEDs I bought to play with. Also it appears to be the case in various consumer products that I've run across. There are LEDs with diffusion packaging. I don't know how well they blend.

Unfortunately you may have to use a diffuser. Just as it would be using a separate red, green and blue LED. But once you get the color balance sorted out using different resistor values it should be as bad. It may help if your project has the dome of the LEDs rust peaking out the top. If you have it all the way down to the bottom of the LED the then the emitter color difference will be more apparent.

I don't recall who posted it,

Guilty.

It was just a rough and ready job with no great measurement I used it on the Arduinocaster http://www.youtube.com/watch?v=ehy9xgl4YCs. This used hidden LEDs mounted part way in styrene to act as a diffuser.

The problem is that the different colours are different LEDs with different light outputs and different efficiencies. This is much greater than the different sensitivity to different colours of the eye.

(Do I get a prize for the maximum use of the word different in a sentence)

You could try roughing up the surface of the LED with some fine sandpaper to help diffuse the colors. Its hard to tell from the ebay picture how opaque they start out. Otherwise I have seen people use white ping pong balls placed over the LED as a diffuser.

You could replace the resistors with suitably ranged potentiometers and vary their resistance until you find values you like, then replace them with resistors of similar values.

What about spray on window frosting paint? Or even just a thin coat of white for that matter.

I think the frosting paint may work better, though. We use layers of clear cellophane rolled in to tubes, inside a polycarbonate tube to diffuse lightsabers blades. I get the feeing that layers of frosting paint would have similar properties. (On a smaller scale).

I've used the sandpaper hack on clear single-color LEDs with good results. I've also ground the top flat with a file, and beveled the top edge, to get better wide-angle visibility from clear "point-source" LEDs.

These are already diffused, though, so that won't do much to help in this case.

Something I just noticed is that the emitters are very close to the top of the case: right below the curve of the "dome". I don't know whether that's a design flaw or a manufacturing defect, but it probably has a lot to do with why the emitters are so individually visible. There would probably be a lot more mixing if there were more translucent plastic in the light path. Certainly that's what I'm used to seeing in diffused LEDs.

Tweaking the resistor values to get the color balance right is a good idea, but it sucks to make such a huge sacrifice in brightness to get there. As I said in my original post, other folks shopping for RGB LEDs should avoid repeating my mistake, and check the specs for different output levels that are closer to eye sensitivity ratios.

Ran

Thanks Mike. I just had your number jotted down on the back of a random piece of scratch paper (someday I'll get organized) & couldn't remember the original source.