LED Lighting Control: Dimming Channels with Resistance and Microcontrollers

[wil3]

Hello. I am building an LED striplight for my room. For the device, I would like to have three separate channels so that I can easily color mix Cyan, Magneta, and Yellow. I know the basics of circuitry and wiring the channels, but I am not sure how to control the channels.

I was considering using a basic potentiometer or rheostat on each channel and simply dimming each channel by varying the resistance. My concerns with this approach are heating and being able to find a rheostat with a resistance range large enough to give me full intensity control of each channel. Does anyone have a better way of doing resistance dimming?

I do theatrical lighting design at a local theatre, and so I am also aware that there are more sophisticated ways of dimming each channel using microcontrollers and such. I believe that this approach would give me much more control over my lighting, but I know almost nothing about making computerized electronics and I would appreciate some advice on the process.

Also, I am using CMY mixing because those are the colors we mix for subtractive color mixing in automated dichroic lights at hte theatre. Are different colors used for LEDs?

Thank you in advance.

 

[negitron]

LEDs are not available in CMY colors--well, except the Y part. LEDs can only be manufactured to produce spectral colors (that's not entirely true, since they could, in principle, do what they do to make white LEDs and use a spectral color to fluoresce a dye of some sort but the demand for such things seems low, so they don't bother). You're better off with RGB.

If you want a better way of controlling your LEDs, look into PWM (pulse width modulation). There are off-the-shelf PWM controllers available which should work for you and if you'd like to try the microcontroller approach, it's fairly straightforward to build your own and you'd only need a modest initial outlay for the programmer to get started.

 

[MATLABdude]

To elaborate on what negitron said, you typically use CMYK colours in print / physical media, because you have to mix toner / paint / whatever together such that the light reflected off of something appears to be a certain colour. When dealing with TVs, monitors, and, say, coloured cellophane for a stage show, you use RGB because you're mixing transmitted light together.

This is a big headache in graphics, because you often have something done in RGB colour space (the light you see on your monitor) which often looks different from the output you get on your printer (well, that's usually because your printer isn't calibrated), and even what you see when you get something back from the (professional) Printers. You usually only make that mistake once or twice, however (or, get a Mac, which has a native CMYK display model--this along with Pantone colour matching, and the first run suites of professional design software--is probably what's made Apple the computer of choice for graphics professionals).

Anyways, back to the topic at hand, you can often find LEDs that have all 3 colours on board (RGB LEDs), for instance:
http://www.lumex.com/product.aspx?id=838

If you go up to a jumbotron, or one of the really bright large-area video displays, you'll see that it's not a really big (and really, really expensive) plasma TV or LCD display, but rather that it's composed of thousands (or even millions) of RGB LEDs:
http://www.big-tv.co.uk/LED_explained.htm

EDIT: I have to quit posting after drinking (or more thoroughly read first posts). Given that you use dichroics, and know the term subtractive colour mixing, you probably already understand the above discussion, but perhaps it'll be of use when somebody stumbles into this thread down the road. My tip: pick up a microcontroller and figure out how to do PWM. (Resistive) dimmer control will not give you very fine grain control, and will be problematic if you want to keep a certain light intensity.

 

[wil3]

Thank you both for some excellent information. I am actually a high school student, so don't worry about sounding patronizing MATLAB, because it is very true that I am a bit of a newbie to LED lighting. I do have a bit of experience patching moving lights and dichroics, which is why I knew about those terms. Also, I agree that Macs are fantastic for graphics editing.

RGB with PWM definitely sounds like my best bet- I have also heard that resistance dimming can cause manufacturing disparities to become very obvious. I saw a decent three channel dimmer on Ebay with the exact specifications I need, but I think I will look into building my own first just because I need to learn more about microcontrollers.

Also, would either of you happen to know any more specific information about dimming protocols outside of resistance dimming? I am specifically interested in theatre light dimming, which I believe uses a protocol similar to PWM but that truncates the sine wave without converting it to a square waveform.

Thanks again.