T3D Re: RGB LEDs

[Tom Hubin <thubin@xxxxxxxxx>]

Bob Wier wrote:
> 
> >GL:
> >
> >    You recently asked:
> >
> >> Speaking of LEDs, has anyone ever come across a three colour 'white'
> >> led, ie. with separately variable red, green and blue inputs?
> >
> >    That got me curious, so I did a search on RGB LEDs and found this
> >    interesting page:
> >
> >http://www.ledtronics.com/ds/5&8mm_full_spectrum_RGB_LEDs/80b/80b.htm
> >
> >
> 
> Hmmm - I wonder how they figure they can generate 256 colors? Although you
> can vary the brightness of LEDs *some*, I wouldn't think it would be
> enough to qualify for 256...
> 

Luminous intensity out of a LED is nearly proportionate to the current
through the LED. Very linear over a large range.

You can have 8 levels for each of 3 colors to produce 512 combinations.
You can have 7 levels for each of 3 colors to produce 343 combinations.
You can have 6 levels for each of 3 colors to produce 216 combinations. 

Controlling the luminous intensity to any of eight levels can be done by
controlling the current to eight discrete values. It could be as simple
as 2.86ma steps from 0 to 20ma for each color. Or the 8 values could be
chosen from some performance criteria.

Pulsing, as you suggest in your next response, can also be used. As long
as the rep rate is greater than 30Hz so that you do not perceive the
flicker. Tens and even hundreds of Kilohertz is no problem for LEDs. 

Controlling the current is easier for a simple circuit. Pulsing is
easier for a microcontroller with only TTL outputs. The choice depends
on the application and the designers personal preferences.

Tom Hubin
thubin@xxxxxxxxx