At 07:16 PM 4/18/2004 -0700, you wrote:
>Hi:
>
>I would like to experiment with a 3 color LED to see what colors it can
>make.
>The commercial color LED drivers seem targeted for cell phones and are
>extremely small surface mount parts making them difficult to work with.
>
>I would like to have good control of the current in the Red, Blue and
>Green LEDs.  The forward drops for the 3 colors are considerably
>different with Blue a little over 3.5 Volts.  This is a problem in that
>a small change in the supply voltage or LED temperature will cause a big
>change in the LED current is simple dropping resistors are used from a
>+5 Volt rail.

A bit of current change won't be all that visible, relative differences
might be important near white, but the eye has a kind of color balance
correction anyway. The Vf voltage differences can be taken care of with
different valued resistors. The LEDs will change output intensity and
color with temperature as well, even if you hold the current constant.

>One approach would be to use 3 independent current sources, each good
>for 100 ma and switch them on and off (PWM) with an 8 pin PIC.

You could do that, though current sinks might be easier.

>Is there a simpler way to do this?

Just resistors and transistors might be good enough. You could sense
ambient temperature and fiddle with the current ratios if that was
necessary.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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