On Wed, Apr 21, 2004 at 10:04:31PM +0000, Matt Redmond wrote:
> Hi All,
>

> My basic EE skills are lacking

You may want to bone up. I've spent quite a bit of time reading the copy
of the Art of Electronics I requested from our school library.

> so I was hoping someone might suggest a solution to this...

Fire away.

>  I have a bicolor LED - one of the 2-lead types where the
> green anode is the red cathode and vice-versa.  You reverse polarity to
> change colors.

Check.

>  What I need to do is drive this with one output pin on my
> PIC.  I swear I'd do it with two if I had them, but I don't.

Understood.

>  The green can
> be on all the time except when the red comes on.  So - steady green and then
> red when my output goes high (or low).

Good. So it doesn't have to be off. That makes it a lot tougher.

>
>
> Any thoughts?

Several. The first is that if you can afford to burn the current you can
simply tie the outside leg to a midpoint resistor voltage divider that locks
the outside leg at 2.5V (presuming a 5V circuit). Then the PIC output can
swing from 0 to 5V, going from one color to the other as it swings. Neither
LED in the bipolar should require more than 2V forward voltage.

A more active approach could be to inject a opamp voltage follower between
the divider and the LED. So you get:

PIC pin -> resistor -> LED -> opamp follower -> voltage divider

The nice thing about this approach is that you can build the voltage divider
with 47K or 100K resistors limiting the amount of current they consume because
they no longer have to feed power to the LED.

BAJ

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