I have been struggling with a prototype of back-to-back dual LED's today.
It looks like it won't be real easy:

1.  I first hooked them up like this:

                    _aDk_
RA3 ---------|           |------50R--------2.5V supply
                    |_kDa-|                        |
                                                        27R
                                                        |
                                                        GND


As predicted the red was way brighter than the green, until I tweaked up the
power supply.  The extra 27 ohms to ground was needed because the red lamp
feeds current into the power supply, which it can't regulate, and the 50 ohm
is for current limiting.


I then tried an arrangement like this:

                                                        5VDC
                                                        |
                                                        47R
                    _aDk_                        |
RA3 ---------|           |-------------------|
                    |_kDa-|                        |
                                                        47R
                                                        |
                                                        GND

This is essentially the thevenin equivalent of a 23.5 ohm resistor in series
with a 2.5 volt power supply.  For some reason, the current and the
brightness seemed more similar in this case.  Current was about 11mA red and
10mA green

I then hooked up two LED's to the same resistor divider,from different port
pins. Predictably each had less current, 9mA.

This whole arrangement is a real power hog.  I am thinking about running the
resistor divider from an unregulated power supply.  I have to drive 8 LED's
from this arrangement, and my calcs say that I'd need a resistor divider
like this to run them all:

    5VDC
    |
    |
    3R
    |
    |-------- 2.5 volt supply
    |
    3R
    |
    GND

Sheesh!  There's gotta be a better way to skin a cat.

--Lawrence

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