> I would suggest an easy remedy to this is to use a bipolar LED
        one of
        > the two-lead red/green types, available at Radio Shack or any
        mail-order
        > electronics house) connected as shown:
        >
        > Gnd----[220ohm]---+---[220ohm]----+5
        >                   |
        >                   +---[LED]------- Port pin
        >
        > that this can be a useful trick for driving more than one
        "independently-
        Hmmm, I was having a discussion about bipolar LED's driven by PIC
        pins in
        one of my classes I'm teaching this summer just this morning. The
        objective
        was to drive 6 bi-polar LEDS (2 pin) using as few PIC port pins as
        possible.
        Each LED should be able to show red,green,yellow, and off.

        The design I hit upon after thinking a bit revolved around
        multiplexing
        the 6 LEDs utilizing a common pin on one side. Something like:

         Port pin---+-[220ohm]--+---[LED]------  Port pin
                    |
                    +-[220ohm]--+---[LED]------- Port pin
                    |
                    +-[220ohm]--+---[LED]------- Port pin
                    |
                    +-[220ohm]--+---[LED]------- Port pin

        And so on. One led can be selected by tristating all but one port
        pin
        on the right side. The selected LED's color can be picked by
        twiddling
        with the left port pin and the one non-tristated port pin.

Actually, I think there is nothing new under the sun really ;)
If you know about LED sign boards, this is how they work! You
seem to have simplified it into a single line! (the initial
poster did it down to a single pixel!) I think it is a nice idea
though!

        The question that popped into my mind during this design discussion
        was the relevance of the position of the resistor. Does it matter if
        the resistor is on the anode or the cathode side of an LED.

        BAJ

Theoretically both are identical. You don't need to put a resistor
to both ends of a bipolar LED either. I do not know how this setup
works with high frequencies (well, we have seen radio beacons
recently), so I cannot comment on that... (Not that many people would
drive their LED's at these frequencies!)

Is there any "catch 22" to this?


| Ahmet ONAT  Kyoto Univ. Japan                                 |
| E-mail    : onat@kuee.kyoto-u.ac.jp                           |
| WWW page  : http://turbine.kuee.kyoto-u.ac.jp/staff/onat.html |
|             My 6 leg walker, RC airplanes & more in home page |