Hi Lawrence,

Not sure exactly what your requirements are but if you can multiplex and
have 9 pins you can easily drive the 8 bi-colour LEDs. This is a trick I've
done before and it only requires a few normal current limiting resistors.
I've done this trick with two single colour LEDs per leg (wired like a
bi-colour), so I'm sure this will work.

PIN1 >------------,
                  |
                 LED
                  |
PIN2 >---/\/\/\---+
                  |
                 LED
                  |
PIN3 >------------+
                  |
                 LED
                  |
PIN4 >---/\/\/\---+
                  |
                 LED
                  |
PIN5 >------------+
                  |
                 LED
                  |
PIN6 >---/\/\/\---+
                  |
                 LED
                  |
PIN7 >------------+
                  |
                 LED
                  |
PIN8 >---/\/\/\---+
                  |
                 LED
                  |
PIN9 >------------+

Basically you have to multiplex the two pins connected to the LED you want
to be high and low one way around gives you red, the other gives you green.
The trick is that all the other pins need to be tri-stated at the same time
otherwise several of the other LEDs will light as well. Do a lookup table
for the 16 individual LEDs for the values of the pins and another for the
tri-state values and then just scan them.

Another alternative could be a 74hc138 3 to 8 decoder. You can multiplex 8
single LEDs or 4 bi-colour LEDs with just 4 pins (3 + Chip select so you can
turn them all off).. You simply use the '138 pin as a pull down for a single
LED (with one limiting resistor for all to the 5V line as only one is on at
once. For bi-colour LEDs you put then across two pins with a resistor so
that you can make either leg of the LED low, the other one will always be
high

I was struggling to fit enough LEDs (5 single and one bi-colour) in a
project myself and rather then take the step up the a bigger PIC I threw in
a '138 and managed to fit them all in. Actually I only used 7 of the pins so
I can turn them all off by sending address "0" so I eliminate the extra pin
for the chip select:

          ,---------,
 PIC1 >---|A   7  Y0|-NC
 PIC2 >---|B   4  Y1|---K LED A---+-----------/\/\/\---> 5V
 PIC3 >---|C   H  Y2|---K LED A---|
          |    C  Y3|---K LED A---|
          |    1  Y4|---K LED A---|
          |    3  Y5|---K LED A---'
          |    8  Y6|---/\/\/\---BiColLED--,
          |       Y7|----------------------'
          '---------'

note I haven't tested this circuit yet, but it should work..

Hope that helps
Cheers,
Ash.

---
Ashley Roll
Digital Nemesis Pty Ltd
www.digitalnemesis.com
Mobile: +61 (0)417 705 718




> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Lawrence Lile
> Sent: Saturday, 2 February 2002 8:55 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [PIC]: Driving a dual LED
>
>
> I have been struggling with a prototype of back-to-back dual
> LED's today.
> It looks like it won't be real easy:
>

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