Mark Willis wrote:

> 3 resistors?  I've tried to fit that, 4 is the only number I come up
> with

  What it the objective?  Minimum resistors?  Maximum drive current?

  This circuit, or the general task of multiplexing nÓ-n LEDs on n I/O
lines really is my "pet" - I think you'll recall references to it in the
past.  A sub-set of the problem is multiplexing 7-segment displays.

  The "optimal" solution involves using one resistor and one SMD NPN
transistor per I/O line.  Was it Mark who had a finger in this one?
Collector goes to *un*regulated +ve supply, resistor between base and
emitter, base driven by PIC I/O and emitter to LED matrix.

  In use, one output is driven high at a time, and as many of the other
lines as LEDs need be lit, are driven low, others being made inputs.

  Software consists of setting PORT register to 1 for the "strobe" line,
all others zero.  TRIS is loaded with zeroes for all lines on which a
LED must *light*, ANDed with the complement of what was just put in the
PORT register (i.e., strobe line is also zeroed).  All others "1"s
(input).

  More detail on request.
-- 
  Cheers,
        Paul B.