Phillip,

I did something like this a few years ago when I first got into PICs.  I
created a 3x3 two coloured matrix using two leaded two colour LEDs.  The
final project was going to be a tic-tac-toe board.

They were hooked up with a 220 Ohm Current Limiting Resistor in Row/Column
format and to turn on an individual LED in the Matrix, the TRIS bit for the
respective row and column was turned on with the colour (Red/Green/Off)
being selected by making either the Row/Column High.

I think this could be done in a very similar manner using tri-coloured LEDs
(eliminating the need for the ULN parts).


Not to many problems and pretty easy to program using a TMR0 Interrupt Handler.
The concerns I had were:

1.  I put a Switch Matrix in with it at the same time.  Pressing a button
would cause the LED at the button to light weakly (this may be desireable).

2.  Memory.  I never came up with a great way to store the 18 bits (two bits
for each LED red/green/off/off) required for the current display value.
This might be a major issue with an 8x8 array and some of the smaller PICs.

3.  I could never tell the difference between the colours (I'm
colour-blind).  Two-Colour LEDs seem to have less differences than other
LEDs (I can usually tell red/yellow/green by the different intensities of
the light - not so with these parts).

Actually, it was number 3 that caused me to give up on the project.

Good Luck, I'm interested in hearing what you come up with.

myke
>Hi All,
>
>I am just starting to think about a project that will require various
>patterns to be generated on a matrix of LED's.
>
>8 * 8 would be ideal, giving 64 LED's but as I will require 4 other I/O
>lines and they will be Tri Colour LED's 7 * 7 (49) may be more
>achievable. Of course the other problem with this type of display is
>that the drive to the LED's will have to be multiplexed.
>
>My thoughts so far are that the port A0 - 6 would drive the common
>cathodes (via an 8* ULN darlington driver) and the Red / Green anodes
>would be driven via ULN's on B0 - 6. Bits A7 and B7 would be used to
>turn on the required ULN anode line. This would then enable LED and
>colour selection.
>
>So far we have 16 I/O's plus the other 4 required lines, means a 28 pin
>package.
>
>Questions:
>
>Any body done this before.
>
>What would be the best PIC to use.
>
>Any one got any thoughts on the multiplexing.
>
>TIA.
>
>--
>Philip Martin
----------------------------------------------------------------
>Royal Quays             If at first you don't succeed, try again. Then quit:
>North Shields.          no use being a damn fool about it !
>                                                        W.C. Fields
>email philip@philmart.demon.co.uk
>
>

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