In SX Microcontrollers, SX/B Compiler and SX-Key Tool, George Herzog wrote:

Everyone knows Ohm's Law E = IxR but forgets about Watt's  P = IxR

The ULN2803 may not be as good a separate transistors when you start mutiplexing strings of LEDs.

It is a neat little device, but by putting 8 lines in one package they share the heat build up.  And Darlingtons run a lot hotter because are really two transistors together.

I suppose the Hex D flip-flops serve a couple of purposes.  They provide a switch outside of the microprocessor that can handle a bit more power and heat.  And depending on how they are used, they can cascade and reduce several microcontrol lines down to one clock line.

As mentioned above, there are lots of matrix projects.  Mostly because there are different ways to do LED matrix and each has its own pro and con.

I suspect it is best to start with one matrix and generate indivdual characters that spell out a message.  The n figure out how to scroll Down.  Then figure out how to scroll Right.  After all that, you can move on to bigger grids or banners.

But as your project grows, the P in Watt's equation grows exponentially.  You will have to figure out a lot of tricks fo the trade to drive all that current drain to the right places.

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