At what point do I need to switch to external power?
If a single pin on the Arduino can handle a limited amount (is it 60mA?), then am I correct in thinking that the transistors act as a middleman, of sorts? ie,
does this mean I need to be concerned with the total mA the Arduino can supply
Sorry if some of these questions seem silly and/or self explanatory
if I made sure I was never powering more than, say, 20 leds at a time, would it suffice?
With regards to sinking/sourcing, am I correct in saying that my circuit (which is presently column scanning) is doing both
Furthermore, does this mean that if I were to accidentally light 8 columns X 8 rows at the same time, it would need to sink up to 1.2A?
(And I'd pop the sink and the 595, and set fire to my Arduino, right?)
Yes a matrix circuit needs both sourcing and sinking of current.Did you see this:-http://www.thebox.myzen.co.uk/Workshop/LED_Matrix.html
Since you had commented on that post, I assumed (perhaps wrongly) that it was correctly wired.
I recall seeing another of your posts saying the 74HC595 can only sink 25mA.
Well without seeing both the schematic and the code it is hard to know what exactly what is going on so there is no point jumping in and criticising something that you don't know 100% to be wrong. (does that surprise anybody? maybe that is not the image I give off).
Then reality kicked in
It's a sod reality isn't it.