Why exactly do i need transistors to drive the rows?
But why are there not also transistors for the columns? You need transistors for the columns too for the same reasons.
I haven't worked much with transistors. but the basic idea is that they amplify current in the base by it's hFE value, right? my guess is that i should let 20mA/750(hFE) through the base in order to achieve 20mA through the collector?
Collector-emitter saturation voltageVCE (sat)IC = 30 mA, IB = 3 mA ⎯0.0420.1V
It all depends on how you drive the matrix. You do not need transistors for the columns because they are only supplying the current for one LED at a time. Where as you do need transistors for the rows because they are sinking the current for the whole row.
i was looking around at digikey and the max7219 seems a bit expensive for my little project, but i found this: TLC5916. It's is a fair bit cheaper and seems to do what i want. am i missing something though?
I saw this when you first posted it and I have been waiting for someone more knowledgeable than me to answer, they haven't so here is my answer....
Do not do this next time. if you know the answer tell them asap .if you are wrong you will be corrected. do not wait for others to answer.