I see this connections for 4-digit display.
All used pins set up on output. But, really, is it correct connections?
Why it used A* input pins, and, digital pins set up on output, it is right?
Maybe all pins need used by digital pins with NPN transistors? Collector is 5V, Base - Arduino digital pin, emitter - resistor with display anode, next, collector is display segment pin, Base - Arduino digital pin, emitter - Ground.
What resistance is needed for the resistor? I think, more than 220 Ohm?
Sorry, but I'm really a beginner in electronics.
(5-1.8)/220=0.014
amps is within the UNO's allowed consumption, so you're fine there. that said, i would invest in amax7219
to free up all those pins and make coding simpler with existing libraries. – dandavis Mar 8 '17 at 16:40