I have built something similar to this.
My LED works fine. The problem is I do not understand how the circuit works. The anode side, display pins 3 and 8, receive power. Lets say for now I have pin 2 of the display going to pin 2 on the arduino board.
Now in my mind the pin 2 on the arduino board should complete/ground the circuit. However the
sketch uses pinmode(2, OUTPUT) and I use digitalWrite(2, HIGH) to turn on that specific led.
How does the cathode side of this CA 7-seg display work?