Another way to do it without external circouts (exept a driver circuit i.e. ULN2803), is software multiplexing.

Set up a timer interrupt to branch approx 2ms depending on how many 7-seg displays used. I have used up to 6 on 2ms (means that each display will be updated in 2ms/6=0.33ms intervals)

1. time: Display LD1
2. time: Display LD2
...
...

Of cource. This requires select-lines for every display, but with a MUX, only two lines will be occupied.


Best regards
John E.