I’m using a 4 digit seven segment display from an alarm clock that only has 9 pins

[PaulRB]

More details on that circuit I built a decade ago.

Useful circuit: Charlieplexed 4 x 7-Segment display

Just wanted to record this little "practice project", in case I need to refer back to it, or its useful to someone else. Normally, 12 outputs from an Arduino would be needed to drive a 4 digit 7-segment display. 4 outputs for the common connection of each digit, plus 8 outputs for the segments. This circuit uses the technique known as Charlieplexing, so needs only 9 outputs. In fact, up to 8 7-seg digits could be driven with these same 9 outputs, which would have needed 16 lines using conventi…