Arduino: Double seven-segment using only 9 pins

A couple of days ago I finally managed to get my hands on a Arduino Duemilanove, and was desperate to find something to try it on.
I got my hands on the double seven segment you see there, and wanted to hook it up so as to be able to print numbers on both digits at the same time. However, since I don't have any sophisticated hardware, and my Arduino doesn't have all that many pins, I had to do some thinking in order to get it done.
Eventually, I managed to hook it up as it appears on the video, using only 9 pins: 1 for each of the seven segments - not counting the decimal point - and one for the common cathode for each of the digits in the seven-segment thingy. I then just swap the voltage on the cathodes from high to low very rapidly, making the current go first to one digit and then to the other fast enough for it to appear to be simultaneous (but it is not!).
I love this!

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• arduino
• seven-segment
• breadboard
• make

[ljandersson]

Strobing the common cathodes is pretty clever! You can probably do it in three pins though if you use shift registers.