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7-segment

Can't figure out how this 4 digit display works

 

Sorry this isn't really a robot related question, just an electronics one.

I was cruising through a local electronics shop. They have lots of salvaged parts. They had a whole box of old 4 digit LED clock displays. Since they were only about $3, I picked one up. My son wanted to build a clock.

After working through all the connections, here's the schematic.