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MAKE subscriber Florin points out this thorough rundown on 7-segment display usage, even covering the basics of multiplexing with the common component -
Since the digits share the same data bus, each digit will have the same number displayed, like the wheel on the picture, to change the number the “data guy” rotates the wheel. So how can we display 1234 you might ask, well wee need another guy, the selector, which will leave only one digit to be seen, all the others are shut off, by synchronizing the “data guy” and the “selector guy ” so they operate at the same time, when the wheel is at the 1111 position, the selector opens the first window, when at 2222 it opens the second and so on. By changing the data and selecting the digits at many times per second the human eye will see a steady image with 1234, the display refresh rate should be above 50 times in 1 second, otherwise the image may flicker.Whether you consider yourself a "data" or "selector guy" (or gal), the tutorial seems handy way to get acquainted with such concepts.
Or just use a MAX7219 IC combined w/ an Arduino:
http://www.arduino.cc/playground/Main/LEDMatrix
Much easier...
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This is how a 7219 works. And it's important to know how your building blocks work so when something goes wrong (and it always does), you're better equipped to troubleshoot.
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I once need 3 seven segment LEDs to count to 999 for a project. I created the wiring diagram which connected the LEDs to three decade counter chips. In my excitement I wired everything together using perf-board and wire-wrap upside down. That is to say I read the diagram rightside up but connected all the wires with the board upside down looking from the bottom. This created a perfect mirror image wiring of the chips and LEDs. When I applied the power it didn't smoke but just turned on as if all was just fine. With one very scary issue...The LEDs displayed 666 all the time! Never counting up or down. I'm not kidding this really happen and can be reproduced simply by reproducing my mirror image wiring error. Once I noticed my issue I rewired the LEDs, decade counters, latches correctly and everything worked just fine. But WOW very spooky!
Todd Harrison
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This ignores the technique of charlieplexing and so wastes 4 GPIOs.
(Not that charlieplexing is problem free mind you)
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