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The project described in this article requires basic electronics skills and is provided for information purposes. Do not attempt to replicate it or use it for any purpose unless you've got the proper knowledge and skills.
It’s incredibly easy to hook up a 7 segment LED display to a computer’s parallel port. I first did it 30 years ago with my original Apple II and the first accessory I got for my Apple, a parallel printer card. The primary purpose for the card was naturally to hook the Apple to a printer, but the manual mentioned that it could also be used as a “general purpose, 8-bit parallel output port.”
Many new PCs, especially notebooks, don’t have parallel ports anymore. They’re easy to add to desktops though, you can get a PCI parallel port card for under $10. Even if your computer has a built-in parallel port it’s still desirable to use a PCI parallel port. That way if you make a mistake and end up damaging the port you’ve only damaged an inexpensive card instead of potentially damaging your entire motherboard.
Step 1: Intro
I used a small solderless breadboard to assemble the circuit; in fact the same one which I used to build the same circuit for my Apple II experiments three decades earlier! (The resistors and 7 segment LED are new, I tossed away the old components many years ago).
Circuits like this have been used for practical applications – a counter to indicate the progress as a computer boots up, an indicator of the computer’s performance level, etc.. I originally made this project primarily to learn a little bit more about electronics and my Apple II, but also because it’s fun. I decided to rebuild the project for a current PC parallel port for nostalgia and also to show some of the principles used with early microcomputers.
In the early days of microcomputers we didn’t have gigabytes of memory or terabyte hard drives. Every line of code was precious so programmers learned from the start to write efficiently. (My Apple II came with just 16K of RAM - less memory than a current high end calculator.) The efficiency even extended to hardware designs. Steve Wozniak came up with the revolutionary idea of using a single timing crystal. It generated signals both for the microprocessor and the color burst signal necessary for color video output. The memory was refreshed by the same circuit which controlled the video. Woz recognized that if he could do something in software rather than hardware it would save money in the long run. Once software is paid for it can be reproduced very inexpensively, in contrast hardware has to be purchased for each unit made.
I’ve seen a circuit for connecting a 7 segment display to a printer port that uses a chip specifically designed to convert incoming data into the correct signals for a 7 segment LED. All you have to do is send your data to the printer port. I’d rather save a chip and do the conversion through software.