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Dimitri, Frank, Kevin and Robin from Eectronique have sent in some details of a project that they have been working on. They had an old early 90’s LED matrix sign and wanted to make it work with a Raspberry Pi. The circuit layout is what you would expect from that era.
Read More: Old Display Reverse Engineered to work with a Raspberry Pi
“I was able to retreive an old (1992 !) bus display (the thing that says the next stops on the buses). It was old: unknown protocol, unknown wiring, not fast at all, etc..
I brought it to the university (uni of Geneva), and, with some friends, we decided to hack it. As we had some RPi’s laying around, we thought it would be cool to hack the display and connect-it to the RPi: we could then be able to display the date, next bus stops, some jokes, etc .
The display itself is very old school: all the cathodes pixels/led on a line are connected to shift-registers (daisy chained). then, the anodes of the LEDs of each line are connected to the transistors. So to display a string, you first have to send the first line, then power the first transistor. Then clear the line, send back the second one, power the second transitor, etc.. Is was very simple to connect to the RPi as the signal is only going from the RPi to the display, so we didn’t had to do voltage-level conversions.
We then wrote some software (in C: the git repo is on the website), to handle the protocol, the sending and a deamon that listens on a pipe.”