So a while ago I saw these floating around on ebay after staying up late one night looking at unique vacuum fluorescent displays I can’t afford online But this one was quite affordable and has the same resolution as the arduboy! It’s a 128x64 dot matrix multiplexed display. I went ahead an bought it, it finally arrived today.
If you’ve never heard of VFDs (Vacuum Fluorescent Displays), you’ve probably at least seen them! They’re often used at cash registers to display your total in bright fluorescent blue or green. They’re similar to neon displays like nixie tubes, but a bit different considering the environment inside the tube is practically the opposite. While nixie tubes are filled with a noble gas (neon), VFDs have all gasses removed from it. Nixie tubes require high voltages applied to a mesh anode, so that certain segments will glow with a orange plasma on whichever cathode is selected. They don’t get very hot, at most around 40C, but they still use a lot of volts, around 140-170V on the anode. Vacuum fluorescent displays, however, run at much lower voltages and use a different method to illuminate segments, but look very similar from afar.
They work much like a miniature CRT TV, without the magnetic fields. In a CRT an electron gun in a vacuum tube creates an electron beam that hits a phosphor screen, and two coils change where that gun aims on the screen, bending the electron beam, and we do this really really fast to make an entire image across the screen. A VFD Uses a much more specific method to illuminate phosphors, but they both are illuminating phosphors with electrons to create a visual effect. In a VFD, there are a few very very thin tungsten cathode wires suspended horizontally across the display. Underneath the cathode is a mesh which will be our interface for selecting segments and brightness. lastly, there is the anode, which is the segment shape coated in phosphor. The cathode is driven at 2.4vAC, so that the wires constantly give off electrons from low heat due to thermionic effect. These are naturally attracted to the Anode, so to stop all the phosphors from glowing, the mesh is ran at ~24vDC. This give the mesh a negative charge, repelling the electrons and not allowing them through the mesh to the phosphors. We can change exactly where and how much negative charge we give the mesh in order to control which segments get lit up and how bright. That’s why for something like the 128x64 dot matrix, it needs to be multiplexed. That’s a lottt of outputs. Almost all VFDs however come with shift registers and some other circuitry already installed either in-glass or on pcb that has been interfaced for you.
Phew! Now that the short tech lesson is over, we can get onto the meat: I now actually have the 128x64 VFD! The listing didn’t have a datasheet but I was lucky in that the part number was very visible in the display itself on the back, shown here.
A quick search on duckduckgo led me to this datasheet first which is pretty good!
This is probably going to be my biggest arduboy project, and I have no idea how long it will take until it’s finally done, but I want to design a driver and interface for this display that could be driven by the 32u4 and a lithium ion battery. It’s probably not going to be easy, but I’ve had a slight obsession with VFDs lately, so I’ll end up working on this until I have a reliable working pcb for it!
If anyone else wants to contribute to this with knowledge or advice, feel free to chime in! Or if you just want to learn along with me and/or try it yourself, keep an eye out on this post and ask lots of questions! I’ll update as I learn and make progress with this I think the first step should be designing a small enough power circuit to step up and down the voltage for the 24vDC and the 2vAC, which will also need a circuit to convert to AC obviously. Though I have seen this used with some minor circuitry and PWM, it seems true AC is better for the life of these things, DC tends to burn it out even in PWM. After that is figuring out how to interface the display with the 32u4 in the most basic ways.