Working with VFDs


We love old display technology, like Nixie tubes, but they’re often difficult to work with because they require higher voltages than digital logic. Vacuum florescent displays (VFD) fall into this category. While not necessarily “old”, they are becoming far less common than LCDs. The main benefit of a VFD is that it actually produces light directly; it doesn’t require a backlight. You’ll find these displays on various players and appliances: CD, DVD, VCR, microwaves, stoves, car headunits, and others.

[Sprite_tm] had written off some VFDs, but recently revisited them with renewed interest. He started by testing what sort of voltage would be required to drive the display. It took 3V for the filament plus 15V to drive the grids. There are VFD controller chips available, but he wanted to get this working with what he had on hand. He had experience with older 40xx series logic, which can be powered by much higher voltages than 5V 74xx. His final schematic has three 4094 serial to parallel chips with an ATtiny2313 controller. A 5V power supply is dropped to 3V with diodes to drive the filament while a boost converter brings it up to 15V for the 4094s that switch the segments. While the code is specific to this display, it would be a great place to start your own project.

12 thoughts on “Working with VFDs

  1. I have a few displays laying around. I might just dig out some old ones I got a few years ago when Nixies were all the rage. My only problem is what to use it to display. anyone have ideas?

  2. it’s worth remembering that vfds can do more than merely display information – as they’re thermionic devices it’s possible to do so much more if you enjoy fiddling for it’s own sake. :-) shows you how to use them as amplifier tubes, for instance – and pretty much anything that can amplify can also oscillate if you want it to – and occasionally when you don’t!

    a nice new-year challenge would be to build a radio using vfds as the only active components – the internal construction of some will allow you to find two or more triodes in a single package once you figure the pinouts, so an am-only receiver for your local station with sufficient output for a small speaker or some walkman headphones shouldn’t be very difficult. ;-)

  3. richard that sounds like good reading to me.

    I wouldn’t know where to begin, but I’m more than ready to live vicariously through others when it comes to esoteric projects like that.

    this itself is a very cool project!
    major stuff from spritesmods

  4. @richard

    Tom Kipgen is inspiration for unusual and gorgeous tube and crystal radio design, and he has two posted, each using one EM83 magic eye tube, the Green Tree and Winking Wedge sets.

    Apparently, they are both based on the first schematic by Jara “Krysatec” (“Rat”) on the following page.

    If anything I make ever looks half as good as Tom’s stuff, I’ll consider my life complete.

    1. Sorry, Boris, but it doesn’t work like that. Consider that if you replaced the filament with a series of n resistors, each of 1/n times the original filament resistance, the voltage drop would be the same across each resistor. Consider also that the current is obviously the same from one end of the filament to the other, and heating depends on current. The fact that the voltage with respect to one supply terminal is different at one end of the filament than at the other end is immaterial.

      1. The voltage different is between the filament (Cathode) and the Grid Voltage (Anode).
        So if you use a DC drive, the difference between the filament and the grid *will* be different for each of the digits. By using a symmetrical AC drive, you even out the difference.

        I used a ZVS to drive the filament as well as provide the high voltage DC.

        1. I’m sorry, I’m still not “seeing” this. “Symmetry” is not the issue here. The issue is whether one end of the filament voltage is common to one side of the grid voltage. e.g. If there’s a transformer in either path, and there is no connection to ground on the output side of the path, there is no problem even if the filament voltage is rectified to DC.

          In any case, I haven’t seen this issue on an 8-digit VFD; the difference in brightness arising from from 15V grid-to-filament at one end vs. 12V at the other was not noticeable to my eye.

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