Hello, in this video I will tell you all about
VFD tubes, how they work and how to connect them to a microcontroller like an Arduino. VFD is an acronym for "Vacuum Fuorescent Display". This kind of display was very common in the
70s until the 90s of the last century, and was widely used in many electronic devices
like pocket calculaters, consumer electronics or dash panels. Nowadays, LCD or OLED dispays are used instead,
but VFDs still live in selfmade retro gadgets like my pendulum clock. I prefer them over Nixie tubes, because VFDs
need a much lower anode voltage, so it is much safer and easier to tinker with them. They are also cheaper than Nixies and I personally
like their turquoise color. I got a set of 6 tubes on Ebay, because this
is the number of tubes you need to display hours, minutes and seconds. These big 7-segment tubes were made in USSR
in the 1970s until 1990s, but there are still big stocks available and so I got them for
under 20 bucks. Let us now have a closer look on such a tube. I increased the filament voltage a little,
above the nominal 1.5V, so you can recognize it by its glow. The filament also serves as cathode. The most obvious parts are the seven segments,
whereas every segment is a seperate anodode. With a voltage of 25V between the filament
and the respective anodes, you can so compose a number or some letters. For example, to get the number 4, you must
put a positive voltage on pin 7, 6, 4 and 3 It works a bit like a CRT: The electrons emitted
by the cathode fly to the anodes, and make the luminiscent coating of the segments emit
light. You can also see the control grid between
the filament and the anodes. With zero or negative voltage, the electrons
are kept away from the anode, while positive voltages let the electrons pass. So you can switch the whole tube on or off. This feature is used for multiplexing. Now the question is how can we drive the segments
with an MCU like Arduino, when we need 25V anode voltage? We can simply achieve this with two transistors. When the MCU puts a low level on T2, the transistor
blocks and so turns T1 on. This means that the segment will also be activated. When on the other hand the MCU puts high level
on T2, T1 will block and turn the segment off In other words, this circuit inverts the input
level of the Microcontroller. But That's no problem if you invert the Microcontroller's
output byte. But the real disadvantage is however, that
you have to solder at least 14 transistors for one tube. Fortunately, there is an IC which perfectly
fulfils this task. The TBD 62783 is a transistor array with 8
channels. The input channel can directly be wired to
the digital outputs of the microcontroller, and the open emitter outputs directly to the
tube´s anodes. The Arduino Sketch is quite simple. A byte-array assigns the values to a bit pattern,
which is then written to the respective output pins I tacked evrything together on a breadboard
and loaded a script that simply counts from 0 to 15 ... That's amazing, but how can I control more
than one tube at the same time? I could of course wire a transistor array
and seven outputs of the Microcontroller to every tube. You can imagine, that a clock with 6 tubes
needs 42 IOs then. And that's where the control grids join the
game. Remember that pulling the grids to ground
turns the tube off, while positive anode voltage turns it on. So we can use the gates to control the tubes
in a way, that only one tube is active at a time. Like the anodes, the grids can also be easily
driven by a transistor array. The anodes are connected in parallel, that
means that the microcontroller shifts out the numbers one after the other. Due to the control of the grids, the numbers
are only displayed on their assigned tube. One after the other. We call this Multiplexing You can consider to place a 4017 decimal counter in front of the transistor array Because this configuration only needs one IO Port for the clock, you can use the IOs dedicated for grid control for other tasks. You can control up to 10 VFD tubes with this circuit. If you like this video, please give it a thumb up, and when you are interested in this or other projects, feel free to subscribe to my channel, enable notifications and you'll never miss anything. So, that´s all for now. Hope you enjoyed this Video. See you next time, take care
