Saturday, January 31, 2015

My experience with Axiris IV-3 shield for Arduino

I recently bought on ebay the PCB for this open source VFD shield, designed and made by Axiris, a company in Belgium. I was attracted by the solid documentation provided on their site, which includes the schematics (even Eagle files), assembly instructions, demo software, files for laser-cutting the enclosure. I should also mention the professional photos, rivaling those of the top sites (e.g. adafruit, sparkfun).

Assembly went smoothly, as expected. I also expected it to work on the first try, like the other two VFD devices I built previously, the Ice Tube Clock and the akafugu VFD clock. I guess I ran out of luck :)
But, as my kid would say, "losing is learning". This became an opportunity for me to actually go one step further than just soldering the components mindlessly (because the instructions are too easy :)

This is how I learned what fascinating little devices VFDs are. They function similar to cathode ray tubes (CRT), used in the previous generation of TVs: the filament gets heated (by about 1V, believe it or not(*)) and frees electrons; a thin grid, supplied with higher voltage, attracts these electrons and speeds them towards the anodes, which are conductors that become fluorescent when hit by these electrons.
VFDs are also similar in functionality to triodes, and probably can even be used as amplifiers (in a MacGyver-kind situation :).

Compared to Numitron tubes or even Nixies, VFDs are more complicated to drive, since they require two voltages: a very low one (around 1V) for the filament (cathode) and a higher one (between 5V and 60V, depending on the model) for the fluorescent anodes and the grid.

IV-3 tubes used in the Axiris shield require about 30V for the anodes and 0.85V for the filament. The power source to generate these two voltages is original (when compared to those in other successful VFD devices). The documentation gives a great explanation on how it works.

In any case, it only "half-worked" for me: I got the high voltage around 32V, but the tubes were still dark.
There were 2 possible causes for this:
  • either defective tubes (with burned filament) or
  • missing low voltage
Not being able to find a "troubleshooting guide", I contacted Axiris. After a few emails, their advice was to check C2 and C3 and eventually play a bit with their values (between 5 and 15 nF). Which I did, with no success.

Needless to say that the design of this power source is out of my league. The filament voltage is high-frequency AC, impossible to measure with a regular multimeter (**). But a VFD's filament, like a light bulb's filament, works with DC as well. So I tried lighting them up with 3 AAA batteries in series (since the 4 filaments are connected in series). I was able to see them glowing (until that moment I didn't even know where the tube's filament was). That did the trick and proved that the VFDs were not defective.

As an alternate (working) solution, I chose to replace the low voltage AC from the oscillator's power supply (which did not work for me) with the PWM voltage from the available D11. I cut the trace to VFIL1 (see schematics below) and re-wired to the anode of diode D1. I took out C2 and C3 (right hand side in the photo above) and connected a wire between VFIL2 and Arduino's D11.

With analogWrite(D11, 200), the 4 in-series filaments are now powered with a measurable 3.2V (3.9V from D11 minus 0.7V voltage drop on diode D1). This is not an outrageous solution when you think of switching power sources driven by pulses from uC used in some Nixie clocks.

With the hack in place, I was then able to successfully run Axiris' demo sketch (shown in action in the photo).
As they say, "all is good when it ends well".

What I would change or improve in this kit:
  • add the ability to adjust the high voltage with a trim-pot; for two reasons:
    • eliminates the restriction to use only a 12V power adapter to generate the required 30V;
    • the tubes' luminescence diminishes over time (I read); a higher anode voltage can bring the brightness back;
  • power the filaments independently (in parallel);
    • if the filament of one tube burns out for some reason, all tubes will go dark; finding which one is defective is not obvious, and will require a multimeter;
  • replace the 8-pin "power" header with a 6-pin header, to be able to plug the shield in the original Arduino 2009 as well; the left-most 2 pins are not used, yet Arduino's capacitors are touching one of them;
  • add a troubleshooting section;
    • include measuring the resistance between VFIL1 and VFIL2 being about 32 ohms; that proves that the filaments are intact;
    • include testing of the filaments with a battery for the common folks that lack an oscilloscope (for those who have one, include some info on the AC voltage);

In conclusion, I find this kit as a great introduction to VFDs. I think it is easier to understand how it works than the Ice Tube clock, since it's all discrete components (no IC driver). It is also a lot more software-hackable, being controlled by an Arduino sketch. It is more hardware-hackable through the use of Arduino shields, which may include an RTC, Bluetooth, buzzer etc.

(*) One can also start a fire with a AA (1.5V) battery.
(**) Nick, I am just one step away from using the oscilloscope you sent me. I promise to write a review on it soon.

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