Monday, October 14, 2013

My experience with the Arduinix Nixie tube shield

This was not an easy project to finish. Anything that could go wrong, it did, due to a rare combination of ambiguous hardware kit design (that's what started it all), and bad luck (software bugs in Arduino IDE 1.0 nevertheless). In the end, I learned a few things, which made me a better person :) :) :)

Please don't take this as a rant, nor as a (negative) review. As usual, the main purpose of the post is to document the experience and eventually help others troubleshoot similar problems they may have with the Arduinix shield kit.

The first issue I had was not getting the high voltage (180V) required by the Nixie tubes. For some reason, the provided schematic and assembly instructions are ambiguous on the exact value of the C3. This made me look at other HV power supplies, with the conclusions captured in this post. Anyone taking a closer look at the Arduinix HV schematic will notice at least 3 differences compared to others using the same 555-based design:

- the very important capacitor C3 has unusually small value (only 47pF, compared to 2.2nF, a much better value, according to the calculations);
- pins 6 and 7 are connected;
- transistor Q5 is shown as PNP (though correctly marked as MPSA42, an NPN transistor).


The assembly instructions, showing 2.2nF for C3, say that the value of this capacitor varies "the most", "as we make slight modifications and improvements to the kit". What improvements in a standard, proven and tested schematic?


What else was there for me to try? Most of the components around the oscillator, of course: the inductance, the resistors, the capacitors. The highest voltage I got was about 80V. So I decided to revert to "the standard". I cut (top side) and re-routed (bottom side, see photo) the PCB traces around pin 6, 7 and R14/16, I replaced C3 with a 2.2nF, and, unsurprisingly, I got the long desired 180V.




Next step was the software. After I uploaded the sample sketch using Arduino 1.0, only 2 digits were lit.
Three hours and a lot of effort and frustration later (even wondering how everybody else got this sketch working), I realized that Arduino 1.0 itself was the problem. Even the simplest test sketch, tried on multiple Arduinos, failed, incredibly, to work!!! (And you can try it and confirm this too.) Here it is:

void setup() 
{
  Serial.begin(9600);
  Serial.println("in setup");
}

void loop()     
{
  Serial.println("in loop");
  delay(1000);
}

Switching to Arduino 1.0.4 solved it. My Arduinix shield is now functional. Making it into a clock is going to take a few more steps though, including hardware (adding RTC, probably by resurrecting Wiseduino+), writing the software (with functionality to set up the time from buttons), and of course, the most challenging of them all, making a proper enclosure.


A few more observations:
- the INS-1 neon lamps are too tall to be used as dots between the IN-17 Nixie tubes;
- it seemed that the little Nixie PCB could be placed at the same level as the Arduino board (and under the shield, as opposed to on top, as it is currently, see the photo above); it has holes that align with those in the original Arduino 2009, but the ICSP connector is in the way though;
- all photos on arduinix gallery show the Nixie board attached to the shield by ribbon cables; I wondered a bit if my solution, using regular headers, was proper;
- this is the cheapest open source Nixie-kit out there.



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