Arduino Nixie voltage booster: 12 V to 200 V

Nixie PSU kits are cheap and robust, but sometimes they can be "too much" for a project: too big, too costly, too powerful . It is the case of single tube circuits, where the current requirement is pretty low and board/case space too.

The Net is full of projects that use the 555 in place of specialised IC's: a PWM signal controlling a MOSFET it's (almost) all that you need. Since my circuits are usually based on Arduino, why not use its embedded PWM generator? Again, there is at least one fully working Arduino code and diagram on the Net, and that's where I started (thanks to Ian of nixieclock.biz).

While I could get clean HVDC, it was too high: even more than 350 V! But but but it would drop to about 200V when current was drawn. It behaved like a far-from-ideal voltage generator with a high internal resistance.

All of this could be fixed with careful run-time trimming of PWM parameters in software after a thorough (software) calibration, but still the firmware would lack the real-time response provided by specialised circuits. So I went a different way.

I used a Zener diode to keep the voltage at 180 V. When the Nixie lights up it "pulls" the current its way, effectively cutting out the Zener. The trick here is to calculate the drop resistor Rz before the Zener in a way that it will load the power supply slightly less then the Nixie. When displaying a digit this Rz becomes part of the anode current limiting resistor you need to add anyway. Since the Nixie sustain voltage is less than the striking/Zener voltage, you need Ranode > Rz with Ranode = Rx + Rz. Just do the math in such a way that the anode current is a few hundreds of microA higher than the "stand-by" current.

Yes, I know I am wasting power into the Zener, but this way I keep control over the maximum voltage if something fails upwards in the PWM chain. Moreover, since I fully control the firmware, I can save power by turning totally off the voltage booster when nothing is to be displayed. Last but not least, as in the original circuit, a resistive voltage divider lets me read the output value though one of Arduino ADCs.

I will share the diagram in a second post on this topic.

From Eagle to KiCad

For Xmas 2016 I gave myself a little time to test KiCad as a replacement of Eagle.

Even if I used it for very few hours, I could not get the right feeling with Eagle interface and most of the boards in my wishlist would not fit the free version limit.

So I jumped in the Hack-A-Day tutorial series on "Creating a PCB in Everything" and tried KiCad. I love their old-school keyboard shortcuts. I love the way the interface responds when drawing a schematic diagram. I could easily (cough... I had to) design my own symbols for Nixie and VFD and their respective footprint.

So far it has been a positive and productive experience with ZM1332/5870S and IV-6 drawn in KiCad, both symbol and footprint.

I need to define a couple more tube display components and then I'll work on my first PCB to send to a fab house.

Meanwhile I keep up-to-date my own KiCad libraries on github.

Nixie Tube Ciapapuer - 3 - final firmware

Lately I have pushed to github a new version of the Nixie Tube Ciapapuer firmware.

The main change lies in the way the random number generator is initialised, that now relies on an external library called Entropy. For simplicity I added the library in the github area too. It relies on small speed differences of oscillators within the Arduino microcontroller.

A second change is that the depoison routine (or slot-machine effect) can go either backwards or forward, and it's all chosen randomly.

That's it.

Ham calls in a stationery store

I was walking in the neighborhood this afternoon and at the stationer's shop window I saw these bags:

They show supposedly U.S. vanity car plates with HAM callsigns. W6HQF seems to be expired, but NCA is not.

Looking up other callsigns is left as an exercise to the reader.

My first Nixie based instrument at work

Since the Philips PM2422A instrument works, it has been put into use. Here it is measuring a 1% (vintage) 10.7 Mohm resistor.

The reading has been jumping between 10.71 and 10.70, and finally settled on the latter value. I wonder if there is a warm up time for these instruments?!

The picture also shows the emulated "1" leftmost symbol.

My first Nixie-based instrument

At the ARI Torino HAM rally few Sunday ago (Nov, 27th 2016) during my third or fourth tour of exhibitors I spotted what looked like a box with Nixies. The box read "Philips PM2422". A quick picture search with the smartphone confirmed my suspicions: a 3 and half digit multimeter, with Nixies. I was even able to power it up right there, and the tubes happily glew.

 

I paid 30€ and I think it was the right price. The seller then realized he could have asked more, I nodded and I replied that I did not negotiate the price on purpose.

 

Once home I opened it, and everything looked fine. No fat electrolytics, no sign of burns. But a surprise: there are only three tubes inside, not 4 or 4+1 as in the PM2421. The most significant digit is a long neon lamp, so there's implicit leading zero blanking. And the minus sign is another small neon lamp. That's it. Cheaper than full feature tubes but effective. I think more recent versions of the same instrument carry four tubes. I got a PM2422/A2, while they went up to /A5.

 

Finally I own a bench multimeter!

 

Oh, by the way, can it be a clock? Well, it can count from 0 to 1999, so as a 24h clock it would work up to 19:59. It is suitable as an office desktop clock since most people leave way before 8 pm. Or simply make it a 12h clock. I want to explore if a simple DAC produces a stable enough reading, soon.

Nixie Tube Ciapapuer - 2 - video

If you are curious how the Nixie Tube Ciapapuer looks, here's a short video:

In the end I dropped the fading effect descried in the previous post and opted to include an "off" step in the sequence.

The object is now sitting on my office desk and pretends to be reading visitors' minds. J One funny moment was when a colleague picked up the box and the "off" moment kicked in, that caused a bit of surprise and worry. I should include an accelerometer and intentionally switch off the tube when the box is moved.

Nixie Tube Ciapapuer - 1

Or, in plain English, "Nixie tube home decor".

Literally: "Nixie tube dust collector."

Why, why would I ever want to build a device with just one Nixie? First of all, to use symbol Nixies like ZM1001 (side view) and B5094 (top view). Then because: housing is simpler than using several tubes, firmware is pretty simple (no RTC or strict timings), circuit is simple, it will result in a compact object.

My first attempt is with a ZM1001 that has 6 symbols, so there are 6 cathodes to switch. The typical 74141 driver has 10 outputs, nevertheless since I am lazy I choose it for this project, thus reducing the connection to uC to 3 data lines.

In order to differentiate from my previous projects and learn something new, this ornament firmware will also:

• randomize transitions and next symbol
• fade-in/fade-out

Fading requires one more control line. One way is to PWM the anode voltage through a PMOS or PNP: this requires a component with high Vds/Vce (that is missing in my junk box). Another way that is doable in this project is to switch the BCD combination to unused 74141 outputs. Either way uses a 4th output from the microcontroller, but the latter is "low voltage" and implemented in the basic circuit "by design."
So, going for fading through switching to unused outputs, a look at the 74141 truth table indicates that I can use the MSB (Most Significant Bit) to switch over to unused outputs (8, 9). Indicentally I had not planned to use those outputs, so now it is a matter of writing the proper software routine.