Years ago I bought a bunch of IN-14 Nixie tubes from the Ukraine and I had them lying around since then. I always wanted to use them for a custom device and so I decided to finally tackle this project and build something that utilizes this almost ancient way of displaying digits, but for now I didn’t want to build a Nixie tube clock (I thought that was a bit of a cliché thing to do and for now I’ve had enough of fancy hipster clock projects), so I thought: Why not build a thermometer for my room that can be activated by clapping? I made it clap activated so that it wouldn‘t be on all the time, because I thought that was quite a waste of energy and I also didn’t want it to illuminate the room, especially during the night.
The Nixie tubes are controlled by an Arduino, which is also responsible for reading the temperature from the well-known DHT-11 temperature sensor.
Nixie tubes and high voltage
Nixie tubes are cold cathode tubes that are filled with a specific gas. Furthermore, they contain a common anode (or cathode) and separate cathodes (or anodes) for every digit or character they can display:
In my case the tubes have a common anode and the digits are separate cathodes. Unlike other tubes from that time (transistors, diodes, …) Nixie tubes usually don’t need to be heated to work properly (hence the name: cold cathode tube).
The only thing they require is a pretty high voltage, typically between 150 and 180V DC. This is typically the main problem when handling these display devices, because it means that you will need a custom power supply or step-up circuit and controllers, that are capable of switching the cathodes on and off without using too many GPIO lines.
12V to 170V DC step-up converter
Let’s start with somehow creating the necessary voltage to make the tubes glow. Luckily the typical Nixie tube needs a high voltage but very low current, which means that it is quite easy and cheap to build such a converter.
Be careful when using this circuit and high voltages in general. They are not a toy and getting a zap hurts a lot in the best case and can potentially kill you in the worst case! Always turn the power supply off before changing/servicing the circuit and make sure to use a proper case, so that nobody touches it accidentally when it is in use!
I used the well-known MC34063 integrated circuit for the step-up converter. This small IC combines everything you need for any kind of switching converter. But in this case we want to use it as a step-up converter in combination with an inductor, like described in this datasheet from texas instruments:
However, instead of using the built-in transistor of the IC, I decided to go with an external transistor, which helped keep the IC cool and also allowed me to have a higher current draw at the output. Furthermore, because It was surprisingly hard to find the right values for all these components to get an output of 170V, I gave up after some days of calculations and tests (The highest I got from 12V was 100V) and decided not to re-invent the wheel. Instead I bought a kit from eBay, that pretty much follows the above schematic with a few tweaks:
But for now, let’s look at the circuit and try to understand how it generates 170V:
The main work is done by the IC. This small chip offers everything, an efficient and simple switching DC-to-DC regulator needs. It basically switches the transistor Q2 on and off at a fixed frequency, which is set by a timing capacitor (1nF in this case). By switching the transistor on, current flows through the inductor L1 and energy accumulates in the coil. Then Q2 gets turned off again and L1 charges the capacitor C5.
R2, D1 and the other transistor Q1 are used to reduce the switching loss of Q2 and to therefore make the whole circuit more efficient.
R1 is a current limiter and R4, R3 and the trimmer are used as a voltage divider to feed a reference voltage into the IC’s pin 5. This voltage reference should be 1.25V and it’s used to determine the output voltage. Fixed values can be used, but a trimmer is not a bad idea, so you can always fine tune the output voltage and therefore the brightness of the Nixie tubes.
Controlling the tubes with an Arduino
So, as you saw earlier, the tubes require a high voltage to turn on. “So how can you switch the tubes on and off with a microcontroller, like an Arduino?”, you might ask.
There are a few alternative routes you can take to achieve this goal. For example dedicated Nixie tube drivers. You can still get new old stock and used ICs, but they can be hard to find and they can be expensive and I don’t expect them to be easier to find in the future, because these are no longer produced.
So I won’t use such a Nixie tube driver. Instead I’ll use transistors and binary to decimal decoders, so that I don’t have to use 10 GPIO lines per nixie tube. With these decoders I’ll need 4 GPIO lines per tube and one line to select between two tubes.
Additionally, so that I don’t need to switch between the tubes all the time with a high frequency, I’ll use flip-flops (which will need one additional GPIO line for resetting) to retain the last input as long as needed:
The full schematic and PCB design files will be available for download in part 3 of this series, where I’ll also talk about the transistor array, that is not shown in fig. 4.
PCB design considerations
While designing this circuit, I found decoders with built-in R/S-Flip-Flops, that are still being produced (for example the CD4514BM96). But unfortunately I couldn’t get these quickly as the delivery time was two weeks and I didn’t want to wait that long. So if your goal is to make a small PCB (or you want to have a small number of different ICs), then you should definitely go with such a chip, instead of using external Flip-Flops.
There are also inverted variants of these decoders. For example the CD4514BM965 is the equivalent to the above mentioned IC, where the selected number will be low instead of high, which is not, what we want in this case. So pay attention to this detail when ordering your parts.
You can use any type of transistor for your array, as long as the ratings match the voltage and current draw of your tubes. There are also transistor-array ICs available, but again, I couldn’t find any that were rated above 100V or that were quickly available.
Everything else regarding the PCB design will be discussed in part 3 of this series.
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