Part 2 - Project Specifications

The Goal

The desired outcome of this project was to create a neat desk ornament that would be visually appealing but also somewhat unique. It also needed to have a compact footprint, needed to be easy to power, and, above all, it needed to be safe (as highlighted in the foreword, this was to be a gift for a good friend of mine).

The Building Blocks

The sections below will outline each of the key issues that need to be tackled in this project.

The Focal Point

Since the primary objective was to provide a neat visual effect in a unique manner, what better way to achieve this was there than to go with a Nixie tube? These are stunning to look at with their neon-orange glow, and have an intricate metal grid inside; all this combined with the vintage aesthetic of their glass tube housing. To further this effect, I opted to go with an IN-19 symbol tube as opposed to one of their standard numerical cousins such as the IN-14 or IN-8 tubes. This has three benefits: firstly, the symbols are arguably much more pleasing to look at than a standard array of numbers from 0-9 (regardless of how randomised they are in the order in which they are displayed); secondly, they are actually somewhat easier to drive than the numerical tubes owing to the reduced number of cathodes (8 as opposed to 10), this will become especially apparent later on; and thirdly, they are often available much cheaper, largely due to the lower demand since one cannot reasonably make a stereotypical 'Nixie clock' exclusively from symbol tubes. As a side note, I opted to use the IN-19V (as opposed to the other '-A' or '-B' variants, although this was down to personal preference.

The Power Supply

The next most important building block would be choosing a power supply for the project. Nixie tubes require a relatively high voltage in order to ionise the neon inside the tube (typically this is specified at no greater than 170V for most standard-size Nixie tubes). This was not the only issue however, any logic circuitry will typically require a low voltage, such as 5V or even 3.3V, and generating two voltage rails with such a range is often challenging. Immediately, two ways of theoretically achieving this are:

Using a high input voltage and regulating it down to a much lower voltage for the logic circuitry, either linearly or by using a Switch Mode Power Supply (SMPS) in a buck configuration or similar.
Using a low input voltage and stepping/regulating it up to a much higher voltage in order to run the tube.
Using the mains as an input source for the tube and using a conventional line-frequency transformer to run the logic circuitry.

Practically, however, only option 2 is really feasible in this scenario. Option 1 is definitely not the way to go here, high voltage sources (other than mains electricity) are likely to be large and/or dangerous and/or expensive and/or difficult to obtain, not to mention there are very few ICs that can handle such a high voltage at the input, regardless of whether they are linear regulators or not. Technically, one could use a resistor and Zener diode to create a crude voltage rail for the logic circuitry, or possibly a discrete transistor + op-amp style regulator if more precision was required; however, the power dissipated by the regulating element would render this highly impractical.

Likewise, going with option 3 and using the mains, whilst mitigating the problem with finding a power source, still introduces the bulk of a line-frequency transformer, as well as the issue of having lethal mains voltage (and current handling) capability which is absolutely a no-go in this project.

As mentioned, this leaves option 2 as the only viable path; however, it does open up possibilities with easily accessible (and safe) low-voltage sources such as USB power, or potentially even battery power. The regulating circuitry can be made small and lightweight as well, and the high-voltage output can be somewhat current limited to far safer levels than any standard mains outlet can be with conventional circuit breakers and fuses.

The Tube Driver

Having established that a Nixie tube is the desired focal point, a compatible driving circuit is definitely called for, exposing any logic circuitry to 180V will annihilate it completely. This will inevitably involve some sort of transistor-based solution and, being a Nixie tube, the conventional manner of driving it involves pulling a single cathode at a time to circuit ground; other cathodes should be left to 'float' (be placed in a 'high-impedance' state). Conveniently, this is exactly the effect created by an NPN transistor in an 'open collector' configuration. As a starting point, this is more than sufficient information, a more tailored solution can be developed at a later stage.

The Logic Circuitry

Again, the main focus with this circuit is a nice visual effect, these are usually fairly complex and involve randomisation (to some degree at least). Nixie tubes also require their cathodes to be periodically cycled through in order to prevent 'cathode poisoning' of unused cathodes, this will likely warrant some degree of 'smart' control logic. The easiest way to implement all of this will be with a microcontroller-based solution, not necessarily a full-blown Arduino board with a bunch of breakout modules and shields, but some intelligence over operations, plus the ease of changing and tweaking the effect with a few lines of code is definitely a luxury.

Summary

Now that the building blocks have been established, and a rough solution to all the issues has been devised, attention can turn towards developing the final circuit. This is what will be discussed in the following parts.

Part 1

Project Contents

Part 3