Gizmos: Nixie Tube Clock

Nixie tube clock

Is there any retro-technologist who doesn’t love nixie tubes?  Originally developed in the 1950s, a nixie tube consists of an envelope filled with a rarefied gas (usually neon, sometimes with a bit of argon), a wire mesh anode, and ten separate cathodes in the shapes of the decimal digits (or whatever the designer wishes).  A constant positive voltage is applied to the anode, and the cathodes can be made to light up by connecting them to ground, which causes a current to flow, making the gas near the grounded cathode glow in the shape of the digit.  Nixie tubes are particularly easy to drive from decimal-oriented circuits such as ring counters, and were widely used in early computers, calculators, and test equipment.  A rapidly-changing nixie tube has a particularly intriguing appearance since each digit appears in its own plane and the digits consequently appear to jump toward and away from the viewer.

By the 1970s, nixie tubes had largely been replaced in the west by LED and vacuum fluorescent displays, but they remained in use in the Soviet bloc for many more years, and new old stock (NOS) tubes from Soviet and Eastern European manufacturers are readily available today at affordable prices.  Nixie tubes are cold cathode devices and have no filament to burn out, so they have long lifetimes: a well-made nixie tube can last more than twenty years in continuous operation.

I have had an inordinate fondness for nixie tubes ever since I first glimpsed one in the 1960s, and when I heard of a source for a nixie tube clock, jumped on the opportunity to obtain one.  The clock pictured above, running next to the computer on which I’m writing this document, is made by Millclock of  Zaporizhzhska, Ukraine.  They sell a variety of nixie-based clocks and thermometers: this is one of their less fancy offerings.  This model is available both in kit and assembled form.  I opted for the assembled version since it uses surface mount components and, although I’ve been building electronic gear since I was nine years old, I lack the equipment and experience for that demanding technology.

Nixie clock: circuit boards

Inside the clock are two circuit boards.  The top circuit board mounts the nixie tubes and blue LEDs for the optional back light.  The nixie tubes are soldered directly to the top board: they are not socketed.  The rest of the circuitry is on the bottom board.  A button battery provides back-up power so the clock need not be reset in the event of a power outage (the nixie tube display is, of course, extinguished while external power is absent).

Nixie tube clock: back view

Here is a view of the back of the clock.  It is powered by a “wall wart” power adapter which delivers 12 volts DC through the power connector at the left (barrel negative, pin positive).   The wall wart has a Europlug mains plug; if your wiring uses a different standard you’ll need an adapter; it runs on either 50 or 60 Hz and any voltage from 100 to 240 V, so that’s all you’ll need.  The two intermittent switches at right are used to set the clock and specify modes.  If the “S” switch is pressed while the clock is running, it will switch from displaying the time to showing the date, then switch back automatically to the time.  Pressing the “M” switch toggles among mode settings as described below.

Nixie clock: set hourThe first press on the “M” button allows you to set the hour.  Each press on the “S” button increments the hour, or you can hold down the button to rapidly advance the hour.  The hour is always set in 24 hour mode, from 0 to 23, even if you’ve selected 12 hour display (see below).  If you don’t want to set something, just press the “M” button to advance to the next setting without pressing the “S” button.

The next press on “M” advances to set the minute, which is set just like the hour.  When you change the hour or minute, the seconds are automatically reset to zero.  You can use this, while watching another clock, to set the clock to the second.  If you skip past the hour and second without changing them, the seconds will not be reset and the clock will continue to run while you modify other settings.

Nixie clock: set day of monthThe day of the month is set next, using the same procedure.

Nixie clock: set monthThe month, from 1 for January to 12 for December is set next.

Nixie clock: set yearNext, you set the year, modulo 100 (for example, 18 for 2018).

Nixie clock: adjust clock rateThe next setting is used to “trim” the rate at which the clock runs to correct it if it gains or loses time.  The default setting of 50 (note that in the Soviet bloc IN-14 tubes used in this clock, the “5” is an upside-down “2”) is the default rate of the clock’s oscillator.  You can adjust it up and down between 0 and 99 to correct the oscillator’s speed.  My clock runs just fine with the default setting.

Nixie clock: automatic date settingNext, we encounter a sequence of mode settings which are specified by single digits.  The first setting controls whether the clock displays the time continuously until the “S” button is pressed to show the date (0), or cycles back and forth from time to date (1).  Use the “S” button to choose the desired mode.

Nixie clock: set 12/24 hour displayYou can choose to show the time in 24 hour mode (0) or 12 hour mode (1).  Note that if you set 12 hour mode, you must still enter hours in 24 hour mode when setting the clock time or alarm.

Nixie clock: set backlightThe clock includes blue LEDs beneath each of the nixie tubes to provide a backlight effect.  You can choose among four modes.  If you choose 0, the backlight is always off; if 1, always on.  Mode 2 turns the backlight on between 6 and 23 hours, while in mode 3, it is on from 22 to 6 hours.  Here is the clock with the backlight activated.
Nixie clock: backlight on
I have remarked that the blue LED is the technological design cliché of the present day, and further I find that it detracts from the mellow retro vibe of the orange nixie glow.  Other than for this picture, I’ll leave it off.

Nixie clock: set anti cathode poisoningIn the clock application, the most significant digits of the hour and minute displays use a subset of the ten digits in the tube.  This renders them vulnerable to reduced lifetime due to “cathode poisoning”.  Setting this mode to 1 causes the clock to periodically cycle among all digits to reduce the impact of this and extend the tubes’ lives.

Nixie clock: activate alarmThis activates (1) or deactivates (0) the alarm clock function.

Nixie clock: beep on hourIf you’d like the clock to beep at the top of the hour (why?) set this to 1, otherwise leave it 0.

Nixie clock: set alarm hourSet to 1 to set the hours of the alarm.

Nixie clock: set alarm minuteSet to 1 to set the minutes of the alarm.

Nixie clock: alarm hourSet the alarm hour (24 hour clock).

Nixie clock: alarm minuteSet the alarm minute (0–59).

After you have set all the modes, the clock will cycle through all the digits to confirm the settings and return to the time display.

To be sure, this clock does not use remote radio, NTP, or GPS to set itself automatically.  You’ll have to reset it if it drifts, and twice a year if you live in one of those benighted jurisdictions where a coercive government makes its subjects change their clocks twice a year.  (Look at us!  We have so much power over you we can change the time!)  But this is retro-technology—none of those things (well, maybe the coercive government) existed in the heyday of nixie tubes.  What I like about this clock is not just the look, but that, sitting on the nightstand, it serves as a night vision friendly night light and its big digits are (more or less) legible to my myopic vision without fumbling for glasses.

One ergonomics note.  Between the hours and minutes tubes, there’s a small neon lamp which blinks every second.  The leads to this lamp are uninsulated, and protected only by the adjacent tubes.  If these leads should be shorted together, they’ll short the high voltage supply, which will take down the display.  If you somehow manage to touch them with your finger (which is very difficult), you’ll get a (harmless) shock.  Be careful not to disturb this bulb.

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Saturday Night Science: Fun with Cosmic Rays

I took an international flight today, and did something I’ve intended to do for some time: monitor the background radiation flux as the plane changed altitudes.  I brought along a QuartaRAD RADEX RD1706 Geiger-Müller counter which detects beta particles (high energy electrons) and photons in the x-ray and gamma ray spectra and displays a smoothed moving average of the radiation dose in microsieverts (μSv) per hour.  The background radiation depends upon your local environment: areas with rocks such as granite which are rich in mildly radioactive thorium will have more background radiation than those with rocks such as limestone.

One important component of background radiation is cosmic rays caused by high energy particles striking the Earth’s atmosphere.  The atmosphere is an effective radiation shield and absorbs many of these particles before they reach sea level, but as you go to higher altitudes, fewer particles are absorbed and you experience a higher background radiation dose from cosmic rays.  Background radiation at sea level is usually around 0.10 to 0.13 μSv/h.  At Fourmilab, at an altitude of 806 metres above mean sea level, it usually runs around 0.16 μSv/h.

I waited until the flight was at cruising altitude before turning on the detector and placing it on my tray table near the window of my window seat.  This was not a high-flyer: the plane was a Bombardier Q400 Dash 8 regional turboprop on a medium-range flight within Europe, with a cruising altitude of 7000 metres  (the plane’s service ceiling is 8229 metres, modest compared to the Boeing 747-8’s ceiling of 13,000 m).  My first reading was:

Radiation monitor: 1.24 μSv/h

Wow!  1.24 microsieverts per hour is almost ten times the usual reading near sea level.  And this was inside the fuselage of an airplane cruising at a modest altitude.

About half way through the flight, we encountered moderately high turbulence (enough to turn on the seat belts sign, but nothing really scary), and the pilot in command requested a lower altitude to try to escape it.  Air traffic control approved a descent to 6000 metres.  During the descent, the background radiation level smoothly decreased.  Here is part way down the slope.

Radiation monitor: 0.86 μSv/h

And now we’re at at the new cruising altitude of 6000 m.

Radiation monitor: 0.67 μSv/h

Finally the plane began its descent for landing.  Here are readings on the way down, with the last one on final approach over water shortly before touchdown on the runway on the coast.

Radiation monitor: 0.20 μSv/h

Radiation monitor: 0.13 μSv/h

Now the radiation level has fallen to that around sea level.  But wait, there’s more!

Radiation monitor: 0.07 μSv/h

This is at an altitude of just dozens of metres, still over water, seconds before touchdown.  Background radiation is now around half the usual at sea level.  (This wasn’t a fluke—I got this reading on several consecutive measurement cycles.)  But think about it: the contribution to background radiation from terrestrial sources (such as thorium and uranium in rocks) and cosmic rays are about the same.  But in an airplane flying low over water, the terrestrial component is very small (since the sea has very few radioactive nuclides), so it’s plausible that we’ll see around half the background radiation in such a situation as on terra firma.  Indeed, after landing, the background radiation while taxiing to the terminal went back up to around 0.13 μSv/h.

It would be interesting to repeat this experiment on an intercontinental flight at higher altitude and through higher latitudes, where the Earth’s magnetic field provides less shielding against cosmic rays.  But the unpleasantness of such journeys deters me from making them in anything less that the most exigent circumstances.  There is no original science to be done here: extensive monitoring and analysis of the radiation dose experienced by airline passengers and crews has been done.  This is a Fourmilab “basement science” experiment (well, not in the basement, but in a shrieking aluminium death tube) you can do yourself for amusement.  If you do this on a crowded flight, your seatmate may inquire what’re you’re up to.  “Measuring the cosmic radiation dose we’re receiving on this flight.”  This can either lead to a long and interesting conversation about atmospheric absorption of cosmic rays, background radiation, and radiation hormesis or, more likely, your having an empty seat next to you for the remainder of the flight.  Think of it as win-win.  There were only seven passengers on this flight (I don’t go to places that are too crowded—nobody goes there), so this didn’t come up during this experiment.

Return Flight

A couple of weeks later, the return flight was on an Embraer E190 regional turbofan airliner. The altitude of the flight was never announced en route, but this aircraft has a service ceiling of 12,000 m and usually cruises around 10,000 m, substantially higher than the turboprop I took on the outbound flight. I expected to see a higher radiation level on this flight, and I did.

Return flight: 5.07 μSv/hour

Did I ever! Most of the readings I obtained during cruise were around 3.8 μSv/h, more than thirty times typical sea level background radiation. (I’d show you one of these readings, but there was substantial turbulence on the flight and all of my attempts to photograph the reading are blurred.) During the cruise, I got several substantially higher values such as the 5.07 μSv/h shown above—more than forty times sea level.

Why was there such variation in background radiation during the cruise? I have no idea. If I had to guess, it would be that at the higher altitude there is more exposure to air showers, which might account for the greater variance than observed at sea level or lower altitude in flight. Or, maybe the gremlin on the wing was wearing a radioactive bracelet.

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