Yet another Nixie clock...?
I think I've seen 1000 Nixie clocks on the net, everyone seems to
build one. But then, I found some Nixie tubes in a surplus store and
once I saw them lit, I (inevitably) designed this clock.
For those who haven't seen Nixies before, they're a display technology
introduced in 1955. They are glass devices containing metal cutouts
of digits (in this case 0-9) which, when given 180 volts, glow a nice
orangey/pinky/purpley glow. It seems impossible to take pictures of
their colour; something about the frequencies of light they produce
make the numbers look yellow or orange, but they're much nicer in
This clock was started in 2007 and finished in 2011... Initially, I
completed the high-voltage driver and control PCBs. Once the clock
worked, was settable and kept time (!), it went into an ugly temporary
case. It was housed this way for over 3 years; finally, at
the Make, Hack, Void
workshop, I made its final case and completed the project. (This is
It's powered from an old 12V iPod power supply and a light sensor lets
it dim itself in low ambient light, or switch off completely in
darkness. (It's designed to be a livingroom/kitchen clock rather than
sleepy-time clock, so the tube life is extended by powering them down
overnight.) Little clicky buttons on the back set the time (or alter
a time calibration setting).
Nixies and electronics
I found some B-5991 tubes (dating from 1968), having never seen Nixies
up close before. The glow, the glow! I had to make something
with them. These are front-view tubes, and arguably the
cheapest/nastiest tubes. Ah, well, they still need attention too, so
I used them.
The sensible thing to do is to use some high-voltage driver chips,
e.g. 74141s which combine a BCD decoder and high-voltage transistors.
I didn't have any of these, so used a string of 74HC595
serial-in/parallel-out shift registers with some MPSA42 high-voltage
NPN transistors. The segments of the tubes are pulled to ground with
the MPSA42s (40 in all), which are driven by the outputs of the shift
A separate PCB contains an ATMega48 to drive the whole affair. This
shifts bits serially through the '595s to select digits to light and
keeps time with an external 32KHz watch crystal. Initially I had
naively used an external 3MHz crystal to both clock the AVR and
generate a timekeeping tick — but such crystals are designed for
short-term frequency accuracy rather than long-term, and the time
drifted horribly. A better solution is to clock the AVR from its
internal oscillator (~8MHz) and drive a timer with the watch crystal,
for long-term RTC accuracy.
The power supply is a switch-mode boost converter, generating 180V.
It is driven by an AVR PWM output, and a voltage divider provides
feedback to an AVR analog input. The loop allows the AVR to regulate
the voltage. Choice of components is very important here; the diode
should be as fast-recovering as possible (lowest switching time), the
output capacitor should have as low an ESR as possible and the MOSFET
should be both fast-switching and have a low 'on' resistance. The key
to a high output voltage is a high current building up through the
inductor, so Ron of the MOSFET is important. I've had
success with the BUZ73 and IRL630A; these can dissipate a lot of
boost converter calculator is useful!
I do plan to present schematics here, but they (like the sourcecode)
will need some tidying up first. Really, though, there's nothing
particularly wacky in this circuit.
The case front is sheet copper, from the scrap merchant. I assume it
was stolen from the roof of an orphanage or church. It's tinsnipped
into shape, polished and folded by hand clamped against angle iron.
The curve was applied by hand also, leaving it a little uneven... but
hey, this is hand-made not machine-made. The rear half of the case is
recycled from the case of an old microwave oven, sawn, folded and
soldered to keep the right angle. Some old scrap iron is glued into
the base to provide extra stability and the base finished with some
non-scratch felt. For the buttons, microswitches are pushed by
filed-down nylon machine screwheads. Finally, the front is secured to
the back using some very strong magnets. The cutouts are finished
with black metal flyscreen mesh.
It looks great in my kitchen. >:)
June 2007 to November 2011
12th November 2011, © Matt Evans