The NixieNeon clock is a product of
my love for clocks and electronics.
Ever since I was a young child,
I have been fascinated with
electronics. I remember watching
my father build nothing boxes with
neon bulbs and 90 volt bateries
which would make you tingle right
up to your elbows when you
touched them (bad influence from
my older brother). As the years
went by, my primary focus shifted to
computers and writing software but
still, I enjoy the smell of rosin when
soldering and enjoy tinkering with
My fascination with clocks was slower to develop. It
was awakened when inheriting several very old clocks
which were once my grandfather’s.
I have wanted a nixie tube clock for some time. In the
last few years I decided I would build one, but not just
another clock driven by a microcontroller where all of the
magic of the time keeping was hidden inside a piece of
silicon. I wanted a circuit where I could watch it count,
like the old skeleton clocks. Scouring through the Internet,
I ran across a circuit using neon lamps in a ring counter
which then drove the nixie tubes. At last my break-through
— a clock circuit which was as retro as the nixie tubes
At first, I tried to do it without a processor. This
worked well, but it had its limitations. First and foremost,
the neon ring counters proved to be somewhat erratic
over time. Every so often, a bulb would not fire when it
should or — just as bad — one would light when it
shouldn’t. Either way, this caused the clock to miscount.
After building several clocks of the original design, I
decided to add a processor to help hide the occasional
miscount; it also helps finding which of the bulbs are
misfiring. To maintain the original goal, the processor does
not have an active role in the clock’s operation. It does
track the time and will periodically make the rings ‘dance,’
42 August 2009
■ FIGURE 1. Completed Clock.
BY JOE CROFT
then reset the clock to what it has as the correct time. The
ring counters still do the bulk of the job. It can also spin
individual rings which hopefully will show bad bulbs.
Ultimately, I want to have the processor to monitor the
rings to detect miscounts.
Developing this clock has proved to be quite the
learning experience for me. From it, I have learned that
neon ring counters are quite sensitive. Neon bulbs do not
have a consistent behavior from bulb to bulb. As well, the
bulbs are sensitive to light. I have also found that neon
bulbs can be easily stressed by excessive current.
Empirically, I have determined even short periods of
excessive current can change their trigger voltage. So, be
kind to your neon bulbs.
In the end, the biggest lesson I have learned is that
time is not on your side with this clock. As the bulbs age,
their characteristics change. Over time, there is the good
possibility that some bulbs in the clock will not fire or fire
when they shouldn’t which will cause it to miscount. It’s
not that the bulbs won’t light; they just will not light when
they should. For me, I can live with this. The processor
will keep the clock on track for the most part. When a
bulb is bad enough or is messing up and making the
clock get way off, I’ll just replace it and wait for the next
bulb to misbehave.