more) lamp will light. We will assume that lamp GL3 is lit.
This will put a 5 to 10 volt drop across R3 which will
charge C3 to the same voltage through D4. If a positive
pulse is applied to “Pulse In,” a negative pulse will be
applied to the ring counter which should extinguish the
GL3. This will bring the voltage level on the resistor side of
C3 to 0 which will force the diode side of the C3 to - 5 to
- 10 volts. As the voltage is re-established on the ring, lamp
GL4 will light up first because it will have the additional
voltage of the capacitor as an advantage over the others.
Once it lights, it will draw enough current through the
150K resistor R11 to lower the voltage on the ring down
below the level that any of the other lamps will be able to
light. The lighting of lamp GL4 will then charge C4 and
the process will continue with each pulsing of “PulseIn.”
To cascade rings, “Pulse Out” can be taken from the
lamp side of any of the 11K resistors which are tied to
ground. It must not be loaded too heavily by the
successive stage or its operation will be effected. One
important thing to remember — as far as the overall clock
is considered — the stage which is used for “Pulse Out”
will be 0 for that ring. This is most important for the
minutes and 10s of minutes rings.
First, a warning! This clock uses 120 volts and 160
volts which are present throughout the board. Handle the
board carefully! The corners are “ground” and the board
can be held by them safely.
This board contains parts which must be installed in a
certain direction. The electrolytic capacitors must have the
positive lead (typically, the long lead) inserted into the
hole with the square pad. There is a small plus sign by that
pad, as well. Diodes must have their cathode inserted in
the square hole. The cathode side of the diode has a
stripe painted on it. The transistors and the regulator have
their outline silkscreened on the board. Be sure to orient
them correctly. The markings on the bridge rectifier are
faint, but they match the markings on the silkscreen of the
board. Pin 1 of the two integrated circuits has a square
pad. Putting in any of the parts backwards will cause
the clock to malfunction or damage to occur to the
component or the clock.
I do not recommend using water soluble rosin core
solder, either. My attempt at building one of these clocks
with it just left me totally frustrated. The water soluble
rosin is conductive and you need to wash the board, then
dry it completely to test the clock as you build it. Any
changes to parts of the rings involve a wash and dry
cycle, as well.
If using the available printed circut board (PCB), the
first step should be to solder in the pins for the nixie
tubes. These are small and will not stay put until they are
soldered. Insert each of the pins through their hole and
then place a piece of cardboard over them and turn the
■ FIGURE 6. Completed Circuit Board.
board upside down and solder them in on the solder side.
Once they have cooled, the board can be moved freely.
Next, the power supplies should be built; first, the low
voltage portion, making sure that 12 volts (may be as high
as 15 or 16 volts) and five volts are made. Then, build the
high voltage portion. Make sure that it generates about
160 volts. Adjust R3 until it does. About 120 volts should
be present on the cathode of Z1.
Next, the miscellaneous parts which have small
quantities should be installed. These include the 51K,
100K, 1M, 470K, and 220K resistors. Follow these with
the 20 pf, .01 µF, .1 µF, and the 150 µF capacitors. Once
these are installed, install the MPSA42 transistors and the
four colored neon bulbs which are a frosted white color
Next, install the three micro switches, the socket for
the CPU, 16 MHz crystal, two 1K resistors, and the J2
header. DO NOT INSTALL THE PROCESSOR YET.
Now we start work on building the ring counters
themselves. One at a time starting with the bottom center
ring counter, work your way around the board clockwise.
We need to test each ring before moving to the next.
In an ideal world, the neon bulbs would have identical
firing voltages. Sadly, they do not. After building a number
of these clocks, I have found that most of them will only
have one or two bulbs which don’t want to work right.
Other clocks have had five or six lamps misbehave.
Typically, the first two rings work when built. For
the rings after the first two, I recommend they be
breadboarded first using a protoboard to weed out any
lamps which don’t fire reliably (to be referenced as finicky
from now on) or lamps which fire at a lower trigger
voltage (to be referenced as erratic). Either of these types
August 2009 45