The nixie tube clock consists of a high voltage power supply, seven rings counters and
an Atmel AVR processor. The power supply is shown in schematic Schematic 1. It
takes 12 volts AC and converts it to DC which drives the Maxim 1771
switching power supply to generate 160 volts. The Maxim 1771 operates by
effectively shorting the 12 volts DC to ground through the coil L1 using
the MOSFET Q1. When it turns off Q1, this releases the coil from ground and
a large positive spike is generated which is then stored in an electrolytic
capacitor C3. The diode D1 keeps the capacitor from discharging back though
the circuit. Resistors R2, R4 and the potentiometer R3 make a voltage
divider which feeds a small voltage back to the Maxim IC so that it can
adjust the pulses frequency and duty cycle to regulate the voltage.
Resistor R5 and zener diode Z1 are used to regulate down this down to 120
volts do drive the ring counters. The AC input is also used to provide a
60hz signal to the first ring as well as an interrupt to the processor.
Schematic 1 - Power Supply
Three of the ring counters divide by 6, 3 others divide by 10 and the last
ring divides by 12. The divide by 6 rings and the divide by 10 rings are
paired to count the 60hz down to 1hz, the 1hz to 1cpm and the 1cpm to an
hourly pulse. Each of the rings are similar. The difference being the
trigger circuit is different for the first two rings than the subsequent
The first two rings use a single transistor to drive them, while a 3
transistor circuit is used to provide clean consistent pulses to drive the
remaining rings. In addition to the different triggering circuits, the
minutes and 10s of minutes rings have half of the diodes replaced with the
base emitter junction of a transistor. Those transistors as well as others
are used to drive the digits of the nixie tubes.
Schematic 2 is indicative of the divide by 10 rings. It depicts the
simpler 1 transistor trigger circuit which is used in the first 2 rings
which divide the 60hz down to 1hz. Notice that the transistor is
capacitively coupled to the ring driving the positive side towards ground
in short pulses.
Schematic 2 - 10 Stage Ring Counter
The ring counters work by relying on the fact that the neon lamps require a
higher voltage to turn on than to remain lit. Schematic 2 will be used to
explain the operation of the ring.
On initially powering up the ring, one
(maybe more) lamps 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 "Pulse
In". 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 nixie 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.
Schematic 3 is indicative of the divide by 6 rings. It depicts the
more complex 3 transistor trigger circuit used in the latter 5 rings. This
trigger circuit uses the first two transistors to clean up the pulses from
the previous stage which then are capacitively coupled to the third stage
which is then tied directly to the ring. This provides a short clean pulse
to ground with each input transition from low to high.
Schematic 3 also depicts the nixie tube drivers. For driving the nixie tubes,
3 of the diodes tied to ground in the standard ring are replaced with the
base-emitter junction of a transistor. When the neon lamp lights, the
transistor turns on which lights the appropriate digit. This works for half
of the digits. To drive the other half of the digits, a transistor is tied
to the lamp side of the resistor through a resistor. The transistor drivers
are only used in the ring for the minutes and the ring for the 10s of
Schematic 3 - 6 Stage Ring Counter with Nixie Drivers
To indicate hours, a twelve stage ring (which is not shown) is used. This
ring is similar to the divide by 10 stage adding 2 stages to the ring as
well as using the 3 transistor trigger circuit.
Schematic 4 shows the CPU of the nixie tube clock. It's purpose is to track the time by
counting the cycles of the 60hz wall current and periodically resetting the
clock to the correct time afeter doing a short dance for the sake of eye candy.
The main purpose of having the CPU reset the clock is to compensate for the
inevitable mis-counting of one of the rings. Unfortunately, as time goes on,
the characteristics of the bulbs change which will cause rings to mis-count.
If it gets bad enough, the ring will have to have a bulb replaced.
Schematic 4 - CPU