The aim was to build a small and simple nixie clock without using a microcontroller (AVR, PIC or so), using only a common primitive logic circuits. What nixie tubes are I describe in detail here. Having no microcontroller (MCU), functionality of the clock is very simple: it displays the time and is set using two buttons.
The clock has a four digit display made up of well-known nixie tubes Z574M from RFT, but you can use virtually any other nixies. TR1 transformer (230V / 9V - 1.5VA) is used together with the circuit 7805 (with a small heatsink) as a supply of 5V for logic. To avoid resetting the clock during power outages, it has a backup battery. The battery has a voltage of 3 to 4.5V. It may be two to three AA or AAA cells or a 3V lithium cell (CR2032). Schottky diode D2 prevents reverse current from the battery to the 74141 drivers and 7805. D3 prevents unwanted battery charging during mains operation. The current drawn from battery during backup is only cca 25uA. The clock is clocked by 32 768 Hz crystal. Its frequency is divided using a 14-bit binary divider 4060 (HCF4060, CD4060, 74HC4060). On its last output there's 2 Hz frequency. The required 1 Hz frequency is obtained using a remaining binary divider in IO2. IO2, IO3 and IO4 are double decimal dividers 74HC390 (CD74HC390). Each decimal divider consists of separate base 2 and base 5 dividers. When necessary, they may be used separately. The resultant frequency of 1 Hz enters IO4 that counts seconds and tens of seconds. Tens of seconds are reset at 6 to 0 using D8 and D9 diodes. Nixies for seconds are not connected, but if required, they can be added (in a similar manner as minute display). Pin 6 of IO4 offers frequency 1/60 Hz. Its falling edge clocks IO3 - counter of minutes and tens of minutes. Tens of minutes are reset when it reaches 6 using D6 and D7, same way as tens of seconds. The pin 6 of IO3 offers 1/3600 Hz frequency. Falling edge clocks hour counter IO2 at the moment when minutes overflow from 59 to 00. Diodes D3 and D4 reset the counter of hours and tens of hours at the same time when this counter pair reaches 24. Binary values are decoded by circuits MH 74141 (SN74141N, 74141, K155ID1). Those decoders decode from BCD code to 1 of 10. They are intended for direct nixie driving. Their outputs are open collector NPN transistor with a 60V zener diode protection. Tens of hours can be displayed without decoder. States are only 0, 1 and 2. If we don't need to display leading zero, just two transistors (T1 and T2) for switching of the numerals 1 and 2 are sufficient. The separator of hours and minutes is a neon glow lamp flashing at a frequency of 1 Hz, switched by transistor T3. The nixie clock is set using buttons TL1 and TL2, allowing 2 levels of accelerated run. Circuits IO1 to IO4 should be in the CMOS version (HC series), so that their consumption is low and thus the clock can be battery backed up. When using conventional bipolar TTL circuits (eg. 74390 or 74LS390) the clock will work, but the logic circuitry consumption will be significantly higher and the battery backup will be problematic. When running from the backup battery, the nixies and the 74141 drivers are not powered up.
The nixie tubes are powered without mains isolation. Mains voltage of 220-240V is about the right voltage to operate nixies. Operation without an isolation transformer is possible if the clock is sealed in a suitable safe box and no live part is externally accessible. The backup battery must be enclosed and inaccessible from outside (don't use battery door). The buttons must be rated at 250Vac, because it's not just a "contact to contact" voltage that matters, but also "contact to human" voltage. Nixie tubes obviously are not exposed, they are safe behind a transparent cover. It is of course possible to add an isolation transformer. Nixies are powered by one half cycle of mains without filtration capacitor. Nixie current is chosen low to extend their lifetime. For Z574M and similar (Z573M, Z570, Z5700M, Z5730M, Z5740M, ZM1080, ZM1080T, ZM1082, ZM1082T) current range from 1.5 to 2.5 mA is permitted. Too small current can cause the cathode not to illuminate completely. When powered by a pulsing current, the peak current must exceed the minimum required value. Pulsed current is a method of nixie dimming. To increase the brightnes, a smoothing capacitor can be added. Resistors R8 - R11 determine the nixie current. Nixie voltage drop is about 140V and the peak voltage of a 230Vac mains is 325V, the resistor has therefore a peak voltage of 185V. The resistors are chosen so that the peak current exceeded the minimum operating current, but not too much. If the current wasn't enough to display correctly, you can reduce resistor values. I chose 82k resistors, which set peak current to about 2.25 mA (average current is about 0.5 to 0.6 mA), which proved to be sufficient for those nixies to display correctly and to be easily readable. Power consumption of this nixie clock is cca 1.8W.
Long term operation experiences:
7. 10. 2015 - continuous operation of the nixie clock started.
7. 4. 2018 - 2.5 years of operation with no failure or signs of nixie wear.