The first working transistor demo just had its 75th anniversary: https://en.wikipedia.org/wiki/History_of_the_transistor How to better celebrate it than having (103 times) fun with transistors? Some time back I discovered in the German magazine Funkschau 1969 #2, #3 and #10 the article "Digital-Uhr zum Selbstbau", a copy of it can be found here: http://www.nixieuhren.de/downloads/digitaluhrfunkschau.pdf It is a Nixie clock design completely based on transistors. I always wanted to build one of these. When I found out that Kicad supports auto routing (freeRouting/LayoutEditor), that became much more feasible. This repository contains the PCB designs as well as OpenScad designs for the case and the spacer for the Nixies.
Warning: Nixie tubes require high voltages which can be lethal. Please be careful!
I'm living in US, so I had to modify the prescaler from 50Hz to 60Hz mains frequency. /60 can be very easy implemented as /10 followed by /6. This allowed to re-use the PCBs of the first and second digit, just the decoding/high voltage transistors aren't populated on these PCBs. Luckily the usual PCB prototype manufactures always send a couple of copies of each ordered PCB, very handy for that.
The Schmitt trigger for converting the sinusoidal mains signal to a square wave feeding the prescaler was converted from Ge to Si. Because the circuit is small, I didn't develop a pcb for it but implemented it on a piece of perfboard.
Power source is a 12VAC 1.5A wall wart, this way no routing of mains voltage is needed. The high voltage for the Nixie tubes is generated by a small reverse operated transformer. Because of the high DC resistance of the transformer the supply is very soft and reaching the correct voltage (about 170V) is a bit critical. Therefore the rectified 12V was added in order to reach the required voltage. If this is needed or not will depend on the type of transformer. The Ge transistors of the 12V regulator were replaced by Si ones. The pass transistor needs a heatsink. A perfboard was used for the power supply.
All components are easily obtainable except for the Nixie tubes. The IN-12 Nixie tube can still be found on ebay. Because the decimal point isn't used, both the IN-12A and IN-12B version will work fine.
The boards are stacked with 25mm M3 spacers and 10mm M3 spacers. The stack will be mounted to the case.
Even more difficult to source than the Nixie tubes themselves are sockets for them. Therefore I soldered the Nixie tubes directly to the PCB. I've seen this method first in Russian test equipment offered on a flea market. For equally spacing the Nixie from the PCB a 3d printed spacer is used (in12b.scad).
The case is based on the fantastic Ultimate Box Maker https://github.com/jbebel/Ultimate-Box-Maker. The pcb mounts and the tabs are using M2.5 metal threaded inserts https://www.amazon.com/gp/product/B07NBPGTY2 .
The letters in the back panel are painted black with a sharpie.
The screws can be installed with an Allen key through the vent. For the picture I didn't have the front panel installed yet, but the front panel needs to be installed before the screws. In this version of the case the tab was 2 mm thick and I broke one off. Therefore I made it 4mm thick. It might be best to print it with 100% infill for added stability.
Things I might change if I'd redesign it:
- Move the Nixies to the center of the pcb for better looks
- Add a second mounting hole to the pcbs on the lower side (I made the usual mistake to first layout the pcbs and then think about the case)
- Change to SMD components. I forgot how tedious it is to bend the leads and clip them.
That was a fun project. It turned out to be quite easy to build a more than 50 years old design today. I fear one of today's "modern" Nixie clock designs based on an microcontroller will be much harder to build in 50 years from now, the toolchain for the controller will not work with Windows 2070, USB20 will be incompatible with USB2.0, ... ;)