I dreaded getting to work with high voltage and transformers. Schematics with dotted lines around an area with words like 'touch this and DIE!' don't bolster my self-confidence. Especially not when I'm doing something for the first time.
Luckily, someone pointed me to the fact that, with an 'electron pump', you can get high voltages from a (relatively) safe wallwart. There are schematics for that kind of thing circulating on the internet, but I decided to buy a kit that does that -- I'd rather concentrate on the driver logic than on the power supply. Maybe next time I'll build the powersupply from scratch, but not this time.
The driver logic for this particular type of nixie is pretty simple: put 170V through a resistor of 33K on the anode, and drive transistors for the various cathodes to let the filament glow up. Of course, you'd need transistors that can deal with the high voltage -- but Futurlec has those for a very reasonable price.
There are BCD-to-decimal drivers, the 74141. Most clocks use these, but that would mean I'd need 4 pins per nixie. At 6 nixies, that would mean 24 pins -- and I have, at most, 11 pins on the 16F628A to do the work with.
Instead, I decided to use 4017's, a decade counter. Instead of the ICs driving eachother, I've decided to drive each 4017 separate, with a common reset line, which nets me 7 lines to drive the whole shebang. Maybe I'll add a DCF77 receptor, so that I can also display the date -- but that's something that can be decided on at a later moment.
All in all, that's a lot of components and a lot of traces. Instead of packing everything very tightly together on lots of stripboard like this guy did, I have decided that I should start etching my own PCBs for this project. Learning the Eagle Layout Editor is taking some time, but when I get up to speed with it, this should prove to be a huge timesaver for larger projects.
Eagle comes with lots of libraries, but you can also create your own library with parts. Of course, there was no IN-14 nixie tube part, so I made one using the Sockulator.
It took me all weekend, but now I have a single-sided PCB layout with a 12-pin header (anode, cathode, number 0 to 9), 11 resistors, 10 transistors and a single nixie. This will be placed on top of the layer with the 4017's and the 16F628A. I think the clock will turn out to be 5 cm wide and 25 cm long -- quite acceptable.
The schematic. I've decided not to use the dots, so they are unconnected. The ordering of which resistor is connected to which pin on the header has been determined while making the layout, to prevent unrouted connections. This is why the pins are not in order -- I'll deal with that when routing the signals of the 4017s to the pin headers 'downstairs' -- I'll have more room there.
The layout. Pin header on top, resistors (upright!) underneath, then transistors, then the nixie. All in all, 5cm wide and 3.5cm long. No wire connections, single sided.
This projects is going to be... interesting.