Alongside building the "BCD-Seven" clock, I got interested in building the nixieclock I've been thinking of for a while. Today, January 4th 2013, I designed the schematic. Note that the powersupply design is not complete yet. It basically consists of a frequencydivider, hours and minutes counters, display drivers, and the four nixietubes themselves. And, of course, a power supply. What's soo awesome about the 7490 is that it resets on 10 automatically, and that it has a built-in AND gate on it's reset inputs. So you don't need any external AND gates! Here's is how it works.
The frequencydivider divides the 50Hz coming from the mains by 3000 to get a one minute pulse (60s pulse) which then enters the 1s minutecounter (of course you could also divide by 50 (or 60 if you're in the US) and then by 60 to get the 60s pulse). As the 7490s reset themselves when reaching 10, I don't need no stinkin AND gates here. The signal from Q3 of the 1s minute counter goes to the input of the 10s minute counter. The 10s minutecounter resets on 6, and carries over the the 1s hour counter from it's Q2 output. The 1s hour counter carries over to the 10s hour counter from it's Q3 output. Because this is a 24h clock, the 10s and 1s hour counters resets when they reach count 2 and 4.
For the BCD to Decimal decoders it's possible to use either 74141s or the russian "K155ID1" which is fully pin compatible with the 74141. I used four K155ID1 ICs as they were a lot cheaper to get hold of.
As you can spot in the schematic, I make use of two 7486 XOR gates on the clock input on both of the hour and minute 1s counters to make setting of the time possible even if the preceding counters carry over is high. The two 74132s schmitt triggers (I have tons of them) I use for debouncing the hour and minute time-setting pushbuttons. There's also a reset button for resetting the hours and minutes counters, that's why I use the four OR gates in the design.
The powersupply is simple. A 7805 regulates the rectified 10V down to 5V. To get the 50Hz I use halfwave rectification, a resistor, and a 4.7V zenerdiode to pull the 50Hz down to 4.7V which then enters the 74132 schmitt trigger which makes the signal all crisp and pretty prior going to the counterlogic.
That's all! A pair of ugly schematics are available down below. If you have any questions, please contact me.