After a lot of testing, it’s looking good!
So here is my more-or-less final schematic. Since I used bits ripped off from a bunch of places and cobbled them together, use it freely for your own clock project. Schematic posted here.
Please of course bear in mind that though I’ve made an effort not to make mistakes on the schematic, they are bound to be there so if you build this and hurt yourself because you didn’t check your work, well that’s your fault isn’t it? You’ve been warned. Some parts of this circuit have potential around 170Vdc which will zap you good!. I very much doubt the the circuit can kill, but I don’t want to see a test of that either. Enough, you get the point.
So I am quite pleased. Although I’ve been unable to breadboard up the entire thing at once, I’ve built it in pieces and it seems to work well. Quite low power also, I reckon the thing draws 150mA or less. I will have to wait a bit to give a measured reading on that as I accidentally melted one of the fuses in my multimeter and will seek a replacement for it.
Things put in, things left behind
There are many things I added to this circuit to give it some functionality and to make it usable. One can set the hours and minutes independently using push-button switches and in this sense works just like an alarm clock. There is also a seconds reset button which will zero out the seconds when whatever your setting it to reaches zero seconds to keep them close to in-sync. Holding down the button also acts as a seconds-hold so that’s quite useful. These three should be push-button momentaries, the type that have three terminals: a normally closed, input and normally open. The seconds reset needs to be double-pole double-throw momentary. All cheap and easy to find.
I’ve helpfully labelled the transistor pinouts as well. As it happens, I seem to have selected ones with every type of pinout combination possible. Infuriating really, why can’t they just stick to one standard one? Doesn’t matter what it is, just make it the same! All of the parts here, apart from the nixie tubes themselves, are cheap as chips and commonly available.
How it works
The circuit is divided into five basic sections. At the bottom left we have the 5V power supply (simple 7805 regulator) which powers all the CMOS chips as well as the crystal oscillator and the vanity LEDs. Moving right we have the 4521 frequency divider IC and the crystal oscillator. The resistors and caps start the crystal resonating and the 4521 divides its frequency down to the desired pulses we need for other parts of the circuit.
At the bottom right is the blue ultra-bright vanity LEDs which I plan to mound under the nixies to give them a nice nuclear blue glow that I’ve seen from other projects. Gotta have that! It pulsates once every four seconds and uses an RC timing circuit to fade it in and out. It should be noted that this circuit causes slight voltage fluctuations elsewhere in the circuit though these seem to not effect it’s function at all.
Above that we have our 12V Supply. I chose a 12V wall wort, since they are frequently over voltage anyway and as it happens it overcomes the 7812s dropout voltage to make a reasonable 11.90V. Moving right we have the small switch-mode supply that steps up the voltage for the nixies to about 170Vdc. This is quite a lovely circuit I lifted in its entirety from stuff posted around the net (see my previous posts for schematic). It is quite reliable and works well. As I mentioned in previous posts, it has a maximum output of 10mA. Since each nixie draws about 1.25mA, and each of the neons that make the separator colon draw about 1mA, we are only up to 7mA. Plenty of headroom. This saved me from having to step up un-isolated mains voltage and makes the whole circuit a damn lot safer.
The third row up from the bottom is where all the time calculation happens. Six 4017 counter ICs are used, one for each digit. Since the clock only has four nixies, the first two (the 1s seconds and the 10s seconds) are left with their outputs unconnected. Function couldn’t be easier: the 1s seconds accepts the 1Hz signal from the 4521, counts to ten then resets, passing a signal to the 10s seconds 4017 IC which counts to six and resets, passing it on to the minutes and so on. The function of the three switches is explained above and the hours in the previous post.
Finally, at the top we have the four nixie tubes and two neon lamps for our display. Each number of each digit (and the pair of neons) are driven by the MPSA42 high voltage transistor which accepts the input from the appropriate counter IC output via a 33k resistor. This works wonderfully. The two neons use the same transistor, but use the 1Hz signal from the 4521 as an input.
That’s it really. Better explanations of how this works are out there written by others more eloquent than I.
If you are going to build this up, I would highly encourage you to examine the circuit closely and test as you build it up. For a beginner project (as that is what I am still) this has some quite excellent educational value.
So now, I just have to build it up. The last challenge is, of course, to select (or build) a suitably attractive case for it. I’m hoping to hunt through a few junk shops for a nice box I can use.