10 Minute Clock

I briefly had a chance to mess about in the lab last night just to continue doing some tests for my nixie clock project. I’m pretty sure I have a firm handle on how the 4017 decade counter works and the timebase is working beautifully. I also did a quick current measurement of my Hivac XN11 nixie tubes to be sure the power supply can handle them.

I first did a simple test of adding to my previous circuit with the addition of two more 4017 decade counters. I had run out of them and was forced (oh poor me) to go out shopping for more. I bought about 20 of them thinking I’ll make good use of these chips in other projects down the road. The result, before I ran out of space on my large breadboard, is a 10 minute clock that counts seconds and one digit of minutes before resetting. Video of the result is below. I used the 16Hz output from the timebase so it’s more exciting than watching paint dry or beards grow.

So yeah, that works. I’ve also been mulling over how to get the hours to reset at 23:59. Though it’s easy to get one counter to reset when it hits any number, it’s more difficult to get two counters to reset conditionally. What I mean by this is that the hours digit must count to 10 twice, then count to 4 while the 10s of hours digit counts only to 2 then resets at 3. When the 10s of hours resets, the 1s of hours should reset when it hits 4.

This I can make possible by linking the reset signals of both chips together though how I will actually do this isn’t quite clear to me yet. The original schematic I’ve had for years, found over at Mike’s Electric Stuff, suggests that I connect the two together and use an RC plus a diode to actuate the reset. The 24 hour clock instructions are written (the schematic is for 12 hour clock) rather than diagrammed so I’m trying to decipher what they mean without melting anything. It would mean that I can ditch the 4013 flip flop entirely from the circuit, being unnecessary.

It’s on the list for this evening’s entertainment. I’ll modify my 10-minute clock to simulate the hours digits for testing.

I had also given some thought to adding two more digits to the clock, which would be easy to do provided I had more nixie tubes. I was gifted a pack of 4 so that is rather limiting. I’ve found a potential source here in Toronto so I may investigate adding a seconds display to my clock. Or I may not. Reason below.

Power to the nixies

The original nixie clock schematic linked above is great I am sure, but it did scare me a bit in that it used un-isolated mains power to both power the circuit and provide a clock for it. I am not quite confident enough to mess with that especially given a certain degree of klutziness of which I am prone. My alternative was to use the schematic for a kit I found that is a power supply for nixies and neon lamps which is pretty cool. It is much safer to my mind and I can use a wall wort to power the thing. One limitation of this little switchmode converter is that I read somewhere it has a max current output around 10mA which is not much at all.

Fortunately, though they require a high voltage to operate, nixies draw very little current. I read somewhere that a typical max current rating of your run-of-the-mill nixie is about 2.5mA. Using the 33k resistor (recommended for the XN11) and 170V I am able to get the things to light up nicely. A quick current measurement from my multimeter gives me the joyous news that it draws a measly 1.25mA. So technically I can power up to 8 of them. Being the cautious sort, I would limit myself to 6 of them and/or a neon lamp to indicate the flashing colon.

I think what I will probably end up doing is just stick with the 4 digit clock and use ultra-bright blue LEDs underneath them to indicate that seconds are indeed being counted. After all, a seconds display is nice to look at when you are playing with a clock, but annoying in every other conceivable circumstance.

The LED under-lighting idea came from photos of numerous nixie clock projects and looks pretty damn cool. It would be a simple matter to add 4 ultra-bright LEDs in any colour I choose, then use a simple NPN transistor to switch them on and off to the 1Hz or 2Hz signal (whichever looks better). The addition of a capacitor would also give them a fading effect rather than an annoying blink. Going to look super-cool!

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