Nixie and Numitron Clock

Project Logs

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Long Overdue Update
Charles Ahrens • 07/18/2016 at 03:23 • 0 comments

It's been 4 months since my last update, and I have been waaaaaay too busy to make time for this project. I restarted it in the past month or so, and I have made some progress.

I tried various combinations of the MPSA42 and MPSA92 transistors, but I could not work out a way to both multiplex the anodes and cathodes (for a total of 16 transistor circuits instead of the 60 if the tubes were not multiplexed). Eventually, I gave up on the transistors and ordered some 74141 chips from Moldova off of eBay. Unfortunately, after waiting several weeks for delivery, the chips have been stuck at the post office while I was out of town and I need to go pick them up.
I also ordered a new IV-9 Numitron to replace the one that I dropped and shattered. Once I had all 6 tubes back, I made the driver circuit with 6 TPIC6595N shift registers to see the max current draw. At 2.5V, all 6 tubes running at once, the max current draw is around 0.55 A. I believe that the wall wart I am planning on using can supply both the Nixies and Numitrons without being over the rated current.
With the DS3231 RTC connected, I started work on the display code that takes the time from the RTC and displays it on the tubes. I developed a specific animation that cycled the commas at the top of each minute, then displays the current temperature in Celcius and Farenheit.

I also redesigned the enclosure after I could not fit the components into the redwood one that I had previously made. Unfortunately, I don't know what else I could use that case for. Luckily, the department of Electrical Engineering was throwing out a lot of interesting 1940s - 1980s tech, so I snagged an old variable resistor (with 3 huge knobs and a solid wood case). I made a new 3D model of the case with the interesting knobs that I pulled from the old variable resistor. I think that I will add some rotary switches to switch between different modes, such as time & date, day of the week, etc and some toggle switches to turn off the tubes late at night so I can sleep.
Circuit Simulation
Charles Ahrens • 03/07/2016 at 05:30 • 0 comments

I used the electronics lab facilities (and software) at my college to simulate the anode switching circuit for the tubes, and it seems to be working. The power dissipated through the MPSA92 is a little worrying, though, since the simulation software says that it will draw about 3.8W, when the datasheet says that the maximum rating for that transistor is 1.5W.
When I have tested this circuit before on the breadboard, the MPSA92 (or the '42) get really really warm (and sometimes burn out) while the tube remains lit regardless of the input voltage on the MPSA42. According to the simulation, this circuit with these resistance values and the voltage lines connected as shown should work to switch the anodes for each tube on. I will test this on a breadboard when I next get a chance, but if it doesn't work, I will just resort to the tried and true method of using 74141 chips to switch the cathodes instead of the discrete transistors and leaving the anodes of each tube set to 180V.
2016 Update
Charles Ahrens • 02/29/2016 at 01:31 • 0 comments

After my last update, I moved back across the country and started my next year of school. The clock sat on my shelf, still wrapped in plastic from when I moved it, and remained untouched for 4 months. Only in December did I finally get enough time to pick it up and resume work on it. As of now, I have completed the wiring of all the tube sockets, but I am having difficulties with the anode switching circuit for each tube. I can get one tube at a time to light if I have the power supply directly connected to its anode, but attempting to have high voltage transistors in a darlington bridge as I designed is not working. I will need to figure out a slightly different plan. I think at this point I will try a few more transistor circuits and if those don't work, I will have to desolder all of the tube sockets and switch to the 74141 chips that are meant for standard Nixies. Unfortunately, these chips would not allow me to use the commas in the tubes, and I really want to keep that functionality.
Knob and Power
Charles Ahrens • 09/27/2015 at 03:43 • 0 comments

I decided on using a rotary encoder with a pushbutton integrated into it as the only source of control for the clock, to switch between the different modes (12 hour, 24 hour, date, temperature, day of the week, setting time, etc.) and I needed to machine a knob to fit the encoder since it didn't come with one. I used the machine shop on campus to lathe a small ~15 mm knob out of brass, with a brass plate that would hold the knob and power plug to the side of the case.
Woodworking
Charles Ahrens • 09/27/2015 at 03:21 • 0 comments

Before I moved back across the country, I designed and built a heart of redwood case for the clock and set some machine screw wood threaders into the middle section so that the front and back plate could mount together without damaging the wood.
After I assembled it, I used a combination of mineral oil and beeswax to finish the surface to a nice sheen. It's soft and smooth, but not slick. I might do a few more coats to make it really shine.
This is the assembled clock with tubes in. Right before I took this, I dropped (and shattered) one of the Numitron tubes, so I will have to order another from ebay.
Circuit Schematic and Transistor Tests
Charles Ahrens • 07/24/2015 at 03:58 • 0 comments

I wanted to create a circuit diagram in order to figure out how I was going to fit all the components on the circuit board and how they were all going to connect together. I don't want to spend the money to get a custom printed PCB, and I don't have the components to do my own toner transfer and etching method, so I will use a solderable perfboard to wire everything up.
As shown in the circuit diagram, the cathodes of the Nixie tubes will be run through shift registers connected to high voltage transistors that will dump the current to ground, while the anodes for the Nixie tubes are connected through another shift register and high voltage transistors in order to supply the 180V supply and multiplex the tubes.
The Numitron tubes, however, will all have their common connection to 5V, then the cathodes will be dumped through the high current shift registers. I have it designed to have 6 of these shift registers, but I might change that to have the Numitron tubes multiplexed as well. I'm not sure yet, I will have to do some tests to determine which method is better.
The test of the high voltage transistors is shown below. I wired these up with a 5V and a 180V power supply to check that the tubes would light with the correct anode resistor value, as well as not burn out either the transistors or any of the circuit. Luckily I ordered 25 of each MPSA42 and MPSA92 (NPN and PNP) so that I can have a few extra when I inevitably burn one out from faulty wiring.
Once I ensured that the circuit would work, I designed a PCB in EAGLE, with the intention of copying it to solderable protoboard with a sharpie and then soldering over it. I bought single sided protoboard, so the soldering will be on the bottom, and the top layer will have loose wires connecting the different chips and bridging gaps. As shown in the PCB, there will be only a single rotary encoder (with a built in button) to navigate the setting of the time, as well as enable me to switch between temperature display, day of the week, and date.
3D modeling
Charles Ahrens • 07/18/2015 at 23:28 • 0 comments

I'm working on a 3D model of the clock in order to determine the style of the case, as well as the spacing of the tubes relative to one another. I had to start with the tubes and sockets and move on to the wood parts of the case. I intend to make the case out of several stacked pieces of wood (I have a lot of heart of redwood left over) with minimal complex cuts so that I can produce the case with mostly hand tools and my table saw. I'm using my student license of Autodesk Inventor to model all the parts.
Wiring the First Nixie Tube
Charles Ahrens • 07/18/2015 at 05:16 • 0 comments

I wired up a connector from the IN-12 Nixie socket to breadboard header pins, with the pins in order for the digits on the tube. The high voltage anode was kept separate so that I didn't accidentally short anything while testing. I then tried to calculate the correct value of the anode resistor for the tube. I started with a supply at 180v and a 10k ohm resistor, but the current in the tube was just at the maximum value, around 3.6 mA, so I used a 100k ohm potentiometer to determine the perfect value. I started dimming the potentiometer until I reached a good brightness. 3.6 mA was too bright and I was worried about the tube, whereas 0.5 mA was too dark, and the number started getting fuzzy. Around 2.0 mA, the brightness was just right, and the number was crisp and readable. I measured the resistance and determined that a value of around 20k ohm would produce about 2.0 mA across the tube (or 2.2 mA with the decimal point and number lit).
Nixie Tube Tests
Charles Ahrens • 07/15/2015 at 08:03 • 0 comments

I have begun tests to try and determine the best anode resistor for the IN-12B Nixie tubes that I have. I used a very nice calculator to try and determine a safe value for the tube to light up, but not to shorten the lifespan of the tube too much. According to the calculator, at 180v and trying to give the tube 1 mA (well below the max of 2.5 mA), a 10 kOhm resistor should be sufficient. When I tested it, the tube was very bright and had a little bluish glow around the orangeish pinkish number. It was difficult to photograph, but definitely there. According to some sites, it's nothing to worry too much about, and just a sign that the tubes were "ultra long life" prepared, by adding mercury to the gasses inside the tube. I'm going to try some other values of the resistor to see if I can get the tube to some sort of sweet spot where it is a good brightness. I might implement a Potentiometer in the final clock design to be able to dim the tubes at night, or I might add a photoresistor to automatically detect the dark and dim them. Or I might do neither and just get used to the brightness.
Power Supply to test Nixie Power Supply
Charles Ahrens • 07/15/2015 at 07:47 • 0 comments

I needed to test the Nixie Tube Power supply, but I didn't have any 12V wall plugs, so it was the perfect excuse to finally finish my ATX benchtop power supply so that my little desk top circuit lab could have a decently regulated 3.3V, 5V, and 12V source (not to mention Ground and a bonus -12V). I followed a tutorial from this great video (I am not affiliated with them at all, it's just an easy to follow guide), where he does a very thorough walkthrough on creating a nice bench top power supply out of an old ATX power supply. I used a red LED on mine to show that it had AC power connected, and a green LED to show that it was switched on and delivering power to the screw terminal. I accidentally shorted out the first power supply that I attempted to do this to, because I forgot to connect the ground from the AC adapter back to the ground on the bottom of the power supply and I moved the top shell into contact with one of the heatsinks inside, shorting out the power supply and welding the top of the case to the heat sink. It wasn't worth saving so I trashed it and used a second old power supply I had laying around.