Nixie Clock Claims To Be Simplest Design
Roseline Dyba
Something that surprised me was the display is multiplexed. For the first two days that I had this clock, I didn't actually notice it. Generally multiplexed displays are dimmer and have a slight flicker that is visible when your eyes move past them - but not this clock. It was not until I pulled out the wiring diagram that I realized that this is multiplexed. Jrgen did such a nice job engineering this that it does not have any of the characteristic dimming problems associated with driving nixies in this way.
Cathode poisoning is a problem with any nixie clock and this design mitigates the problem in a few ways. First, Jrgen provides two transition effects. The first waves the digits across the display and the second simulates a slot machine. These are shown in the video review above. Second, Jrgen has built in a feature that allows the clock to scroll the date in addition to the time. Lastly, the display itself can be dimmed or turned off entirely at night.
In the kit components, he thrown in a lead forming tool. While this is not 100% necessary for assembling the clock, it does help with the overall final build quality. I appreciate that he included one because it means that the only tools needed for assembly are a soldering iron and a hex shaped screwdriver.
Organization is key in a kit like this. Jrgen meticulously packaged the components in a dozen little bags, hand wrote labels on each one, and even included some spare parts. The bags are numbered and referenced in the manual during assembly.
There are two instruction manuals provided. The first one covers the clock operation. It is clearly laid out, color coded, and easy to understand. Each function is defined and instruction on how to set or change values are provided for each effect, setting the date, etc.
[Engineer2you] built a nixie tube clock and claims it is the simplest design. We felt like that was a challenge. In this design, the tubes are set up as a matrix with optoisolators on each row and column. With 60 segments, the matrix allows you to control it all with 16 bits. There are six columns, each corresponding to a digit. That means each row has 10 lines.
Optos? Incidentally its feasible to just use LDRs but these are made of unobtainium these days.
Some clever folks made a rotary version using a gapped disk punched with holes.
Bonus is that it can display custom figures simply by having unused lines.
ah, i assumed we were talking simplest hardware design, i.e. minimal components. Anyway, wifi is ubiquitous and there are more than 4000 public NTP servers available (check out the NTP Pool project). NTP also provides for a very accurate clock, since it always keeps it in sync within a couple of milliseconds.
Well, I suppose if you were to trust that Merriam Webster fellow. It just seems a bit counter intuitive of a term for what is considered a cold cathode device where nothing is burning, no fuel is consumed, just warm glowy plasma. By the same definition do we ignite an incandescent filament?
Modern language is so full of nonsensical derivative gibberish from a thousand dead languages! We need some standards, man! And by standards, I mean one!
I vote that we stick to the original word used by our primitive ancestors when fire was first discovered;
In my senior year of high school, the October 2006 issue of Nuts and Volts featured an article about a Nixie tube clock. The previous issue had discussed a high voltage power supply built around a PIC microcontroller. I had been experimenting with PIC microcontrollers for quite some time before and I was intrigued by the prospect of using a PIC microcontroller to generate a rather high voltage from a logic-level supply. I didn't have much use for it, though. After seeing the Nixie clock article, I now had the perfect application for it. So, I bought several tubes off of ebay, direct from the Ukraine, and got to work.
The most important part of the clock is the nixie tube. I wanted to make a clock with a low part count and a small physical size, so I settled on a four tube clock. I did some research online to determine the most optimal drive configuration for the least complexity hardware-wise. Many designs biased the anodes directly and then used either ten transistors and a demultiplexer chip or a special high-voltage demux chip per tube. I definitely liked the hv demux chip idea because it means I need one part instead of 21 (demux + 10 transistors + 10 bias resistors). Another site that I looked at added anode-side switching and then used only one driver chip. I really liked this idea, primarily because the driver chips are expensive and, just like the tubes, I had to order them directly from the Ukraine. Replacing three driver chips with eight transistors and eight bias resistors seemed like a fair trade.
In researching how I would build my clock, I looked at many pictures online of other clocks. Some were completely over-the-top, some were too simple. I wanted a sleek, modern looking clock that isn't too difficult to build. One of the ideas I liked about one of the completely over-the-top clocks is blue lights underneath the tubes. The little hole in the bottom of the spacer on the bottom of the tubes that I bought happens to be just the right size for a 3mm LED. So I bought a bunch of 3mm blue LEDs off of ebay.
To allow calibration, Timer Counter 2 is clocked from GCLK1, which is sourced from the CAL input on the card. The 1PPS input from the RTC is fed through the external interrupt controller and event system on the SAM, and is used to capture the value of TC2. This basically makes a frequency counter.
The schematic and layout were done in Eagle. The combined PCB panel size (160x70mm) is too big for the freeware Eagle tools, but I have one of Cadsoft's non-profit licenses which allows 160x100mm dimensions and up to 6 layers. I've been using Eagle as long as I can remember for hobby projects and have a huge number of parts created.
The PCBs were ordered from dirtypcbs.com - I've ordered from these folks several times now and they've never let me down. For some reason ENIG only ended up costing $2 more with this order, so I went with it - gotta say I love the look of ENIG and black solder mask together.
Pin sockets are Harwin H3184-01. These are 6mm tall, with 5mm of that length extending into the PCB. Subtracting the 1.6mm PCB thickness, they protrude 3.8mm into the 6.35mm clearance between the two PCBs - this means that any parts on the bottom PCB must be at most 2.95mm tall. I set the rule that any parts underneath the pin sockets must be 2mm high at most.
Finding pushbutton switches was a challenge. Typical right angle switches are too tall to fit between the boards, and I wanted a protruding activator that pokes through the side of the case so I wouldn't have to fabricate pushbuttons. I also wanted a through-hole switch for mechanical rigidity, which ruled out SMT options. Ultimately I ended up finding a TE branded switch with suitable dimensions and cut back the top PCB to clear the switches.
The case was designed in Sketchup. The eventual plan is to machine a case out of aluminum bar stock - I'm currently trying to figure out Sprutcam, however I may just manually machine it on the Bridgeport at work.
For a mechanical prototype, to make sure everything fits together nicely and all my holes are in the right place, I had a case 3D printed. I used the 3dhubs.com service and had it 3D printed by a local guy with a 3D printer. The surface finish on the clock isn't perfect...
I'm running the clock with an Atmel SAMD20, ARM Cortex-M0+ controller. I used this exact chip to run a lighting system for a Burning Man art car last year, and was very happy with the chip for that role, and thanks to that project I've got a bunch of software written already for SERCOMs/TCs/etc. Plus the chip is fairly cheap (cheaper than an Atmel AVR with the same pin count, even), and I like the Atmel Studio tools, so why not?
For in-circuit programming and debugging of the ARM, I'm using a Segger J-Link EDU debugger. I highly recommend buying genuine tools, for the simple reason that they work when you need them to and you can get help when they don't. The EDU costs $60, and its only limitations are some "you can't use this for profit" legalese and a nag screen that it displays every now and then when you fire up the thing.
I've also got a FTDI FT230XS USB-to-UART bridge on the card, which I use to provide a simple text menu for messing around with the internals of the clock and code testing. The FT230XS comes in a small TSSOP, has a built-in oscillator, and you can configure its spare pins to do cool things like report that you're plugged into a dedicated charging port.
This RTC is a bit spendy, but it has a bunch of features I like. It does temperature sensing/compensation, which makes it +-1ppm accurate over a wide temperature range. It does frequency control by changing the crystal load capacitance instead of adding/dropping 32K clocks, which helps with on-the-fly calibration. There is a ISL12022M part available which integrates the crystal already, however the wide SOIC package was too tall/wide to fit mechanically so I went with the separate-crystal part instead.
To back up the RTC I'm using a BR1220 lithium cell, held in place with a Keystone coin cell clip. This cell has a 35mAh capacity, which at the 1uA typical discharge current of the RTC can keep the time backed up for 35,000 hours or about 4 years with the clock unplugged. Or about 40 years with it plugged in. It'll do.
I originally stuffed an A5100 GPS receiver module and a MMCX antenna connector in the location where the backup battery currently sits, and had plans of the hour/minute buttons being time zone adjustment buttons and the clock being self-setting. But an external wired antenna is annoying and would have to be located near a window, so I decided to just yank the GPS and go to the "battery and decent RTC" solution instead. There's 31.6e6 seconds in a year, assuming I can maintain +-1ppm accuracy on the clock crystal that's +-31.6 seconds of error in a year; and since I'm setting the clock twice a year anyway for daylight savings, and the clock doesn't display seconds the error is worst case about 1/4 of a minute. That'll do.
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