The Numbers, Mason! How Many Nixie Tubes Can One Project Use?
After staring at theUnion Square clock about 100 times, I decided to replicate it at home, so that I can watch the digits scroll by on my desktop. I named the clock after one of my favorite quotes from the video game Call of Duty: Black Ops, where the protagonist is politely asked for the meaning of a certain set of numbers which are being displayed on Nixie tubes. I feel it fits the piece, as even some native New Yorkers mistakenly believe that the display is the debt clock or something (that is over in Midtown). The clock is read like so: hh:mm:ss:msm:ss:mm:hh, which means that the time is read normally at first (military time), then it goes into milliseconds, then backwards milliseconds, then seconds, minutes and hours until midnight, in that order. In the example picture, it is 19:30 with 9 seconds and 9 tenths of a second, the middle is usually a blur (set to hundredth of a second), then its 4 hours, 29 minutes, 50 seconds and 9 tenths of a second until midnight. After doing some experiments with 7 segment LED displays, I quickly realized that trying to use them (think 1990 alarm clock) was too blocky and would not give the same organic feel of the real clock. So I turned once again to nixie tubes, specifically the IN-14, chosen for their size/price ratio. The tubes for this one were procured from Ukraine. They arrived safe and sound, lovingly wrapped in wool! Great price too. Thankfully I bought 16 of them (one of them dropped and broke while I was cleaning them). These beasts run on 180VDC at 2.5mA each. This tube/chip combo is very common, so I posted a schematic/pcb layout with Arduino code at the bottom of the page. Bless this mess over between each chip and its tube on the schematic. I decided on driving them directly, as opposed to using multiplexing methods. This is because the 74741 chips used to drive the tubes came with the tubes when I bought them, and I would have no other use for them. I created the 15"x4" PCB to hold all 15 tubes and supporting circuitry behind it. The power supply this time was sourced from Taylor Edge; on ebay he goes by xtes. The timekeeping circuit is identical to my previous nixie clock: an Atmega168 controls the show, with a DS1307 keeping time. The only difference is that the sheer number of tubes made for some creative problem solving. Each 74141 chip requires 4 data lines, for a total of 60 inputs, and the Atmega had only 17 spare ones, after SDA and SCL were used for RTC communication. Digging through my array of spare chips, I decided on using shift registers. 74141 chips are BCD->NixieSegment chips, so I can use one 8-bit shift register per two 74141 chips. I drive 12 of the 15 74141 chips with this method. The center three chips are driven directly by the ATMega, because they need to be updated much more often.
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