June 6, 2011 3 Comments
A few weeks ago, I was sitting around with a friend of mine who told me he was interested in building a nixie clock. We spent some time looking at parts, which caused me to gain interest in the project as well. Over the next few days, I began working out how I would approach this project.
The first issue was a matter of voltage. Nixie tubes require a huge voltage (roughly 180 volts). This is a big difference from the LED lights that I’ve been working on in other projects. Unlike a regular 7-segment LED which can run off of the logic board’s 5v line, the nixie tube w0uld require a separate power supply. Off hand, I have no idea how to build a power supply that can produce that kind of voltage. I suppose I could do some research, but I can imagine that amount of research just for the power supply of a project would be a deal breaker. Luckily, I found a supplier who sells power supply units specifically for nixie tubes (see resources in the price breakdown below). The power supply requires anywhere between 9 and 30 volts, so a 12 volt wall wort will power this just fine. First challenge solved.
The next issue was how to address each character in each of the nixie tubes individually. Again, nixie tubes don’t work like a 7-segment display where a combination of 8 digital lines, easily controlled by an 8-bit shift register, each turn on a part of the character. Instead, a nixie tube has 11 lines; 1 common ground and 10 inputs (one for each character). This means that it takes 10 different signals to control the value shown in the nixie tube. Being greater than 8, a regular shift register can’t be used. Going back to the supplier of the power supply, I found an interesting IC: the 74141 nixie driver. This chip has four inputs and ten outputs. A parallel combination of the four inputs creates 16 (4^2) possible logic codes. Each one of the first ten possible combinations is mapped to a single output. For example, if lines 1 and 4 are on and 2 and 3 are off, the #8 output pin goes high.
Each of the four nixie tubes will be driven by a dedicated nixie driver IC, thus 4 driver chips are used. Each driver chip has four parallel inputs. This means there are 16 digital inputs required to drive all four nixie tubes. Sure, there are micro controllers that have 16+ digital outputs, but I don’t want to go that route. Instead, the 16 digital inputs will be driven by two daisy-chained shift registers. Daisy-chaining two 595 shift registers means that I can feed a 16-bit output over three digital outputs on my micro controller: serial, clock, and latch. By doing the logic this way, three pins on my micro controller can produce 10,000 (10^4) different numerical combinations among my four nixie tubes. Obviously, this is a clock, so most of those values won’t be used.
My last concern was how to pipe the 180 volts from the power supply to the nixie tube while driving them with 5 volt logic. Since it is a regular on/off situation, usually an NPN transistor would do the trick, but the standard NPN transistor has a maximum load voltage of 60 volts. The required 180 volts is WAY to great for this solution. I considered a solid state relay, but at $4 a piece times 40 channels, that gets expensive. After digging around for a while, I found a high-voltage MOSFET that I think will work nicely. The Fairchild FQN1N50C has a maximum voltage of 500 volts. The drain pin will connect to one of the character pins on the nixie tube, the source will connect to ground, and the gate will attach to the corresponding output line on the nixie driver IC. A resistor will actually sit between the driver and the MOSFET, but I haven’t yet done the math to figure what resistance it needs to be.
Since this is a clock, my micro controller needs to know what time it is. I will use the DS1307 real time clock module which is available at sparkfun. This will connect to the micro controller via I2C, which only requires two lines: data and clock. It also requires a 5 volt and ground line, which is shared with the micro controller.
To produce the 5 volt power for the micro controller and the real time clock module, a simple pair of 100uF capacitors and a 5 volt regulator are used as a power source. The source for this combination will also be the 12 volt plug on the wall. When it is all said and done, it looks something like this:
So where do I get the parts? Mostly online. Below is a chart of where each part is obtained. I’m not sold on the style of nixie tube I have listed, but it is an option. I don’t have the resistors listed yet since I need to figure out what resistance I need. I’ll probably get these from the surplus store down the street for 10 cents a piece, so add $4. Similarly, the regulator and caps are cheaper there, so I’ll probably pick them up there instead of ordering through Sparkfun.
|FQN1N50C MOSFET||40||0.414||16.56||See description|
|595 Shift Register||2||1.5||3.00||See description|
|74141 Nixie Driver||4||3.99||15.96||See description|
|Nixie Tube||4||12||48.00||See description|
|Nixie Tube Power Supply||1||19.99||19.99||See description|
|MCU (Teensy)||1||16||16.00||See description|
|Real time clock||1||14.95||14.95||See description|
|100 uF cap||2||0.35||0.70||See description|
|5v regulator||1||1.25||1.25||See description|