Nixie tubes are one of the first digital displays. Meters became old school! They are actually little neon lights, with anodes in the shape of the numbers. This allows for a much more pleasing font than segmented displays, like L.E.D.s. One of my most prized possessions when I was younger was a Hewlet Packard voltmeter with a Nixie display. The day it fell of the bench and broke for good was a sad day indeed.
B-5870 Nixie tube
I've always been a tube fan. The first really serious electronics project I ever built was a tube superhet short-wave radio. Took me three years! But I've always had a soft spot for the warm glow of tubes ever since. One day while surfing the net looking for a WWVB receiver to use for my seismographic station, I ran across a page that had a Nixie clock. That was it, I had to make one. Just after I had decided to make the clock, someone offered to sell me a WWVB receiver and decoder cheap. Serendipity! And thus, the Atomic Nixie Clock came to be.
The Atomic Nixie Clock!
The system works like this. WWVB is the call sign of the atomic clock in Colorado. It transmits at a very low frequency, 60kHz. This allows the signal to travel far, but requires a sensitive receiver to pick it up. Then the signal must be decoded. In most areas, one cannot get reception all the time. So, to perform this function, I used a model 321B receiver/decoder from Ultralink. I had a little trouble getting the unit to work, but Ray got me going.
Not only does the unit receive and decode the WWVB signal, but it has an accurate clock of it's own. This allows it to keep track of time during periods of poor reception. This is a great little unit. Unfortunately, they don't make it any more. They do make an improved, and thus more expensive model.
The WWVB reciver and 433mHz transmitter
Once the WWVB signal has been received and decoded, a PIC12F675 micro-controller queries the 321B for the time. It then passes the data to a 433mHz transmitter, that I got from Bruce at Rentron. Bruce is really helpful, he basically taught me how to program PICs via e-mail.
So now we have time being broadcast at 433mHz all over my house. I did it this way because I had two projects, the seismograph and the Nixie clock that both needed the 321B. So, I had to figure out a way to split the signal, and I happened to have the RWS/TWS433 transmitter and receiver pair laying around. As well, any other project that I want to have a cheap and very precise time base can use an inexpensive receiver and have accurate to far less than a second time! Muhahaha!! And I really get a kick out of calling the time and watching the Nixies turn right on the beep. No buttons to set the time, and it does daylight savings time, of course. The lazy man's clock!
One the clock end of things, there is a 433mHz receiver. A Basic Stamp BS2SX is used to run the clock. It queries the receiver for data, then passes the time to the shift registers. Jon Williams, from Parallax (makers of the Basic Stamp) helped me out on the code by showing me some cool programming tricks to improve my clunky code. Heck, after he was done with it, I only had to add a dozen lines of code or so! You can download the code HERE.
Nixie tubes require fairly high voltages to operate, around 170VDC or so. Each tube has at least 10 pins, one for each digit and the cathode. So, specialized chips are required to operate them. There are a number of modern options, but I ran across a stash of the original chips that were used with Nixies, the SN74141N. This chip handles switching the high voltage, and reduces the number of inputs required for each Nixie to four, a substantial improvement. I have some for sale, US$1.50 each plus postage.
While reducing the I/O required to 4 pins per Nixie is nice, it still leaves us with 24 I/O pins needed for a six digit clock. That would mean using a micro-controller with at least that many pins plus some for other functions. So three MM74HC595 shift registers are used to reduce it to two. Now that we can handle!
Whew! Look at that rats nest! The SN7141s can be seen in a row behind the Nixes. Back row, from left to right, is the receiver, the three MM74HC595s and the Stamp. No wonder the motivation to develop printed circuit boards, eh?! All the other stuff on the board is not really used by the clock, just me messing around with various features.
I have been working on getting the Atomic Nixie Clock off of the breadboard. I have decided to make the clock with three layer of circuit board. The top two layers are shown here.
The top board is basically a holder for the Nixie tubes. The second has the SN74141s mounted on it. The boards nest together like this:
Eventually, I will find time to finish the third board, which will hold everything else, and reclaim my breadboard! It might be a while, because I am still fiddling with the circuit, adding features and so on, and time is as always at a premium. I haven't bothered to make a schematic or post the code, as I really doubt anyone else is going to make a clock like this. If anyone is interested in code, circuit or PCB artwork, e-mail me.