If you missed part one please click http://neilb.wordpress.com/2007/03/10/nixies-and-pixies-and-numitrons-oh-my/
Let’s take a look at the anatomy of a typical NIXIE tube clock. In this part we look at NIXIE tubes and common methods to drive them.
Acknowledgements – I have provided links to several peoples web sites in this article. I thank them in advance for providing data and examples on their web sites.
NIXIE Tubes
So, let’s start with the NIXIE tubes. As they are no longer made many types are slowly become more and more rare. The B7971 tube have become very hard to find. By all appearances warehouses of some NIXIEs look to have been found in the old Soviet Union. A search for NIXIE on www.ebay.com will reveal several vendors selling tubes.
NOS or New Old Stock tubes. Many of the old Soviet tubes are what are known as NOS meaning literally that although they are old they have never been used, the net effect being they are like new.
You should be cautious of NOS tubes. It has been a common habit over the years to place a bad tube into the box the replacement came from. Sometimes these bad tubes in what look to be NOS boxes are sold as NOS. Sometimes NIXIEs are in flats that hold multiple tubes. NOS tubes in flats will all tend to be the same date code, if used tubes were placed back in the flat the Date codes tend to be mixed. Check with the seller to makes sure they are NOS.
Date codes. Most NIXIEs are marked with the manufactures name and a code that indicates when they were made. For example 7826 is most likely the 26th week of 1978.
Used. These are tubes that have been used. NIXIE tubes have a finite life time. They fail in several ways. The filaments that form the numbers will slowly erode. Often the filament will erode depositing the eroded material on the inside of the tubes glass darkening the tube. Sometimes the filament will break rendering that digit inoperative. Multiple insertions into a tube socket can cause the seal between the tube and its pins to leak allowing the internal gases to leak out killing the tube. Tubes with flying leads (wires coming from the tube) that have been de-soldered can also leak at the wires due to mechanical or heat stress.
A good write up on cathode poisoning can be found at http://www.tube-tester.com/sites/nixie/different/cathode%20poisoning/cathode-poisoning.htm
Many of the tubes coming from the old Soviet Union are coming from ‘factory cartons’. These are typically NOS and are generally safe to buy.
Top, side or inverted side views. Some tubes are designed to be viewed from the top, others from the side of the tube, Inverted side tubes require the pins/wires to be oriented up in use.
You can find examples of these styles of tubes at http://www.tube-tester.com/sites/nixie/trade01-nixie-tubes.htm and http://www.tube-tester.com/sites/nixie/trade03-nixie-tubes.htm – More examples of tube types can be found at http://webx.dk/oz2cpu/clock/nixie-collection.htm
Specialty NIXIEs. Most NIXIEs can display the numbers 0,1,2,3,4,5,6,7,8 and 9. There are however some NIXIEs that display special characters such as +/-, or only 0 and 1. Other tubes display scientific symbols or electronic symbols. Make sure whatever you order can display the required digits for your application.
You can find examples of special character tubes at http://www.tube-tester.com/sites/nixie/trade02-nixie-tubes.htm
Invereted fives. Some tubes used an upside fire (5) for the number two (2). Personally I think this looks awful. Again, knowing what you are ordering will help you in being satisfied with tubes. This was done to make the tube cheaper to manufacture.
The packaging of the tubes is important; I’ve seen pictures of tubes where nearly every tube was broken in transit. Don’t spend big money on tubes then wimp out on the shipping costs. Check with the seller to see how they ship them and if they will replace broken tubes, most will.
Driving the tubes
NIXIE tubes require about 180 volts to work. Most modern microcontrollers and drivers chips can’t deal with this high a voltage so some kind of driver electronics are required. (Future posts will cover power supplies.)
Direct Drive – Each NIXIE tube is individually driven, this is typically the simplest to implement.
In the era of NIXIE tubes special ICs (chips) were made to drive the tubes. The 7441 and 74141 were common. The IC is passed a 4-bit Binary Coded Value using standard logic levels, the IC selects one of ten outputs, and each output is connected to one number inside the tube. Recall that ‘0’ is also a number the tube needs to display it as well so we have ten total outputs. These parts haven’t been made for decades and have become hard to obtain. However there are again old Soviet parts that fill the same function. Searching ebay for K151Id1 will usually result in several hits for driver ships.
Data on driver chips can be found at NIXIE Driver ICs.doc
Using one driver IC per NIXIE tube implements the display portion of a direct drive NIXIE clock, a four tube clock would use four driver ICs, a six clock would use six.
Some kit designs use high voltage transistors to handle the high voltage, some use modern chips that can handle the high voltage. These tend to be the exceptions and are beyond the scope of this article.
Multiplexed Drive – Each NIXIE is turned on individually for a brief moment in sequence, this happens fast enough that the eye can’t see them flicker. It’s generally believed that by multiplexing the tubes they will have a longer life.
The same ICs as is listed above can be used. Only one IC is required for clocks with up to four tubes. Basically each NIXIE has the high voltage switched to it with the correct BCD code being sent to the driver IC. This of course requires coordination of high voltage switches and the driver IC. Every multiplexed clock I have encountered uses a microcontroller to accomplish this, the multiplexing logic being implemented in software within the microcontroller. In a four tube multiplexed design each tube is powered up for ¼ of the display cycle time.
Clock with more than four tubes are generally multiplexed in what is known as a 2×3 scheme. Basically the first three tubes are multiplexed together while the last three tubes are separately multiplexed together. Two of the driver ICs mentioned above would be needed, one for each group of three tubes. This results in sufficient brightness as each tube is powered up 1/3 of the display cycle time. If all six tubes were multiplexed together they would only be powered up 1/6 of the cycle time, this scheme typically results in dim tubes.
Some people have been successful with 1×6 multiplexing schemes so please don’t take my advice as absolute – please see http://www.webx.dk/oz2cpu/clock/nixie-clock.htm for a great example.
More technical info on multiplexing can be found at http://www.decodesystems.com/multiplexed-nixies.html
In part 3 we will examine the Microcontroller.
