How To Build a Nixie Tube Clock

I have always been fascinated with Nixie tubes. These tubes were the predecessors of the 7 digit LED (Light Emitting Diode) and LCD (Liquid Crystal Displays) of today.Actually, in the early 1990s, as an Electronic Technician I was still supporting an electronic weigh system used in steel mills that used Nixie tubes in the display. We were retrofitting these weigh systems with LED displays to make them look more modern.

​What is a Nixie tube?

A Nixie tube is an electronic component used to display a fixed set of characters, typically letters or numbers. It looks like a vacuum tube but functions more like a neon lamp. The tube has one anode or positive terminal and several cathodes, one for each character you want to display. Individual characters can be displayed by applying 170 volts DC between the anode and one of the cathodes. A current limiting resistor of around 47K is required between the anode of the Nixie tube and DC power supply. Each Nixie tube only requires a few milliamps to work. The Nixie tubes I used for this project are Russian made Type IN-12A . This Nixie tube uses an inverted 2 to display 5, most likely to reduce manufacturing costs. A Nixie tube typically displays characters in an orange or reddish glow around the character shaped cathode.

​Where do I get Nixie tubes?

• http://www.allspectrum.com/
  All Spectrum Electronics, based in Van Nuys California, this is where I purchased the IN-12A Nixie tubes and sockets for this project.
• http://www.rutubes.com
  www.rutubes.com, based in Moscow Russia, I saw they were recommended as a source for Nixies tubes for a project on the Jameco electronics site. I don’t know their reputation and would place a small test order first.
• http://www.ebay.com
  eBay has Nixie tubes, be careful and read the description fully before bidding, pay special attention to the country the seller is located.

​Where do I get the rest of the parts for the project?

Most parts like resistors and capacitors I already had. Most vendors require a minimum order when you order discrete components. So say I only need one 1Meg Watt resistor for a project. I would have to purchase at least ten to meet their minimum requirement.

• http://www.jameco.com
  I used Jameco as a part source for my project. This company is very "Electronic Hobbyist" friendly.

​Giving Credit where credit is due!

The Nixie Clock discussed in this Lens is not of my own design. It is based on schematics from Mike Harrison in the UK. My creativeness was used in the implementation of Mike’s Nixie Clock design. Below is the web link to his site.

• http://www.electricstuff.co.uk/nixclock.html
  Parts list and theory of Mike's Nixie Clock.
• http://www.electricstuff.co.uk/nixpthschem.pdf
  Direct link to the schematic I used to build my Nixie Clock from Mike Harrison’s website. The circuit uses a transformer-less voltage doubler design in order to provide the 230 Volts DC needed for the Nixie Tubes.

​Caution, only experienced electronic hobbyists should attempt to build this circuit!

This project employs a transformer-less power supply design. It can be quite dangerous to work around. This circuit can cause potentially life-threatening shocks if you are not careful. The line (120 Volts or 220 Volts AC depending on the circuit configuration) or “mains” voltage is converted to high voltage DC (between 250 and 300 volts) and applied directly to the Nixie tubes. Low voltage DC (4.7 Volts) powers the clock circuit. Extreme caution should be applied when taking measurements or working on the Nixie clock when powered on. I actually built an isolation transformer and plugged the Nixie clock into it for safety whenever I was testing with it powered on. Please keep in mind, this circuit is perfectly safe once assembled in a well insulated chassis.

​My Isolation Transformer:

An isolation transformer is used to transfer electrical power from a source of alternating current (AC) power to some equipment or device while isolating the powered device from the power source for for safety. Isolation transformers provide isolation and are used to protect against electric shock.I build my isolation transformer using two 120 Volt to 28 Volt step down transformers. I connected both secondaries of the transformers together. The first transformer stepped the line voltage down from 120 Volts to 28 Volts while the second transformer worked in reverse, stepping up the output voltage back to 120 Volts. These transformers can be used in the unconventional fashion due to the low power requirements of the Nixie clock. I added a switch to the input and fuse to the “line” current side of the circuit. I also fused the output of this circuit. In addition, I added a green indicator light so I know when the circuit is “Hot”.

​The Division Bell:

I broke the Nixie clock circuit up into five circuit boards:Power Supply board - provides the necessary voltages to the other circuits.Lower Digit Board - Drives the Nixie tubes that display seconds.Upper Digit Board - Drive the Nixie tubes that display minutes and hours.Colon Driver Boards - There are two of them, these drive neon bulbs to blink at a 1HZ interval to act as the colons between hours, minutes, and seconds.

​Power Supply Board Build:

Here is the complete Power Supply board. I used pre-drilled perf board cut to 2 inches square. The connections were all made using point to point wiring. Finally, I used our Brother label maker to label the connections. The connection points were just loops of copper wire. The input is 120 Volts AC, the output is 250 Volts DC to drive the Nixies tubes and 4.7 Volts DC to drive the clock logic circuit. The Sync output is a 60 Hz signal used as a time base for the clock logic circuit. The ground is common for both input and output voltages. The input line is also fuse protected.

Power Supply Board Testing:

The next step was to test the power supply board. I connected the input of the power supply board to the output of my homemade isolation transformer. I then measured the output voltages. As you can see from the meter reading, the high voltage output is 305 Volts DC without a load. The low voltage output was 4.3 Volts instead of the anticipated 4.7 Volts, still high enough in voltage to drive the Nixie clock logic circuits.

Nixie Tube Testing:

The next step was to test the Nixie tubes ordered for this project. This also was a great test to find out if my power supply board was up to the challenge of driving a Nixie tube.

Digit Board Assembly:

I used the Radio Shack Matching Printed Circuit Board (Catalog Number 276-170) for the lower and upper digit board assemblies.These printed circuit boards are pre-drilled and have copper foil connecting the holes in the same fashion as a modular breadboard. They worked great for this project!

Building the Lower Digit Board:

Here is a picture of the completed Lower Digit Board. If you recall, I said that this board drives the two Nixie tubes representing seconds. It also drives the neon bulbs that acts as colons between hours, minutes, and seconds. You can also see terminations for the 4.7 Volts and Ground. In addition, the Sync input gets a 60Hz signal, which is used as the time base for the circuit.

​Lower Digit Board Testing:

I used a regulated power supply to provide the needed 4.7 Volts DC to drive the circuit. The homemade signal generator to the left provided the 60Hz sync signal. I used a LED in series with a 470 ohm resistor as a logic probe to make sure the Decade Counter Integrated circuits used in this circuit was taking the 60Hz signal, and dividing it to provide 1Hz and 1/60Hz outputs

Building the Colon Driver Board:

There are two of them, these drive neon bulbs to blink at 1HZ intervals to act as the colons between hours, minutes, and seconds. The boards are pre-drilled perf board cut in a roughly 1 inch by 2 inch rectangle. They contain a driver transistor and related resistors.

Colon Driver Board Testing:

I used alligator clips to connect the Power Supply board to the Lower Digit Board. The 1Hz output of the Lower Digit Board drives the neon bulbs on the Colon Driver Board to blink. Both Colon Driver Boards were tested in this fashion.

​

​The Display Case:

I ordered a "Collectors ShowCase Plastic Display Case" (13.8 x 6.5 x 5.5 inches) from Amazon. This case will show off the innards of my Nixie Clock.

Nixie Tube Mounting:

I used two dowel rods and two wooden clothespins to build the Nixie tube bracket.

​Nixie Tube Mounting (Continued):

Here is the finished Nixie Tube Mounting bracket. It will be held in place by two wood screws to the black plastic base of the display case. After initial fitting, I took it apart, including removing the Nixie tube sockets and Colon Driver Boards, applied several coats of Polyurethane then sanded it with a fine grit sandpaper. Finally I painted it with a flat black acrylic paint before re-assembly.

​Addition of resistors to the Digit boards: - I needed to find a place for the 33K resistors that connect to the Base of the driver transistors. The driver trans

Upper Digit Board (Added 33K resistors are circled in red)

​Machining the Display Case Base:

The next step was to determine the placement of the printed circuit boards, Nixie tube mounting bracket, and time set switches. I also drilled three holes for the power cord and wiring to the time set switches. I used grommets to prevent wire chafing.

​Mounting the Nixie Driver Transistors:

I came up with a creative way of mounting the Nixie driver transistors. I soldered the transistor's Collector’s directly to the Nixie tube socket solder tabs. The Emitters were connected to the round piece of bus wire that will go to ground. Eventually the Base of each transistor will be wired to the proper 33K resistor mounted on the Lower and Upper Digit Boards. The purple wires you see in the picture connect the Anode through the 47K current limiting resistor to the 250 Volt DC connection on the Power Supply Board.

​Wiring the Time Set Switches:

There are three switches:Fast Time SetSlow Time SetSeconds Hold I mounted them to the base of the plastic display case then routed the wires through a hole I drilled earlier to the Lower Digit Board. A little bit of hot glue keeps the wiring in place.

Wiring Phase 1:

I wired the AC cord to the Power Supply Board. This is the thick brown cord in the picture, I then connected the power supply output to the Nixie tubes and the Upper and Lower Digit Boards. Finally, I wired the Base of the driver transistors connected to the Nixie tubes displaying seconds to the Lower Digit Board.

​Testing Phase 1:

At this point I made sure the colons between hours, minutes, and seconds were blinking at 1Hz intervals, and that the Nixie tubes displaying seconds were counting properly.

Wiring Phase 2:

I wired the Base of the driver transistors connected to the Nixie tubes displaying minutes to the Upper Digit Board.

Testing Phase 2:

I made sure the Nixie tubes that display minutes were counting correctly. In addition, I made sure the three switches, Fast Time Set, Slow Time Set, and Seconds Hold were working correctly.

Final Wiring:

The last step was to wire the Base of the driver transistors connected to the Nixie tubes displaying hours to Upper Digit Board.

Final Testing:

The last step was to test all aspects of clock operation. Using the fast and slow time set buttons. I made sure that hours, minutes, and seconds incremented properly.

​Electrical Isolation Test:

I used metal standoffs to mount all printed circuit boards. The metal screws of the standoffs are exposed under the base of the display case. To test electrical isolation, I connected the clock to an electrical outlet then I connected my meter, set to AC Volts, between the ground connection of the AC socket and each standoff screw exposed under the display base making sure that no hazardous voltages exist.

​Complete! - Here are pictures of my completed Nixie Clock with the plastic display cover installed. I use four brass set screws to keep the cover in place. I lo

​Video of my Nixie Clock in action!