You Can Build Your Own
Nixie Tube Clock
Old Style "Nixie" Tubes bring a whole new elegance to electronic timepieces

by Spike Tsasmali


Nixie Tube Clock in Crystal Case
  Difficulty=4 Howls

Have you ever noticed your digital clock? Or the clock on your VCR? Looks like all the other digital clocks around doesn't it? Well, if you want a bit more from your modern timepiece, you have to go back in time to get it.

Huh? What? What do you mean? Back in time? Well, most modern digital clocks use 7-segment format displays. The kind like on most calculators. Some may be LED style, some may be LCD style and others may be vacuum-fluorescent, but they all have one thing in common -- the same boring 7-segment format.

However, way back before the dawning of the pocket calculator and digital wristwatch, computing devices used a kind of display which "glows" with fonted numerical characters. Dubbed "Nixie" tubes by their creators at Burroughs Corporation, these "tubes" contain 10 tiny wire formed numerals inside a glass envelope filled with neon. When electricity is applied to the tube, the neon would glow around the wire shapes giving off a highly visible orange numerical readout.


This animation shows the formed fonted numerical characters of the Nixie tube.
Photo by Mike's Electric Stuff

Since neon requires a high voltage to operate and the Nixie tube was inherently fragile, most designers discontinued using these readouts by the early 1970's and began designing with the lesser evasive LED, LCD and VF displays. Companies such as Burroughs, National, Amperex and NEC continued manufacturing Nixies for many years for replacement purposes despite their lack of use. Nowadays, these tubes are becoming rare thus making any project using Nixie tubes special in its own way and definitely nostalgic.

Nixie tubes were made in many sizes ranging from 1/2" digits up to 4" digits! Most Nixie projects are best built with smaller displays because they are easier to work with but you can choose a larger display if you like.


A Nixie Clock using 1" Burroughs 5092 Displays


Angle View of Nixie Clock Project using 3/4" NEC CD-66 Displays

Building a Nixie Clock

There are many styles of Nixie clocks being built. This project chooses to use a glass display case for the project box because not only does the display case give the final project a high degree of elegance, it also well protects the delicate tubes from damage and keeps unwanted fingers away from the high voltage power supply.

Many different circuit styles have been used for Nixie clocks, from an assortment of TTL and CMOS chips to microcontrollers. For this project, the PIC 16C62A microcontroller was chosen as the timer/counter because the use of a microcontroller gives more flexability in design than fixed TTL or CMOS counters would. Also for advanced experimenters, the PIC code can be ported to a larger PIC with more features and port pins. Adding accessories like Alarm, GPS timebase or external relays can easily be added to the software with very little changes to the hardware.

The tubes used in the clock shown in the photo are NEC type CD-66. Since most Nixie tubes were made on a "standard" pinout, other tubes may fit the PC board layout without change. However if you cannot find a suitable tube, you may wish to alter the PC board layout using EasyTrax before you print out the final artwork. Printed circuit layouts for using NEC's CD-66, ITT's GNP-7, Burroughs 5441 and National 841 (WANG Labs tube) are provided and can be downloaded from the Download site. If you choose to use a tube not mentioned above then you will have to alter the PC board layout and the software used on the PIC microcontroller (more on this later...).

Before We Begin...

Nixie Tubes require +160~+200VDC to operate, therefore caution must be taken when handling the power supplies used for the tubes. If you are not familiar with handling high voltage, please refresh yourself before beginning! Although most of the high voltage is on the secondary side of an isolation transformer, the shock you can get is quite nasty and may cause serious injury if you do not take necessary precautions!

What You Will Need

The complete list of parts for the Nixie Clock Project is given in Table 1. You will also need common shop tools, solder, etc.

Before you can begin the project, you will need to download and install EasyTrax CAD software. If you are not already familiar with EasyTrax, then please take the time to read through the Tutorial on how to use EasyTrax.

You will also need to download the Nixie Tube Clock Project Schematic Diagram and Printed Circuit Layouts. The schematic diagram is located on the 1 Mid Layer of the layout file (read the Tutorial for more information).

If you need help with making PC boards, please read the article, Making PC Boards.

Table 1.
Parts List for Nixie Tube Clock Project

DescriptionQuantityDesignators
.1uF NPO or Z5U Bypass Capacitor1C1
.1uF/200v Mylar Capacitor2C2,C3
220uF/200v Radial Elec1C4
2700uF/35v Radial Elec1C5
1K ohm 1/8 watt49R1,R2,R4,R5,R6,R14-R57
47K ohm 1/8 watt1R3
3.9K ohm 1/8 watt6R7,R8,R9,R10,R11,R12
1Meg ohm 1/4 watt1R13
2N3904 NPN Transistor1Q1
MPS-A42 NPN Transistor44Q2-Q45
4MHz Ceramic Resonator (3 pin)1Y1
PIC16C62A (see text)1U1
74LS374 Octal D-Latch6U2,U3,U4,U5,U6,U7
7805T 5V Regulator IC1U8
Bridge Rectifier 1A 400v2Z1,Z2
Nixie Tube, CD-66, 5441, N541, GNP-7 (see text)6V1,V2,V3,V4,V5,V6
Transformer, 6v Dual (Dale PL12-03)1T1
Transformer, 120v-120v (Signal ST-3-120)1T1
Mini Data Entry PB Switch2S1,S2
Miniature Slide Switch1S3
Socket for U1, 28 pin Skinny DIP Machine Tool1*
Sockets for Nixie Tubes (if required)6*
Heatsink for U81*
Brass Standoffs, 4-40 Dual Ended x 5/8"6*
Chrome Standoffs, #2 x 5/8"2*
Line Cord, 2 wire #181*
Rubber Grommet, 3/8"1*

A Complete Kit of parts INCLUDING etched, punched, plated, silk-screened PC board and 6 BRAND NEW OLD STOCK ITT GNP-7 Nixie Tubes is available from Lupine Systems for $289.95. Please send inquiries to WOOFY for more information!

A glass display case is also needed to complete the project. The PC board layouts used on this project are designed to fit the Glass Display Cases Dual Baseball Display Case. You can order one of these cases by clicking on the link above.

Construction

Printed Circuits

You will need to download, print, etch and punch the appropriate printed circuit for the tube you have chosen to use. If you have chosen a tube other than those on this list, you may have to alter the layout and the software to accomodate the new tube.

Table 2.
Printed Circuit Downloads for Nixie Tube Clock Project

Tube UsedDownload This File
NEC CD-66nixbox.pcb
Burroughs 5441nixbox54.pcb
ITT GNP-7nixboxg7.pcb
National 841 (WANG)nixwang.pcb

If you need help making the PC board, read the article, "Making Printed Circuits".

Carefully assemble BOTH PC boards. The Main board is a DOUBLE-SIDED board and will require soldering on both sides of the board. All VIA holes will require a small wire to be inserted from TOP to BOTTOM and soldered on each side. A machine-tool type socket is recommended for U1 because these type sockets make soldering on the top layer easy. Simply solder the socket completely on the SOLDER SIDE first, then go back and heat each pin you need to solder on the TOP layer by touching the tip of your soldering iron to the pin on the BOTTOM of the board, then let the heat conduct through the pin to the TOP side. Apply the solder to the pin of the socket on the TOP side and use the pin's heat to solder it to the circuit pad.

Fully assemble the Main Board EXCEPT for the Nixie Tubes. You will install these later.

The power supply section is single sided and will not require soldering on both sides. Completely assemble this board but do NOT attach the line cord.

Software and Programming the PIC Microcontroller

You will need to download the appropriate software to match the tube you have chosen to use. If you decided to use a tube other than those on the list you may have to alter the software to accomodate the new tube.

Table 3.
Software Assembly File Downloads for Nixie Tube Clock Project

Tube UsedDownload This File
NEC CD-66nixbox.asm
Burroughs 5441nixbox54.asm
ITT GNP-7nixboxg7.asm
National 841 (WANG)nixwang.asm

Download the software and and PROGRAM the PIC microcontroller. This is an ASSEMBLY FILE and will require you to run it through a Microchip PIC assembler to create the .HEX file needed to program the PIC. You can download a copy of MPLAB FREE from the Microchip Technology website. When you are ready to program, set your programmer to program the PIC with Brown Out Detect OFF, Code Protect OFF, Power On Timer ON and Oscillator to XT. Once programmed, install the PIC in the socket for U1.

Note: If you plan on using a Nixie tube other than those on the above list -- AND -- the tube you choose has a different pinout, then DO NOT PROGRAM THIS PIC YET. Read on and learn how to MODIFY the software so you do NOT have to change the PC board layout to fit your chosen tube.

Once both PC boards are assembled, you will need to stack them together. First, locate the pad near Q1 marked ZC and solder a 2" length of bare wire through this hole and out the bottom side. This wire will extend down to the lower board later. Next, install 2 brass standoffs on the PARTS SIDE of the Main board at holes ST1 and ST2. Install 2 chrome standoffs on the PARTS SIDE of the Main board at holes ST3 and ST4. Assemble the Power Supply board to the Main Board by first feeding the 2" bare wire attached earlier through the hole on the Main Board next to U1 Pin 21. Once the wire is threaded through, use 4-40 and #2 screws to attach the Power Supply board to the respective standoffs mounted on the Main Board.

Locate the two pads marked +5V and GND at the top edge of the Power Supply board. These pads also show up in the exact same place on the Main Board. Use a piece of #22 gauge wire to connect the pads on the Power Supply board to the pads on the Main Board.

Using a short piece of STRANDED INSULATED #22 gauge wire, connect the pad on the Power Supply board marked +200v to the pad on the Main Board marked +200v. Also connect the adjacent GND pads together. These pads are located on the LEFT side of the Power Supply and Main boards.

Installing the Nixie Tubes

You are now ready to install the Nixie tubes. If you are using used tubes, be sure to test them before installing them! This will save you a ton of grief later!

Carefully install each Nixie, beginning with the LEFTmost tube and working RIGHT. You may have to "play" with each lead wire a bit before you get the hang of putting all 14 wires through the holes. Once you have all of the leads of the first tube in the board, solder ONLY TWO of the lead wires, one across from the other FROM LEFT TO RIGHT. This will allow you to ALIGN the tubes to the same height and angle before you solder the tube permanently in place. When all tubes are mounted, adjust each tube so that the display is even height and all of the tubes are straight from front to back and left to right. Loosen one of the two soldered lead wires if necessary so you can move the tube around some. Once you have alignment, solder all of the other tube leads and clip off the excess lead lengths.

Install two brass standoffs on the SOLDER SIDE of the board at holes ST5 and ST6. Earlier you installed two standoffs at locations ST1 and ST2. Remove the screws from the SOLDER SIDE of the Main Board holding these standoffs to the Main Board. Using a screw cutter, make two threaded studs 5/16" long. Screw these studs about halfway in to the standoffs at ST1 and ST2. Then screw two more standoffs onto the open ends of the studs.

Prepare the glass display case by drilling a 1/4" hole in the glass mirrored base at the location marked "LINE CORD PASSTHRU" (Use the 1 Mid Layer printout as a drill template to aid in locating this hole). You will need to use a DRILL PRESS and a CARBIDE DRILL to drill glass. Be sure to wear safety goggles (NO EXCEPTION!!) during this process! Once the hole is drilled, fit a 3/8" rubber grommet into the hole. Next, drill another hole 1/4" through the wooden base perpendicular to the hole in the glass. Then feed the line cord through the hole in the base about 5 inches and tie a knot. Pull all slack back through the base and then feed the end of the line cord through the grommet, through the PASSTHRU hole on the Main Board and solder the two wires to the pads marked AC IN on the Power Supply board.

Your Nixie Clock is now complete!

Use push button S1 to set Hours and S2 to set Minutes. Slide Switch S3 selects between 12 or 24 hour time format. You will need to set this switch BEFORE you plug in the clock.

Making Changes to the Software

The Nixie Clock is designed for 110v 60Hz operation. You can change this to 220v by using a step down transformer for the voltage, and by changing the Interrupt timebase for the 50Hz. Using MPLAB, create a new project with nixbox.asm as the Assembly Node. Open the assembly file and locate Line 219,

FREQUENCY   EQU   120

Change this line to

FREQUENCY   EQU   100

then REASSEMBLE the file.

You can also alter the software to accomodate different Nixie tubes without changing the PC board layout. To do this, locate line 140, the beginning of the Transponential Data Table. This table is used to define which "output" goes to which "segment". Alter this table to match the tube you are using then REASSEMBLE.

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