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The Retro Pi Nixie Clock

TLDR: Tubes that Light up Numbers, High Voltage, Russian Parts, Raspberry Pi! What Fun!

Nixie tubes are about the most compelling way to display numbers ever invented. They have light up numbers with a friendly orange glow. The Raspberry Pi is this cool little computer that can do so many things with so little effort. Why not put them together and make a retro clock that gets its time and date from the internet?

A word of warning before getting too much further into this. This project uses 200V-400V which can be lethal under the right circumstances. Construction of the high voltage circuits should be undertaken only with them powered off and off long enough for the high voltage capacitors to discharge.

If you have not heard of Nixie tubes, they are first type of numerical display from the dawn of the digital era. Invented by Burroughs Corporation, they were standard equipment in many types of measurement devices and industrial controls from the late 1950s to the late 1960s. The LED 7 segment display was introduced in the 1960s and its acceptance brought an end to the use of Nixie tubes in the west. However, they lived on for many years in the Soviet Union and Eastern Bloc countries. The Nixie tubes available for sale today are old stock as they have not been made for decades.

Nixie tubes are built with vacuum tube technology. Inside the glass envelope are 10 cathodes (the – terminals) in the shape of the numbers 0-9 plus a piece of fine wire mesh that forms the anode (the + terminal). To light up a number, connect the anode to the + of a power supply through a current limiting resistor and the number you want to light to the – of the power supply. Sounds just like what you do to light up a LED, right? It would, except for the part about 200V to light the thing. Actually, the Nixie tube is a cousin to the neon lamp and works just like one of those.

Won’t 200V bake the Pi?

To operate the 200V Nixies, it is a good idea to keep the 200V far away from the Pi. The internal circuitry of the Raspberry Pi operates at 5V and below. The General-Purpose Inputs and Outputs (GPIOs) on the Pi can only handle 3V. What to do? Time for another peek back in history.

Remember that the Nixie tube was invented in the late 1950s. This was well before integrated circuits were invented and not too long after transistors were introduced. At the time, digital logic was made with individual transistors (see the article in Nuts and Volts about making a clock with individual transistors), vacuum tubes or mechanical devices (yes, really!). The first popular family of integrated circuit logic chips, known as the SN7400 series, was introduced by Texas Instruments in October 1964 and later second sourced by many companies. These parts became wildly successful because they implemented hundreds of logic functions from simple gates to complex arithmetic functions. They also had a part, the SN7441, which could take a 4-bit representation of a decimal number, so called Binary Coded Decimal (BCD), and convert that to 1 of 10 outputs that can drive a Nixie tube directly.

Ah ha, you might say, problem solved! Almost. When the Nixies died out in the west, so did the SN7441. There is some dusty old stock available occasionally on EBAY and NTE has a part, also old stock, the NTE7441. Now back to the Soviet bloc. They could not get the SN7441 with the cold war and all, so they made their own. From Russia with love comes a virtually identical part, the K155ID1 (К155ИД1). This part is still available from Bulgaria on EBAY, surplus from the Cold War.

Interfacing to The Pi

The SN7441 requires 4 inputs to light up the Nixie tube with the appropriate number. We have 4 of these, so 16 digital outputs will be required. An easy way to get these outputs is with a PCF8574 I2C parallel port expander chip. The PCF8574 is to the I2C world as the shark is to the ocean world. Simple minded, fast, and easy to understand. It can only do one thing: read or write an 8-bit register connected to 8 pins. There are 8 possible addresses for these chips, selected by 3 address pins, starting at hex 38. The two chips in the system use address (hex) 38 and 39. The two of these will give the 16 outputs we need. In addition, it will remove the 200V for the Nixies one more step from contact with the Pi.

On the Pi, the I2C bus is called the SM bus. The clock program is written in Python because python is easy to use and understand. Also, Python has a good module to support SM bus.

You might ask why not use the GPIOs? There are more than enough pins to allocate the 16 needed to drive the SN7441s. If we were to use the GPIOs, it would require quite a bit of code to set them up and put the data out on the pins in the right format. By comparison, it only requires 4 lines of code to setup the SM bus and one line of code to write 8 bits (two digits) to one of the PCF8574s.

We don’t give up on the GPIOs completely though, we do use them to drive the 2 neon lamps for the colon when time is shown and 3 neon lamps when the dash is shown for date. The respective GPIO lines drive MMBTA42 high voltage transistors which handle the 200V nicely. It takes almost as much code to light up 2 groups of neon lamp punctuation as it takes to run 2 digits of Nixies.

The Pi uses the Linux operating system and has WIFI capability. Linux will access the internet and get the time and date for you as part of its normal function. The time and date are simple to get to from Python.

So, there you have your interface from 2 retro 200V Nixie tubes to the Raspberry Pi: two SN7441 ICs and a PCF8574. The next hill to climb: making suitable power supplies.

With a Few Cheap Tricks You Get a Power Supply

An AC adapter is a convenient way to get a step-down transformer and line cord all in one. This power supply uses a 12.6V AC center tapped transformer. By bringing the 12V center tapped AC into the box we can do a couple tricks.

The first trick is to use a two-diode full wave rectifier (D1 and D2) for the Pi 5V power supply. It produces about 8V which is filtered by C1 and regulated down to 5V for the Pi by a simple 5V 3A voltage regulator chip, the venerable LM323.

For the second trick, a 12.6V to 120/240V stepdown transformer is operated in reverse to produce the high voltage for the Nixies and the neon lamps. A transformer is just as happy to step up or step down as long as the proper voltage is applied to the windings. In this case, applying 12.6V to the 12.6V winding will yield 120/240V. The two 120V windings are connected in series to produce 240V which is rectified by BR101 and filtered by C102 to make about 330V.

The third Trick is a TL432 used as a voltage regulator. The TL432 is a slick little part. In a 3-pin transistor package is a micro-power op-amp, 2.5V precision voltage reference and an output transistor. It can be used as a programmable Zener diode, comparator, shunt regulator or as here, a voltage regulator. But look ma! There is no power supply pin!

The pins on the TL432 go by the usual names for a Zener diode: cathode and anode. In addition, the pin to set the voltage is called the reference. If it is put in a circuit with the anode grounded and a series resistor from the cathode to a supply voltage, the cathode will be at near the supply voltage until the reference pin goes above 2.5V. When that happens, the cathode will go to 2.5V. The op-amp gets its power from the grounded anode and the 2.5V (or more) from the cathode.

Normally, the TL432 is used for low voltages. The part itself only rated at 36V. So, the next trick is to make it work at 330V. That can be done by powering it from the handy Pi 5V power supply and teaming it up with our friend the MPSA42 transistor, the surface mount version of which, the MMBTA42, was used on the display board to drive the neon lamps. The MPSA42 is an NPN transistor and it has a cousin, the MPSA92, a PNP device.

When the power is first turned on, the output capacitor will not have any voltage on it. That means that the voltage on the output of the voltage divider made up of XXX will not have any voltage on it and the reference pin on the TL432 will not have any voltage on it. Since this pin is below 2.5V, the cathode will have 5V on it. Q101 will be turned on with the 5V coming from the cathode of the TL432. The resistor R109 and Zener diode Z1 bias the emitter of Q101 so that it can turn off when the TL432 is in regulation. When Q101 turns on, the voltage on its collector will fall which will reduce the voltage on the base of Q103 turning on Q103 and Q104. This will then charge the output capacitor through R108. R108 is there to keep Q104 from blowing up as it charges the output capacitor during power up.

As the voltage on the output capacitor starts to rise, the voltage divider output will start to rise. When the voltage on the reference pin of the TL432 approaches 2.5V, the cathode voltage will begin to come down. This causes Q101 to reduce its collector voltage which in turn makes Q103 and Q104 conduct less. As all of this happens, the output comes into regulation.

This power supply can be easily adapted to other voltages by simply changing the voltage divider to output 2.5V when the output is at the voltage you want.

Finding Parts is an Easter Egg Hunt that Spans from Your Closet to Around the Globe

Just like an Easter Egg hunt, there are some items that are easy to find and some that take more effort. Most of the parts in this project can be found at the usual gang of suspects: Digikey, Mouser, Newark and so on. The Nixie tubes can be found on EBAY. If the B5092 Nixie is not available, the circuit can be used with the only modification being the pinout of the Nixie. There is a vendor in Bulgaria that sells on EBAY who has some similar Russian Nixies and the Russian version SN7441. The 12-pin socket is also available on EBAY from a vendor in China.

The case is a 11” x 8” x 4 ½” “stash” box available on Amazon. Buying the box already made saves a lot of time, but a homemade or other suitable box can be used. The clear Plexiglas (acrylic) is a common item that can be purchased at Home Depot. Orange Plexiglass is somewhat more unusual but can be found on Amazon.

If you haven’t thrown out that old PC, dig it out of the closet and pull out the IDE disk drive cable. The clock needs the same 40 pin ribbon cable as the disk drive. Sometimes, these cables have 3 connectors on them. If yours doesn’t, simply press a third one on at the right place for the display board.

Do Judge a Book by Its Cover

Most people have never seen a Nixie tube display and will be wowed by the box that has the amazing orange numbers. Having a nicely made box will impress people as much as the technology inside. A beautiful case will let you bring your clock out of the workshop and into the living room and create a great conversation piece.

If you use the stash box, the only things that need to be done to it are:

  • Route out the display window
  • Put a hole in the back for the AC cord
  • Mount the transformer
  • Attach rubber feet

Routing out the display window can be done with a router or a mill. If you happen to have access to a CNC router at a school, machine shop or maker space, that the best of all. The Nixie Clock in this project was made on a CNC mill. The opening in the box should be made with a 1/8″ lip around the inside for the Plexiglass to sit on. Similarly, the Plexiglass should be routed with a 1/8″ lip on it. When it is done, the Plexiglass should be a press fit in the case.

Holding the Plexiglass while routing it can be problematic. The easiest way to hold it is to first cut a piece that is about 3/4” bigger than needed. Then, tape it to a piece of wood that size. Blue painter’s tape works well for this and it won’t leave any adhesive.

The piece of Plexiglass holding the display and Pi was cut to size on a table saw and the mounting holes for the display boards and Pi were drilled on a drill press. This piece of Plexiglass was secured to the case with strong double-sided tape.

The Software: Don’t Panic!

Solder is the preferred programming language for most people that like to build stuff. But don’t panic if you don’t know how to program. The Raspberry Pi makes it easy.

The Pi will have to be set up and the program in the listing will have to be loaded. To set up the Pi, you will need a mouse, keyboard, power supply and monitor. Again, that old PC you dug out of the closet comes to the rescue. You can use the USB keyboard and mouse from it. Newer TVs have an HDMI port which will connect directly to the HDMI port on the Pi. You can also use a VGA monitor from your old PC with a HDMI to VGA converter. These are available at Walmart, Bestbuy or on Amazon among others. The final part you will need is a 5V 2A AC adapter with a micro USB connector on it. The easiest way to get that is to use a regular USB cube charger and get a USB to micro USB cable at the local drug store if you don’t have one from an old phone.

A Raspberry Pi model 3 B+ was used for this project. A plain model 3 will also work. When you buy it, get a micro SD card with NOOBS (New Out Of the Box Software) loaded on it. The Pis are available from many suppliers on line including Adafruit, Newark and Amazon.

After all the cables are connected, be sure that the Micro SD card is inserted with the contacts facing the circuit board. Apply power to the board and a torrent of messages will display on the screen and a short time later a desktop like a Windows desktop will appear.

The clock gets its time from the internet. The internet needs to be enabled before the clock will have the right time. You probably noticed that there are no buttons to set the time. That’s because the internet time is accurate, and it knows your time zone. Click on the WIFI logo and enter the password to your network. This will enable the WIFI and the WIFI logo will change to reflect the signal strength. The time displayed in the upper right corner of the screen will change once the Pi has synced to the internet time.

To get the Python program that runs the clock, down load it from the Nuts and Volts website www.nutsandvolts.com/programlistings/clock.py. The Pi browser (world icon) is a version of Chrome which works more or less like the version for Windows. After you have located the listing, copy it to the clip board by highlighting the text and pressing control C, just like windows. Next, open the Python programming environment by clicking on the Raspberry icon in the upper left of the screen and selecting programming and Python 3. In the Python environment, click new. Click in the upper left corner of the just opened window and press control + V. Your listing will appear. Press control + shift + s to open the save as window. Name the program “clock” and click save. The program will be saved in the home/pi directory.

Only one last step to do. Let’s make the program auto start when the Pi boots up so that it will not require as screen, keyboard and mouse. To do that, the file that holds the commands that are to be executed after the graphic user interface comes up, needs to be edited to add the command to run the clock program. Open the “terminal” (>_ icon on the top of the screen) and enter the following command (paying attention to what is upper case!):

Sudo nano ~/.config/lxsession/LXDE-pi/autostart

This opens a simple text editor called “nano” and displays the contents of the autostart file. This editor does not use the mouse, everything is done from the keyboard. Go to the end of the file by pressing the down arrow past the last line of text. Add this line at the end:

@sudo /usr/bin/idle3 -r /home/pi/clock.pi

Press control + x to exit. You will be asked if you want to save the changes, type Y. You will be asked to save in the file …..autostart. Press enter. You are done.

Where to go from here

The clock uses very little of the Rasperry Pi’s resources. It can do many things besides run the clock. Some things may need a keyboard/mouse and monitor. There are a variety of wireless keyboards with a mouse pad that will work well. If you have a television that has an HDMI port, you can use that as a monitor.

With a television and keyboard/mouse you can access Youtube, Netflix and Amazon to play videos (some additional software may be required). The Pi comes with a good suite of applications loaded on the NOOBS micro SD card. These include a browser, spread sheet, word processor, data base as well as a host of handy accessories like a backup program.

The Nixie Clock will make a great addition to any living room or den. But be warned, If you set it next to your TV, getting caught up in just one more episode of Star Trek will no longer be an excuse to not do that chore you promised to do at 8:30.

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