This is a superficial review of the few schematics I encountered while building Nixie clocks, VFD clocks and Geiger counters (no tube amplifier just yet). Although the schematics seemed basic at a glance, they usually ended up being a challenge (that is, they rarely worked right away) for me. That's another reason I am trying to cover them here, so I can use this post as consolidated reference any time I need it.

Tubes require high voltage to work. Some (Vacuum Fluorescent Display) need 40V, others (Nixie) 180-200V, and some others (Geiger) even higher, 400-1000V. The high voltages are generated these days by switching-mode power supplies. Essentially, there is only a handful of popular solutions, and each DIY tube kit picks one of these, based on size, power requirements, cost.

In principle, a switching mode power supply, also known as "boost converter", uses a square wave oscillator ("switch") to create magnetic energy in an inductor, then releasing it as high voltage.
Some scientific explanation (with formulas) can be found here, some practical advice (with schematics and photos) here. Adafruit has a very useful online boost calculator.

1. One of the most popular solution for the square wave oscillator is by using the ubiquitous 555. This is inexpensive, but requires some tweaking and adjusting (values of resistors and capacitors). The schematic is standard, but there seem to be a few variations.
The one below is from Ronald Dekker.


Frank clock (from Pete's Nixie kits) uses an almost identical schematic, but a different set of values for R2-R3- C4 (used for setting the frequency). In the end, the oscillator frequency is about the same at approx 30kHz, calculated with formula  f = 1/0.693/C4/(R3+2R2)  (in the schematic below).


Same 555 is used in Arduinix, but in a different configuration, although still as astable oscillator. This one has an extra HV capacitor (C4) in series with a resistor (R15), whose exact purpose I don't understand. The oscillation frequency is also weird, according to the above formula, with C3 at 47pF, should be 1.5kHz. No wonder this did not work for me.


Another almost identical HV power supply for Nixie tubes is used in the recently-kickstarted "Nixie tube shield" (for which I pledged $15 for the PCB, and yet to receive it).


And finally, 555 is also used to generate the higher voltages required by Geiger tubes, as used by BroHogan (and MightyOhm). The frequency of oscillation is 4.5kHz (f = 1/R1/C2). (I built several Geiger kits from BroHogan and they were all trouble-free.)



2. Other solutions use specialized chips like MAX1771 and MC34063.
Shown below is the high-efficiency boost converter from Nick de Smith (sold by ogiLumen), based on MAX1771.


Akafugu's VFD Version 1 clock uses the same MAX1771, to generate a lower 50V (for VFD tubes).


For MK2, Akafugu switched to using MC34063 chip (schematic not published yet).
The same chip is also used in their Nixie clock (schematic shown below), to generate 180V. This HV circuit has its own (all SMD) board, which I assembled it myself and worked without a glitch.



3. Yet others use a PWM pin of a microcontroller. This method requires the processor to be connected and programmed in order to generate the high voltage. The solution is cheap (saves an extra chip), smaller in size (again, one less chip), and also seems to be highly efficient.

Below is the HV schematic used by Adafruit's IceTube clock.


Some of the microcontroller-based boost converters have feedback (close loop, with PWM adjusting to the voltage output, if I am not mistaken), as are those from Cogwheel and Satashnik (shown below, respectively).


As with any analog electronics circuit, troubleshooting a HV supply is not easy. A suitable tool would be an oscilloscope, allowing for the measurement and adjustment of the frequency and pulse width. Once these are cleared, the high voltage could be adjusted usually from the trim pot. To modify the voltage range, try different values for the inductor.

I just finished assembling the latest version of akafugu VFD clock, the "MK2". Per, from akafugu, generously sent me the board for testing/review. It comes will all SMD components pre-soldered, and the processor pre-programmed. The rest of the components, all through-hole, I sourced on my own.
Here is a photo of the kit as I put it together (I forgot to include the through-hole resistors though).



Assembling was straightforward thanks to the combination of good board design and easy-to-follow instructions. The result is shown in the photo below.



MK2 has some improvements over version 1:
- newer processor (Atmega32U4, as the one used in Leonardo);
- 5V power source (vs 9V previously);
- direct sketch upload from Arduino IDE (no hacking required);
- integrated support (on-board 24LC256 eeprom) for four-letter-word feature;
- all SMD parts (quite a few actually) come pre-soldered;
- directly compatible with the existing (version 1) VFD display shields;
- better clearance with the VFD display shield (because parts are shorter in height).

In conclusion, MK2 is another winner for akafugu.


I also recently assembled the Ice Tube clock from adafruit. I bought the PCB(s) a while ago (when they were still available for sale on adafruit site) and I sourced the parts on my own. I followed the legendary assembly instructions, but things did not go without a glitch for me. Here is what I learned in the process:

• The Ice Tube clock is not Arduino-compatible as I initially thought (see here). It cannot be programmed from the Arduino IDE and it does not have an FTDI interface/connector. The processor (I used Atmega328, but ATmega168 should work with the base software too) uses the internal 8MHz oscillator and an external 32kHz crystal. This crystal, in conjunction with TIMER2 (see file iv.c), is the heart of the clock "mechanism". (More details on the design can be found here.)

• The high voltage for the tube is produced with impulses generated from an output pin of the processor (instead of using an external chip, like may other designs). Therefore, high voltage is not there until the chip is correctly programmed.

• Programming the chip was a bit of a challenge; I used the makefile to build the hex file from the source, but programmed the chip with avrdude directly, as shown in this screenshot (click for bigger picture).

• IV-18 tubes bought on ebay, even when they are new (with the wire terminals intact) can be defective. Out of the 5 tubes I got, 4 had a blackened spot on the back and one had a white spot, as shown in the photo below. Guess which one did not work.

• Inserting all 24 wire terminals of the IV-18 tube in their holes is a challenge. I would much rather prefer the solution adopted by akafugu in their VFD clock, with half terminal holes, opening in a big empty circle (as shown below, photo from akafugu).

• IC sockets, particularly PLCC28, although bought from digikey, can be "defective". Unlike the DIP IC sockets which press each IC pin on two sides, the PLCC28 sockets actually press against the IC pins on one (external) side only. It took me many minutes to figure out that the tube was not lighting up because one socket pin was loose and did not make contact with the IC's pin.

• MAX6921 VFD driver used in the Ice Tube clock is more than twice as expensive as HV5812 VFD driver used in MK2. Yet, adafruit's Ice Tube Clock is still the cheapest complete (including enclosure) VFD clock kit out there.

Overall, building the Ice Tube clock was a pleasant and rewarding experience, which I recommend to kit-builders at any level. This clock looks compact, sleek, stylish, and stands out in my clock collection.

There aren't many VFD clock kits out there that are software-compatible with Arduino (and by that I mean the "Arduino sketch" being compilable in Arduino IDE and uploadable through USB-FTDI).
Most of these kits come with the microcontroller pre-programmed, so they can be built quickly and easily, with minimal effort (totally understandable instant gratification; nobody wants to end up with an expensive dud). Upgrading the software usually requires familiarity with the microcontroller toolchain (compile-build-flash), plus an ISP programmer. (Note: The only processor I am familiar with is AVR/Atmega.)

I was going to build myself an Arduino-based VFD clock, one that can take a sketch through the FTDI cable. Instead of starting from scratch (choosing a schematic, making the board etc), I decided to try one of the only 2 open-source VFD clock kits I found that are based on Atmega processor and :

• Adafruit's Ice Clock
• Akafugu VFD modular clock

I settled for the VFD modular clock because the Akafugu people offered me the pair of PCBs in their kit (for a very reasonable $19, shipping included) and also because I already had most of the parts, including the two SMDs (that come pre-soldered in the kit: ATmega328 and HV5812) and the IV-17 VFD tubes.



















I built the clock following their great assembling instructions, but, as you will see, not without hitting a few stumbling blocks on the way.

I should clarify that this post is not a review of the VFD modular clock kit (since I did not have the kit), but just a record of my observations. It was my choice to not use the kit, thus forcing myself to try to understand the schematic and the software.

Since my goal was to make the VFD modular clock Arduino-compatible, I was aware of the challenges awaiting me:
- burning the bootloader for ATmega328 with internal 8MHz oscillator;
- connecting a makeshift FTDI header;
- adapting the software to work in Arduino IDE;

Once I figured out the fuses, burning the bootloader was straightforward. For this purpose I created this section in boards.txt:

intclock328.name= ATmega328 Internal clock 8MHz
intclock328.upload.protocol=stk500
intclock328.upload.maximum_size=30720
intclock328.upload.speed=57600
intclock328.bootloader.low_fuses=0xE2
intclock328.bootloader.high_fuses=0xDA
intclock328.bootloader.extended_fuses=0x05
intclock328.bootloader.path=atmega
intclock328.bootloader.file=ATmegaBOOT_168_atmega328_pro_8MHz.hex
intclock328.bootloader.unlock_bits=0x3F
intclock328.bootloader.lock_bits=0x0F
intclock328.build.mcu=atmega328p
intclock328.build.f_cpu=8000000L
intclock328.build.core=arduino
Then I checked that the bootloader works, by uploading a simple sketch that outputs on D9 (which is connected to the buzzer). Before doing this, I had to connect the FTDI breakout to the board. Luckily (more probably intentionally), all of the required pins (Rx, Tx, Vcc, Gnd, Rst) are broken out on the female header, so just using wires worked, as shown in the next photo. This is not a nice solution though, since the top board (with the VFDs) needs to be removed before every sketch upload. (Hopefully the Akafugu team will also include an FTDI header in the next revision.)



















After I soldered all the parts on the base board, I realized the biggest problem of them all: some parts (sourced by me, and obviously of different size than those in the kit) stuck well above the two lateral headers, so the display shield (upper board) could not be plugged in the base board. Almost a showstopper at this point. The solution was to replace the tall 330uF/16V capacitor, bend both 47uF capacitors sideways, replace the 330uF/50V with a pair of thinner 100uF, push both inductors as much into the PCB as possible, then cut the top plastic wrapping on one of them. These, combined with the male headers not being flush to the display board when I soldered them, did the trick. I managed to have the clock looking as intended by its creators (if you don't look too closely), as shown in the next photo.



















The Akafugu designers tried to minimize the size of the clock, leaving little room for flexibility (that is, ability to use a broader range of components, of different sizes eventually). The sizes for the chosen components (especially the capacitors and the inductors) are really unique, any deviation would lead to the boards not fitting together.
Using (smaller) SMD components would not be a good solution, since these would also need to be pre-soldered (or otherwise potential non-SMD-soldering clients would be excluded). The only compromise I can think of is enlarging the base board a little bit, keeping in mind that this 4-tube version is the smallest of the display boards, all others extending laterally beyond both sides of the base board.

And finally, the last challenge: the software. I started from the original C code published here. This was written for the avr-gcc compiler and produces a hex file, which is then flashed onto the processor (no bootloader needed, nor provided) using an ISP programmer.

I changed the code (available here), mostly cosmetically, to compile with Arduino 1.0. It does not require any other external libraries and it includes its own I2C/Wire functions (does not use Arduino's Wire library).

In the end, I have an Arduino-compatible Akafugu VFD modular clock that looks just like the original one (I need to add the spacers though).

The Nixie tube, a neon-filled tube with a series of 10 cathodes shaped like numerals, is a classic display for any build wanting a unique, vintage, or steampunk aesthetic. We shouldn’t be surprised a factory in China is now turning out Arduino-compatable Nixie modules (English translation, but don’t get your hopes up), but there it is.

The modules are based on the QS30-1 Nixie tube capable of displaying the digits 0 through 9, and include an RGB LED behind the tube for some nice additional illumination. According to the manual, the modules themselves are based on a pair of 74HC595 shift registers, and are ‘stackable.’ By applying 12 volts to a pair of pins and connecting another 5 wires to an Arduino, it’s possible to drive as many of these Nixie modules as you’d like.

[Paul Craven] got his hands on a quartet of these modules and is planning on building a steampunk style alarm clock as a personal project. [Paul] was able to get the modules up and running fairly quickly, as seen after the break.

While they’re most certainly not the cheapest option, if you’re planning a build with Nixies, this probably is the easiest way to get a vintagey, steampunkey numerical display.

Filed under: arduino hacks, led hacks