Small steps

Sometimes a project stalls for a while. A long while. You know you will get back to it. You just have to compete all the other tasks in life first. But you know that you will not quit. You will finish the project - oh yeah. All that it takes is a little discipline. And a few years.



My decatron-nixie-HF-radio is one such project. I know I will finish it. Its just going to take a while. Today I surprised myself after a hiatus of 10 months and finished a 9 V regulator for the 500 V step-up DC/DC-converter for the decatron tubes.



It was only a 10-minute build. And it was not pretty. But it was an important step towards the (one time in the future) finished project. Stay tuned. The front panel is soon to be finished.

Dekatron tubes controlled by arduino

Interfacing dekatron tubes with a microcontroller is fairly easy, once you understand how the tubes work. Threeneuron's Pile o'Poo of Obsolete Crap provides the necessary background information and schematics for making this work.

I used two russian OG-4 tubes. I prefer the orange look of the neon tubes rather than the purple look of the argon filled OG-3 tubes. The latter tubes just look to modern for my liking. On the above picture you can see how I have mounted the tubes on a rig alongside two Magic eye tubes.

The schematics is more or less directly from the Threeneurons page. It uses two output pins from the arduino, and one input-pin. My high voltage supply is somewhat unstable, so I used a zener diode to protect the input pin from over voltage.

The above picture show the PCB, soldered Manhattan style. The high voltage power supply is from Ebay, and works best with less than 10V on the input side, but can provide up to 1000V. The current is however in the microampere area, hardly enough to kill a mosquito, and just enough to drive two dekatrons at 450 V. There is also some other outputs on the board providing around 170 and 250 V respectively.

I am going to use the dekatrons as part of the display solution for my homebrew RF transceiver. Now I have the Nixie display, magic eyes and the dekatrons under control (the radio itself is not finished yet). Even my cardboard mock-up is looking great!

The above video show the prototype assembly.

The dekatron code is as simple as this. Notice that the input pin is not used in the below code.

int DekIn1=13;
int DekOut11=12;
int DekOut12=11;
int DekIn2=6;
int DekOut21=5;
int DekOut22=4;

int count=0;

void setup()
{
  pinMode(DekOut11, OUTPUT);      // sets the digital pin as output
  pinMode(DekOut12, OUTPUT);      // sets the digital pin as output
  pinMode(DekOut21, OUTPUT);      // sets the digital pin as output
  pinMode(DekOut22, OUTPUT);      // sets the digital pin as output
}

void loop()
{
  if(count<30)
  {
    digitalWrite(DekOut11, HIGH);
    delay(10);   
    digitalWrite(DekOut21, HIGH);
    delay(50);                 
    digitalWrite(DekOut12, HIGH);   
    digitalWrite(DekOut22, HIGH);
    delay(10);  
    digitalWrite(DekOut11, LOW);  
    digitalWrite(DekOut21, LOW); 
    delay(10);  
    digitalWrite(DekOut12, LOW); 
    digitalWrite(DekOut22, LOW); 
    delay(10);
    count++;
  }
  else if(count>=30 && count < 60)
  {
    digitalWrite(DekOut12, HIGH);
    digitalWrite(DekOut21, HIGH);     
    delay(10);                 
    digitalWrite(DekOut11, HIGH); 
    digitalWrite(DekOut22, HIGH);    
    delay(5);  
    digitalWrite(DekOut12, LOW); 
    digitalWrite(DekOut21, LOW);       
    delay(5);  
    digitalWrite(DekOut11, LOW); 
    digitalWrite(DekOut22, LOW);  
    delay(5);
    count++;
  }
  else
    count=0;
  
  
}

Building Farhans Minima Transceiver Part #1

Introduction

I have started my most ambitious electronics project so far. After building my Softrock RXTX I was eager to learn more about RF designs, and HF radios in particular. I came across Farhan, VU2ESE, via Soldiersmoke and decided to build a transceiver based on his Minima.

There are three published versions, and many variations in between that are produced by others. All versions are based on a Si570 PLL Local oscillator and a discrete component BFO.

The first version uses mostly discrete components, KISS-mixer and discrete component audio amplifier, The IF is 20 MHz and it switches between two filters to cover the entire HF band.

The second version uses a FST3253 mixer and a TDA2822 audio amplifier

The third version uses a standard diode ring mixer and a TDA2822 audio amplifier and an IF of 24MHz. I have seen a few versions of the first Minima verison around the web, but I have not seen any of the other two. 

Building my own version

I plan to base my design on the third vesion. It seems to be the simplest of the three, although it does not cover the whole HF spectrum. Since the IF is 24 MHz and it uses only one LPF it is limited to the frequencies below 24 MHz.

To make things a bit more interesting, I have planned to make a few alterations to the Minima 3. First, I will use a Si5351 as the local oscillator instead of the Si570. I used the Si570 in the Softrock RXTX so this gives me the opportunity to try something new. In addition, it contains three oscillators so one can serve as the LO while the other can serve as the BFO. N6QW has advocated for this several times, so why not give it a try.

Further, I will use ADE-1 double balanced mixers for both mixing stages. I will also use a standard LM386 as audio amplifier as I have many of those in the junk box.

Display and front panel

The Minima design and the software from Farhan is based on a 16x2 display.

I have used the standard 16x2 and 20x4 LCD displays in several projects previously: My 3D-printer, my timelapse device, a Shruthi-1 synth, a LXR drum machine, and more recently, my SWR-meter all use this display technology. This time I want to try something different, something more in the spirit of homebrew oldschool radios, and something more difficult. We are talking, Nixie tubes, Dekatrons, tuning eyes and neon indicators. No freakin LCD display or even a single LEDs on this rig.

My initial front panel is made in Front panel express. I have printed a test version on paper and glued it to a piece of cardboard. The purpose is to verify that all the bits and pieces fits to the panel before I order the final panel from Schaeffer.
In the above picture, the eight nixie tubes, one dekatron and one tuning eye is mounted temporary.

I think it will look great when the front panel is all finished. For the time being, we have to use our imagination, but nothing beats the warm glow from nixie tubes and dekatrons. The nixie-board from the above picture is one of my previous creations.

Audio

Moving to the radio itself, I started out building a LM386 audio amplifier. Homebrew hero N6QW always recommends to get the audio part going first, so here we go. The circuit has only 20dB gain at the moment, and I will have to increase the gain to at least 40dB later on, but it is enough to get started.

The sound from the little chip is not bad, and I hooked it up to my iPhone for testing. The speaker is a simple 8 ohm 8cm speaker from Tayda.com.

Low pass filter

The next stage I built was the low pass filter.

As there are no build manuals for homebrew projects, I decided to simulate some of the circuit modules. This is simple in LTSpice and gives valuable insight.

The simulated frequency response seem reasonable for a sub 24MHz receiver.

I used 0805 surface mount capacitors for the LP filter. I made some room on the PCB for a second LP or BP filter, should I have the urge to go above 24 MHz in the future. I hooked up the ADE-1 to the Si5351 via a 6dB pad. The output is terminated to a 51 Ohm resistor for testing. I am not sure whether the 6dB pad is necessary, since it is possible to adjust the Si5351 output power to drive the required 7dBm to drive ADE-1 from the Arduino software.

Future work

There are many parallel projects going on in this build, there are dekatrons and neon tubes that should glow, there are front panels and mechanics to resolve, and there are amplifiers to solder, and crystals to characterize. Hence, I did not want to collect everything in one big and totally confusing posting at the end (will it ever end?). The next stage is to finish the IF amplifiers and the crystal filters. I hope that others will take on building the third version of the Minima as well. If you are one of them, I would like to know. Stay tuned.

Nixie tube experimentation with IN-12

Introduction

I have always wanted to try out Nixie tubes in one of my projects, and here we go. Nixie tubes come in many different sizes and there are both top view and side view nixie tubes out there. I settled on some russian IN-12 top view nixie tubes as they are cheap, and should fit nicely on a front panel (for example on a radio).

The IN-12 was originally used in russian multimeters, radios, frequency counters. This R-155A Brusnika Radio Station serves as a good example on how nixie tubes were used in the cold war era.

Recently, this particular tube has been used for many hobby purposes such as clocks, geiger counters, etc. While I am mentioning it, the major page for steampunk and Nixie-fans is Bad dog designs having loads of excellent clocks using nixie tubes. Ok, enough drooling over other designs. Back to my modest attempts.

Testing a nixie

I purchased a 110-180V power supply kit from ebay. I highly recommend this kit from seller lumos-sk. It comes with an excellent build manual and was very easy to assemble.

The assembled power supply was set to 170V...

...and the IN-12 glows fine.

A PCB for four IN-12 tubes.

To control the nixies, I made a PCB in Eagle. The circuit uses two 74HC595 shift registers to drive four K155ID1 decimal decoders.

The schematic is shown above.

It is possible to cascade several of these boards to drive eight, twelve, sixteen, etc Nixies from just three pins on the microcontroller. I ordered the PCBs from OSH-park, and they were excellent.

However, I should have read Kevin Ryes blog before creating the PCB. I just took it for granted that the Eagle IN-12 part was correct, but as Kevin Rye found out, it is not. The pin numbering on the silk screen is wrong and the anode is connected to pin 5 instead of pin 1. However, if one ignores the numbering, my PCB works if the IN-12 is mounted on the back side of the board (i.e. mirrored). 

Originally I planned to use IN-12 sockets (purchased from Ukraine) but since the IN-12 part was mirrored, the mounting holes were on the wrong locations and it made no sense to use the sockets. In addition, the parts I got were used, so I had to desolder chunks of 30 year old wiring. Totally boring work. So instead, I went back on Ebay and ordered some pins from the old Soviet Union (see picture above). They were excellent for this purpose.

To test the shift registers, I copied the Arduino code made by Imperkins over at Instructables. The only change was to add support for an additional shift register.

The above film shows the Nixie PCB in action.
As mentioned, it is simple to cascade several PCBs to control more nixie tubes. My secret plan for the future is to use eight nixie tubes as a frequency display for a HF radio, such as Farhans Minima.

Further Reading

1. http://scott-bot.com/nixie-bot/
2. KiCAD library
3. IN-12 datasheet (in Russian)
4. IN-12 geiger counter (with laser cutted parts).
5. IN-12 clock (shares Eagle files and svg of front panel cutout)