[Upcycle It] Nixie Display #9 - Hardware hacking

Last week I told you my plan to switch to the other Edison with the Mini Breakout BoardMini Breakout Board and mount this board inside the display case. This weeks blog has a lot of images, to show the process of putting it all together.

Updating and installing the EdisonEdison was not a big issue, but then I came at a point that I completely overlooked. The GPIO levels of the Mini Breakout BoardMini Breakout Board are 1.8V, instead of 5V like with the Edison Kit for ArduinoEdison Kit for Arduino and GROVE kit. So first action was to select and order logic level converters. For this post I had a lot of help from the schematic and hardware guide of the Mini Breakout BoardMini Breakout Board, as wel as from the picture below, which I found on the internet.

Logic level converter

After some digging on the internet I found a couple of links to a Sparkfun bi-directional level converter with four channels which according to the references works for the Edison (https://www.sparkfun.com/products/12009). Sparkfun also has a very nice and instructive guide for this board: https://learn.sparkfun.com/tutorials/bi-directional-logic-level-converter-hookup-guide

I ordered it immediately and three days later it landed on my desk:

Each channel uses a single N-channel MOSFET and a couple of pull-up resistors to realize bi-directional level shifting:

(image courtesy of Sparkfun)

First action now was to connect this small board to the Mini Breakout BoardMini Breakout Board.

LV is connected to the 1.8V IO Voltage on the Edison, while HV is connected to the 5V voltage which drives the PCF8574PCF8574 ICs. LV3 and LV4 are connected to the SCL and SDA respectively on the Edison side, the same for HV3 and HV4 on the e PCF8574PCF8574 side.

To be honest, due to the size of the pads and the very small distance between them this was not a strait forward task, and I really needed my magnifier lens as wel as some desoldering braid.

I soldered three header pins to J19, pin 1-3, which are NC, 1.8V and GND. I also soldered wires to the I2C lines at J18-6 (SCL) and J17-8 (SDA). Header pins also are soldered to the high voltage side of the level converter.

Next I soldered the low voltage side of the level converter to the Mini Breakout BoardMini Breakout Board.

Before continueing I wanted to be sure that this level converter functions properly, so the board was connected to the counter and tested. After a first attempt in which I accidentally swapped the SDA and SCL it again showed the seconds tapping past on the nixie display .

Interfacing the button

As you have seen on the images, there is a button on the front pannel of the counter. I expect its purpose was to reset the counter, but in my case it was not connected. On the main circuit board I found some pads where it possibly was connected to, but someone did remove the wires. On the images below you see the back of the front pannel, with on the left the back of the button, as wel as the loose pads.

Having a button is very useful, my plan is to use it for switching between de different display functions. For this I added pin headers to the button and an GPIO input on the Mini Breakout BoardMini Breakout Board. GP182 was the most convenient, so I added a pin header and pull-up resister to the 1.8V IO power, as can be seen on the pictures below. This way the button can be connected between this pin header and GND.

Power

Another question is how to power the board. According to the specs the counter has a power supply with 224V to drive the nixie tubes, 12V for a function which is not clear to me and 5V for the logic. The 12V appeared to be 14.7V, but its power rating is far to less to power the Mini Breakout BoardMini Breakout Board via J21. So I need the 5V output for powering the Edison. There is a big transistor mounted to a aluminium plate in the power supply unit. I can't see the type, but  it looks like a 2N3055 or similar, which can drive several amps. During normal operation this transistor isn't very warm, so I expect the power supply can provide enough power to both the counter and the Edison. Therefore I decided to power the Mini Breakout BoardMini Breakout Board from the counters 5V. The easiest way to do so was by cutting a USB cable in pieces and solder the wires to the counters main board. I also placed some header pins to the 5V and GND, for connections to the level converter and button. The USB cable is securely fastened to the power supply with a ty-rap.

Assembling the boards

The counters electronic and mechanical design is rather convenient. It almost looks like it is designed for hacking. There is plenty free space to add stuff, like in this case the Mini Breakout BoardMini Breakout Board. The front and backplane are connected using metal rods with holes. On the bottom side the PCB is mounted with screws through this holes, on the top the holes can be used for hacking .

I drilled an extra hole and mounted the Mini Breakout BoardMini Breakout Board with spacers to the upper left rod.

Final result

Finally the other I/O extender boards were built and inserted and all connections were put in place.

Although I have no idea why, it should be very easy to bring the display back in its original state. Just remove the three I/O extender PCBs and the Edison and replace the 7490 counters and the counter is up and running again.

This concludes the blog of this week. The hardware is almost ready now, so three weeks left for finishing the software.

Stay tuned!

Links

• Mini Breakout BoardMini Breakout Board:
  • Schematic: http://download.intel.com/support/edison/sb/mini_edison_breakout_hvm_8_26.pdf
  • Hardware guide: http://download.intel.com/support/edison/sb/edisonbreakout_hg_331190006.pdf
• Sparkfun level converter:
  • https://www.sparkfun.com/products/12009)
  • https://learn.sparkfun.com/tutorials/bi-directional-logic-level-converter-hookup-guide