These display panels were lying around for quite some time now, waiting for an hypothetical interest. I decided to gave them a bit more focus after the interest shown on the dittohead's page : https://hackaday.io/page/3002-giant-nixie-panel-from-ussr The challenge here is in several parts. - Finding documentations and translate them. - Driving the elements with >360 volts. - Find a solution to drive the 128 inputs from few microcontroller GPIO - Ultimately make it controllable with I2C/SPI
There is different kind of these panels, some in a green dot 32x32 matrix, others are alternating green/orange 64x64, or even an alternating Red/Green/Blue 64x64 matrix.
I'm not sure if the light is emitted from a phosphore as the VFDs or by a gas as the Nixie tubes. However by looking closely, a volume is glowing withing each pixel elements, and not a surface, so it seems to be a gas (maybe neon, argon, mercury vapor, etc).
Usually dot matrix displays use shift register, demux, counters to scan the column while driving the rows. In the most multiplexed option, only 2/3 timing and one data lines are needed. A frame sync, a column clock and a pixel clock. Then the pixel data is synced one by one. Resulting a rather high frequency toggling on the data line.
I would like to try to send the data 8 by 8, so for a 100Hz refresh rate, instead of 409.6KHz, it gives 51.2KHz. Doing so will reduce the logic components needed as well.
Before going further, I would like to test driving the anodes 8 by 8. The circuit to generate the 360V pulse need to charge a small capacitor, and my little power supply had to handle the current peak to charge them 8 by 8.
So I cabled 8 anode drivers on the breadboard but had to use the FMMTA42 SMD transistor for 4 cathodes as I didn't have enough MPSA42 transistors. I tried the timing parameters as if the all screen is driven. To summaries, the cathodes are switched one after the other at a 156us rate (100Hz * 64), then wait 10ms as if the screen is at 100Hz. And the 8 anodes are charged during a 8th of that time as there will be 8 block of 8 pixel to drive per cathode.
I think it needs some adjustments, mostly because I forgot here to introduce a small pause between the cathode switch. It results that some pixels are flipped with a small delay. However, it seems to be promising for more pixel driving.
The brightness could be improved by increasing the refresh rate, but I faced unstable pixel ignition. It is maybe due to a too short time to charge the capacitors.
Besides, the brightness is correct, below is in a room with the direct sun luminosity (3:00pm):
And then the same with two light bulb:
I spent few hours to draw a small PCB to drive 8 anodes and 8 cathodes. I've just put a logic buffer in order to have a 'chip select' line. That way I can quickly drive a full screen with 32 GPIO. It's a lot but doable with a microcontroller. The aim is to check rather easily if this driving strategy is valid before continuing. If it's working I can keep the 8x8 driver PCB and think about a more efficient logic.
In the view of making a PCB to hold the 64 anode drivers plus the 64 cathode ones, it is time to test some SMD components in order to make it as compact as possible.
For the transistor, there is apparently a equivalent from Diodes inc, the FMMTA42, 0.03€ in 250 quantity. The high voltage diode can be a bit expensive in smd format. However NXP has the BAS21VD, which is 3 diodes in a SOT package, 0.06€ in quantity of 100.
I would like to take some margin with the passive components regarding the max voltage rate. It appears resistor and capacitors are more easy to find in 1206 format than 0805 for a >200v grade.
Fortunately I have convenient adapter board to make some test on a breadboard to ensure the components works:
Here the second anode is driven with the SMD components while the others with the previous ones. No difference is visible, so maybe it's time to start KiCad. And start to think about some shift registers and demux to drive these circuits without the need of 128 GPIO!
Even before receiving the panel from ebay, I sought information on internet about how to use these displays. There is of course a short datasheet supplied with it, but no clear schematics for a driving circuit. The datasheet is shown in the dittohead's page.
I found someone managed to display a space invader on the 32x32 version and made a video of it :
I tried to extract a schematics from the video, but the type of the transistors and some values of resistor and capacitors are unreadable.
I deduced the following circuit, where the 360v needed to ignite the elements are produced by switching in serial with a 180v source a capacitor charged at the same 180v.
So I started to reproduce on a breadboard the circuit for one anode, and put one cathode to the ground. I used a 'nixie' high voltage power supply (by lumos.sk, nicely done by the way), some stupid guesses for the resistor and capacitor values, small diodes, and MPSA42 transistors.
I got the 360v short pulse when I put the transistor Q2 base to 5v.
And it lights up.
But a bit too bright, in the sens of I'm damaging the elements. It is clear too much current is drawn, or the pulse is too long. After few minutes of running the pixel got dark, event not powered, the burn is visible.