Nixie Matrix Display

I had the idea for this project during Christmas. I am drawn to the less popular Nixie tubes and found that Nixie indicator bulbs are cheap. So in a moment of impulse I bought 200 with the idea of creating a matrix display. Unsure how I would build it at this point, but it was a fun challenge. 

My original plan was to use a series of 16-channel PWM ICs - PCA9685 - all driving mosfets to PWM control each bulb. I spun up a board and tested the idea. 

The PCB worked first time, driving each Nixie tube from a channel from the PCA9685 (mounted on the reverse). 

With the concept proven, it was time to build a frame. I decided to build an oak frame with an MDF insert painted matte black so as not to produce any glare.

I cut the MDF and drilled holes for the wires on a CNC, and used good old hand tools for the oak frame.

  

To keep each bulb, I opted for a small sticky pad, providing just enough stick to hold the bulb close to the MDF backing with the legs threaded through. 

It was all going so well... until I came to wire the bulbs.

The wiring became a mess. I decided on a new approach since I didn't need 16-bit PWM to control a Nixie tube, which only has about 4 bits of dynamic range. I found the Microchip HV5122, 32-channel shift register capable of handling 220V, more than enough for our Nixie tubes (also used in my 6 Tube Nixie clocks). My display would only require four shift registers, with PWM being bit-banged.

 

I created a massive PCB with a high-voltage supply in the middle and two shift registers. The board accepts 12v to generate the required 170v, and an external microcontroller drives the registers. 

In short order, using two of these new PCBs, the display was fully wired up and working. I opted to drive the display from an STM32F030 Nucleo board, which allowed me to write a new frame to the display every 1ms—fast enough to let me bit-bang PWM modulate the bulb brightnesses. 

I don't think the gif does the effect justice, but it makes a hypnotic addition to the living room. It has an almost lava lamp quality.

I have a few more side quests for this project, and I hope to make more of these displays in the future.