HTMA Week 5: Nixies and VFDs

Nixies and VFDs

During this week, I remembered that I had always wanted to build a nixie watch, or at least a nixie clock. Something like this:

So I decided to pivot my final project to doing just that. I’ll spare the details of my research into nixie tubes for a separate page dedicated to the final project, here are the important bits:

• Nixie tubes use 180V, which surpasses the 50V threshold for the class.
  • I’m looking at using IN-17 tubes for their small size and closeness to the reference image.
    • These require
      • No more than 170V to strike
      • 120V-170V to maintain
        • 1.5mA current

A Nixie Tube

• An alternative is to use Vacuum Fluorescent Display(VFD) Tubes, which only require 12V to 30V to operate, but has a slightly different mode of operation than nixie tubes.
  • I’ve purchased 6 IV-22 VFD Tubes off of eBay for $36.44.
    • These require
      • 1V-1.32V for the filament
        • 100mA nominal current consumption
      • 22V-30V for the grid
        • 6-10mA grid current
      • 22V-30V for the anode segments
        • 4mA of anode segment current

Vacuum Fluorescent Display Tubes

• To minimize re-doing work when I eventually switch to nixie tubes, I’ll try to design the PCB as if we’re using nixies where possible.
  • This means we’re using the TPIC6B595 Power Logic Shift Register instead of regular shift registers.
    • These ICs run on 5V logic, so we need to use the 74HCT04 in order to convert 3.3V from the RP2040 IO pins to 5V.
    • We pull the SRCLR pin high to 5V because we won’t have to clear the registers, this also saves one pin.
  • VFDs still need at most 30V to power the grid and anodes, so we make use of the TLV61046A for its small footprint and efficiency.
    • We can eventually use this as an input for a boost circuit to generate the needed 170V.
    • To program for 28V output with maximum 6mA output, we need the following setup
      
      - However, we won't implement this in the schematic for this week. We'll use it for the next prototype when we get the VFD tubes in hand.

For now, we’ll go with the following schematic to control 2 LED diodes.