YouTuber Charles Lohr has come up with a novel and low-cost way to drive Nixie display tubes, turning an ultra-low-cost RISC-V microcontroller into what he describes as a "software-defined flyback power supply."
"For years I eyed the cheap microcontroller sector," Lohr explains. "A month or two back I head about the CH32V003 from WCH [Electronics]. It was just right. A 40MHz RISC-V processor with 16kB of flash, 2kB of RAM, DMA [Direct Memory Access], lots of peripherals, a single-wire debug interface. It can even run at 5V like a [Microchip] AVR! I am a sucker for tiny parts, so I just had to get the QFN at $0.12. For other packages, like the SOIC version, it can dip down below $0.10."
Having the part is one thing, but it's not much without a project to go with it. Rather than blink an LED, though, Lohr wanted to do something a little more interesting: use the microcontroller to drive a Nixie tube, a cold cathode display device dating back to the mid 1950s and named from the prototype's identifier as "Numeric Indicator Experimental No. 1." Inside the gas-filled tube is a wire-mesh anode and a number of cathodes, shaped into numbers, letters, or application-specific symbols and which glow when energised.
To get a Nixie glowing, though, takes more than just connecting up a simple low-voltage power supply, which is where Lohr's project comes in. "These boards […] use a 12-cent CH32V003 as a closed-loop software-controlled flyback controller that [uses] a 3-cent FET [Field-Effect Transistor] and this itty-bitty 30-cent transformer, one cent diode, and two cent smoothing capacitor to produce the 180V at 2.5mA or so needed to power the Nixie tubes — all for under 50 cents in parts!"
The microcontroller is set up to take control signals from a host and handle the closed-loop control system, triggering FETs on each cathode to illuminate particular numerals. "The flyback works like a boost converter," Lohr explains of the circuit's other critical component, "except that, because there's a 10:1 turns ratio on my transformer, it gives me 10x the voltage."