Jul 17, 2023
Charles Lohr Turns a $0.10 RISC
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
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."
Lohr's full video is available on his YouTube channel, while the project's PCB design files and firmware can be found on GitHub under the permissive MIT license.