A VFD (Vacuum Fluorescent Display) is sort of like a combination between a 7-segment LED display and a CRT (Cathode Ray Tube). The seven anode segments that form each digit in a VFD are coated in phosphor. A heated filament cathode generates electrons, and an anode matrix is used to direct those to collide with the phosphor. When they do, the phosphor lights up brightly. VFDs were commonly used in consumer electronics before LEDs became common, because they were bright enough to be easily read. Instructables user Pakue has a tutorial on how they used one to make a retro wristwatch.
Pakue was inspired to build this wristwatch by a similar project from a few years back called ChronodeVFD and built by Johngineer. Unfortunately, Johngineer didn’t provide many details how how his ChronodeVFD actually worked. Pakue used the same vintage IVL2–7/5 VFD that Johngineer did, but had to figure out how to make it work on their own. In the Instructables post, Pakue explains how they were able to get the VFD working, and eventually how they designed a custom PCB to turn it into a wristwatch.
First, Pakue needed to identify the pins to drive the VFD, which they did with a combination of testing and datasheet references. They then designed an ESP32-based PCB to work with the VFD. The entire wristwatch is powered by a single AA battery, but the ESP32 module, VFD filament, and VFD anodes all required different voltages. Pakue used one boost converter to push the AA battery’s 1.5V up to 3.3V for the ESP32. That 3.3VDC also heats the filament, which usually requires 2.4AC, by modulating the power with an H-bridge. Finally, the anodes are powered by a second 24V boost converter. The finished wristwatch probably won’t run long on the single AA battery, but it will look great while it does.