Vacuum fluorescent displays (VFDs) are largely obsolete these days, and have been replaced with technologies like OLED (Organic Light Emitting Diode) displays. However they were still very popular until just a few decades ago, thanks to the crisp, bright, high-contrast light they were able to create. They aren’t mass-produced any longer, but it’s easy to find them used or as new old stock (NOS). If you can get your hands on an IV-21 VFD tube, sjm4306 has designed a tiny clock to take advantage of it.
This timepiece utilizes a single IV-21 VFD tube, which has eight seven-segment digits housed within a single vacuum tube. IV-21 VFD tubes were originally produced in Russia, but can now be found on websites like eBay for reasonable prices. When compared to Nixie tubes, the IV-21 — and most VFDs in general — doesn’t require particularly high voltages. Depending on your exact usage, they can run on anything from 30–60V. sjm4306 already had some IV-21 VFD tubes on hand, and decided to design this clock to use them.
If you want to build your own, you’ll need to have a pair of custom PCBs fabricated. One contains all of the various control components, and the other is used to mount the VFD tube parallel to the first board in order to keep things as compact as possible. The board has a Microchip ATmega328P, which controls the VFD’s segments through conventional multiplexing. Power is provided by a simple 5V micro USB port, and the voltage is boosted on the PCB. The total power consumed with all of the digits turned on is just 0.75 watts. sjm4306 hasn’t yet designed an enclosure for the clock, but it would be simple enough to make one yourself with a 3D printer.