Vacuum Fluorescent Display Watch

That whole project started a while back with an hackaday article from 2014 in which [Johngineer] build the 'ChronodeVFD', a wristwatch made from an old soviet vacuum fluorescent display. It kind of triggered the 'shut up and take my money' reflex in me, but as it wasn't for sale and didn't have any design details available I quickly had to scrap that.

https://www.instructables.com/id/Vacuum-Fluorescent-Display-Watch/

The watch was designed with the following specifications in mind:

• Run (briefly) from a AA alkaline battery
• Compact size
• Wifi & Bluetooth
• Easily programmable

The brain of the watch is an ESP32 (Wroom-32 module) as it can be programmed via Arduino and has build in Wifi/Bluetooth and a very low power sleep mode. To interface with the ESP32 a FTDI USB-Serial converter was used.

The most challenging part of this project was to get the different power supply correctly designed. The VFD needs 24V for its anode and ideally 2.4V AC for the filament. The ESP32 also need its share of 3.3V at more than 240mA when heavily using wireless communications. All that has to be squeezed out of a 1.5V AA alkaline cell.

Early on the 2.4V AC was replaced by just using 3.3V on the filament and modulating the output with an H-bridge to not burn up the filament. It actually can survive 3.3V for a bit, but quickly turns into a light bulb...

The 3.3V is generated using a MP3120 boost converter, that is specifically designed to work with single cell alkaline batteries. It can theoretically go down to 0.8V, but in practice only at very low currents. But it has a built in linear regulator, that allows the use of batteries with higher than 3.3V voltage like 14500 lithium cells.

24V for the VFD comes also from a boost converter MCP1663. This one is less efficient due to the high step-up from 3.3V to 24V. The display also works at lower voltages down to 16V, but loses much of its brightness.

To switch the 24V on the display anodes 2 high-side switching ICs TBD63783A with a 16 bit I2C expander (MCP23017) are used. This can also be done with discrete PNP & NPN transistors, but due to limited board area I opted for the more integrated solution.

The whole design and layout was done with KiCad5.

https://www.instructables.com/id/Vacuum-Fluorescent-Display-Watch/