In short, the build consists of these main stages:

1. Salvage and identify the recycled calculator VFD display.
2. Reverse-engineer the filament and high-voltage supply requirements.
3. Build the ATmega328P + HV5812 VFD driver section.
4. Implement multiplexing, blanking and brightness control in firmware.
5. Add the single-AA NiMH / USB power system.
6. Solve VFD ghosting and blanking timing issues.
7. Add firmware-controlled VFD oscillator inhibit. The ATmega drives a BC547 transistor, which pulls down the ZTX851 oscillator base drive through a 100 ohm resistor. This allows the firmware to shut down the VFD high-voltage and filament converter when the display is intentionally turned off.
8. Add power-path stabilization for the 5 V logic rail. A capacitor on the shared 5 V rail prevents the ATmega from resetting during the transition between the internal 5 V branch and USB power.
9. Add status indicators for 5 V, USB, VFD high voltage and filament activity. The 5 V and USB LEDs are simple hardware indicators, while the HV and filament LEDs are firmware-controlled. HV is sensed through a 1 MΩ / 47 kΩ divider on A4, and filament activity is sensed through a similar divider on A5.
10. Finish the mechanical construction, auxiliary LED/support PCB and front-panel styling.

This Hackaday project page is mainly used for development logs, design notes and technical updates. The Instructables page contains the full illustrated build log.