[Planning] Soyuz: A Nixie Tube Wristwatch Project

Overview

Soyuz (Союз) is a modular wristwatch project built around two NOS (New Old Stock) Nixie tubes. The goal was to turn the Nixie tube — one of the great feats of Cold War-era technology — into a brand of its own.

Key Specs (Targets)

Problem Recognition

There’s no proper Nixie-tube wristwatch brand in Korea, and even overseas brands are hard to get without importing directly. Soyuz aims to be a wrist-worn platform built around a user-swappable Mezzanine module structure, where a single body can accept multiple kinds of display modules (Nixie / Numitron / VFD…).

Design

Form Factor

With a 35mm front sapphire, a Nixie tube roughly 14mm thick, plus the PCB and high-voltage circuitry, the case comes out to 40mm × 40mm × 18mm or larger — very much on the oversized end for a wristwatch. Even so, the goal was to make it feel like an object worn on the wrist, not just a watch.

Case

• Current design: eSUN PLA+
• Planned prototype: 6061-T6 aluminum CNC, untreated hairline finish
• Production options: black/gray/titanium anodizing or SUS304 PVD
• Front window: 1–2T curved optical-grade sapphire, AR coated
• Strap: 24mm standard (racing strap)

Module Interface

The front Nixie module currently connects to the main body via a 2.54mm-pitch 8-pin connector, which carries all of the following over one connector:

• 170V HV (tube anode)
• 5V logic power
• SPI (shift-register data) — LE, DIN, CLK
• (future addition) an I2C line for module status reporting

This structure lets a user pull out a module and swap in a different type (Numitron, VFD) without tools, while the main body can read the driving profile of whichever module is installed.

Circuit Design

Overall Block Diagram

Display Driver

I chose the HV5523 (Microchip, a 32-channel high-voltage shift register rated up to 220V). Rather than a 1970s-style K155ID1/74141 BCD-to-decimal driver, a shift register was chosen for form factor and for driving more than 16 pins.

MCU

The prototype uses an ESP32-S3-SENSE module.

• BLE 5.0 + Wi-Fi 2.4GHz
• ~10μA in deep sleep

Charging / Power Management

• Qi 5W receiver: BQ51013B (TI)
• Li-ion charging uses the ESP32-S3’s built-in charge/discharge module
• System LDO: AP2112-3.3 (low-dropout, 600mA)
• Protection: built-in OVP/OCP plus an external PTC

Sensors

• IMU: MPU6050 6-axis accelerometer. Triggers an INT pin on jerk (wake-up) gesture detection
• Mercury switch: an extra switch added as secondary redundancy

PCB

A 2-layer, 1.6mm FR-4 stack-up.

• L1 (Top): signals, components, 5V power
• L2: solid GND plane
• Main body PCB: 38 × 38mm
• Nixie module PCB: 30 × 18mm

Firmware Architecture

Built on ESP-IDF + FreeRTOS. The overall flow boils down to 5 tasks and 1 ISR.

Task Layout

Anti-Cathode-Poisoning

If a Nixie tube displays the same digit for too long, an insulating film forms on the unused cathode surfaces (spotting/poisoning), causing misfires. To prevent this, a daily “nuking” routine cycles through all digits (00–99). This runs as a routine executed once per day.

Time Sync

• When BLE-connected: synced to the phone’s clock at 1-second resolution
• Offline: ESP32’s built-in RTC (±20ppm) — roughly ±50 seconds of drift per month

OTA

Uses ESP-IDF’s esp_https_ota API. The partition table is a dual-slot layout — factory + ota_0 + ota_1 — with automatic rollback on boot failure. The Flutter phone app downloads the ROM file from a server and pushes it to the watch, which verifies the checksum before applying it.

Power Profile

At 100 wakes of 15 seconds per day, runtime is roughly 72 hours.

Assembly

Soyuz is built from 3 subassemblies.

1. Nixie Module (front, detachable)

• 2× Nixie tubes (socket-mounted, user-replaceable)
• HV5523 driver
• 8-pin, 2.54mm connector

2. Main Module (body)

• ESP32-S3-SENSE
• MAX1771 HV boost
• BQ51013B
• MPU6050
• 1000mAh Li-Po cell

Assembly Sequence

1. SMT the Nixie module PCB → hand-solder the Nixie tube pins
2. SMT the main PCB → flash firmware → test power/charging/BLE pairing
3. Seat the main module + battery in the lower case → apply a silicone gasket
4. Attach the front module → sapphire window + upper case → fasten with a torque driver
5. Final functional test: time display, BLE connection, wireless charging, IMU wake-on-wrist, anti-poisoning routine

Roadmap (as of May 2026)

Trademark registration for “Soyuz” is currently complete.

Closing — What’s Next for the Modules

The plan is to keep any display module compatible as long as it matches the main body’s interface (18-pin, 170V/5V/SPI/I²C), and I’ll close this out with the two candidates under consideration for the next module.

Numitron (IV-9, IV-16, RCA Numitron 3015)

The Numitron is a 7-segment incandescent display tube developed around the same era as the Nixie tube. A tungsten filament is shaped into 7 segments, giving it the same familiar letterform as the 7-segment digits on a typical digital clock.

Circuit-wise, it’s much simpler than a Nixie tube:

• No HV boost (170V) needed — drive directly at 4.5V or with a simple 5V boost
• The main body interface stays the same

Power draw is higher than a Nixie tube (the filament must stay heated continuously), but with the HV circuitry gone, overall system efficiency ends up about the same.

VFD (IV-18, IV-22)

A VFD (vacuum fluorescent display) glows cyan: electrons emitted from a heated cathode are accelerated toward a grid and strike a phosphor-coated anode, producing light. Since the voltage differs, a voltage-switching mechanism keyed to an EEPROM setting would need to be added.

Circuit-wise:

• Filament: 2.5V AC or PWM driven
• Grid/segments: roughly 30V (HV5523) — just needs the MAX1771’s output voltage lowered
• Supports multiple digits (8-digit, 16-digit) → extended usability like alarm time, date, or short message display

Coming Up

Circuit design, firmware development logs, and the production process will keep being shared as a series on this blog.