Start nixie clock project

Notes on the RetroModding IN-12 v2 clock (pinout, programming header,
plan for ESP_UniClock) and the maker's v2 schematic.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CLAUDE.md

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+# Nixie clock: notes for Claude
+
+An AliExpress WiFi nixie clock (4x IN-12 + 2x IN-3 neon dots, 8x WS2812) that is being
+moved from its closed stock firmware to ESP_UniClock (https://github.com/gpeter62/ESP_UniClock)
+with ArduinoOTA and Home Assistant (MQTT) support.
+
+## The device
+
+- On the network at **10.1.1.97** (WiFi `dlink-iot`). Stock web UI on port 80, nothing else
+  open: no OTA, no API, no MQTT.
+- Board: RetroModding (Konstantin Repnikov) "NixieIN12v2 HV2.0", stock firmware **SV1.3**
+  (©2020). ESP-12 (ESP8266, MAC 30:83:98:99:BF:DE, Espressif).
+- **Powered by USB-C 5 V only** (no DC jack). Most likely the maker's "v2.0L lightweight
+  5 volt version". The user says the PCB looks different from the 12 V v2 board.
+- **No buttons fitted**, only the light sensor.
+- Stock settings (2026-10-25): time zone +01:00, EU DST on, NTP `time.euro.apple.com`,
+  brightness auto, 24 h. Stock DST might not work: check in late March.
+
+## Pinout (from the 12 V v2 schematic, `docs/schematic_v2_12V.jpg`)
+
+Not yet verified on this 5 V board. The stock firmware image is the same for both, which
+suggests the GPIOs match.
+
+| Function | GPIO |
+|---|---|
+| 74HC595 DATA / CLOCK / LATCH (OE tied to GND) | 13 / 14 / 15 (15 has 1k pull-down) |
+| DS3231M RTC SDA / SCL | 4 / 5 |
+| WS2812 data | 12 |
+| Photoresistor (LDR, 10k to GND) | ADC (A0) |
+| BTN1 / BTN2 (not fitted) | 0 / 2 |
+
+Two chained 74HC595s make one 16-bit word, all bits active-high:
+- U1 QA..QH = cathodes 0..7, U2 QA = cathode 8, QB = cathode 9 (via 2x ULN2003, shared by all tubes)
+- U2 QC = P1 (neon dot 1), QD..QG = anodes A1..A4 (MMBTA42/92), QH = P2 (neon dot 2, and the decimal
+  point if jumper JP2 is closed)
+- The display is multiplexed: one anode on at a time. Which physical tube is A1 is unknown.
+
+## Programming header
+
+Header on the PCB: GND, GPIO0, RST, RXD, TXD, 3.3V. The USB-C port only supplies power.
+Flashing is done with a CH340C board on **COM7** (COM3 is another device; leave it alone).
+Wiring: GND-GND, adapter TXD -> clock RXD, adapter RXD -> clock TXD, adapter 3.3V not
+connected (the clock is powered from its own USB-C). No auto-reset: to enter flash mode, hold
+GPIO0 to GND, tap RST to GND, then release GPIO0.
+
+- esptool is installed for the user (`python -m esptool`, v5.6.0).
+- `firmware/stock/` holds the stock flash dump. **Never overwrite or delete it**; it is the
+  only way back to SV1.3 (`write-flash 0 <dump>`). Dumps are gitignored because the flash
+  holds the WiFi password; never commit them.
+
+## Constraints
+
+- The 170 V tube supply is live whenever USB-C is connected.
+- Hardware is only reachable via the user: anything that needs RST/GPIO0 or looking at the
+  tubes must be asked of them. Builds can be checked here, but runtime behaviour can't.
+- Show the exact plan before making changes to the device (flashing, saving settings).
+
+## Plan
+
+1. Dump and inspect the stock firmware (version strings, pins if possible).
+2. Write a test sketch that steps through the 16 shift-register bits, to verify the pinout
+   and the tube order.
+3. Add a 74HC595-multiplex driver + CLOCK config to ESP_UniClock, plus ArduinoOTA and
+   Home Assistant controls (display on/off, brightness, LEDs); its stock MQTT only
+   publishes sensors.
+4. Work continues in VS Code (PlatformIO).