A modern implementation of the Berlin Clock (Mengenlehreuhr) using an ESP8266, WS2812B LEDs, WiFi, NTP, and adaptive brightness via LDR.

This project focuses on robust time handling, stable WiFi behavior, and clean separation of logic, avoiding fragile timezone hacks and unreliable auto-connect mechanisms.

• ✅ Correct Berlin Clock layout and logic
• 🌐 WiFi configuration via WiFiManager (fallback only)
• 🔁 Reliable WiFi reconnect using stored credentials
• 🕒 Accurate time via NTP (UTC)
• 🌍 Custom EU Daylight Saving Time (DST) calculation (no system TZ dependency)
• ⏱ Internal UTC-based RTC (rtcBaseTime + millis())
• 🌙 Nightly configurable NTP resynchronization
• 💡 Smooth second indicator fade (cosine + gamma correction)
• 🔆 Automatic brightness control via LDR
• 🎨 Centralized color definitions (no magic RGB values)
• 🔴 Quarter-hour markers (15 / 30 / 45 minutes) highlighted correctly
• ⚙️ Clear state machine design

• ESP8266 (tested with LOLIN / Wemos D1 mini)
• WS2812B LED strip (24 LEDs total)
• LDR (photoresistor) + voltage divider
• Stable 5V power supply for LEDs
• Common GND between ESP8266 and LED power

• Each LED in the 5-minute row represents 5 minutes
• LEDs at 15, 30, and 45 minutes are highlighted as quarter-hour markers

• Upper row: 5-hour blocks
• Lower row: 1-hour blocks

• Single LED with smooth fade animation (not simple blinking)

This project does not rely on ESP8266 timezone handling, which is known to be unreliable.

• NTP always provides UTC
• Internal clock runs in UTC
• Display time is calculated as:
  • Winter: UTC + 1 hour
  • Summer: UTC + 2 hours (EU DST rule)
• Daylight Saving Time is calculated manually:
  • Starts: last Sunday in March at 01:00 UTC
  • Ends: last Sunday in October at 01:00 UTC

• Deterministic behavior
• No dependency on setenv(), tzset(), or libc timezone tables
• Correct across resets and long runtimes
• Easy to understand and debug

1. On boot, the ESP8266 tries to connect using stored credentials:

   WiFi.begin();

2. It waits up to ~20 seconds.
3. Only if this fails, the WiFiManager configuration portal is started.

WiFiManager’s autoConnect() was found to be unreliable on ESP8266 in combination with WiFi stack resets and led to:

• "No wifi saved, skipping"
• Credentials not being reused after reset

Therefore:

• WiFiManager is used only as a configuration UI
• Connection logic is handled explicitly

• Stable reconnect after reset
• No fast-failing attempts
• No accidental credential erasure

• The LDR is read once per second
• Brightness is mapped linearly and clamped between configurable minimum and maximum values
• LED brightness is updated dynamically without blocking the main loop

• Board: LOLIN (WEMOS) D1 mini (clone)
• Flash Size: 4MB (FS:2MB OTA:~1019KB)
• Flash Frequency: 40 MHz
• Flash Mode: DOUT
• CPU Frequency: 80 MHz
• lwIP Variant: v2 Higher Bandwidth
• Debug Level: None

• First upload: Erase All Flash Contents
• Subsequent uploads: Only Sketch

This ensures WiFi credentials are preserved while still allowing clean initial provisioning.

• No battery-backed RTC
  → After power loss, valid time requires WiFi and NTP.
• Daylight Saving Time rules are hardcoded (EU standard).
  Political or regional changes would require a firmware update.
• GPIO0 (D3) is a boot-strap pin and therefore not ideal for WS2812 data output.
  Recommended alternative: D2 (GPIO4).

• State machine based architecture:
  • STATE_WIFI_CONFIG
  • STATE_WAIT_FOR_TIME
  • STATE_SHOW_CLOCK
• Time handling is strictly separated from display logic
• Color configuration is centralized and independent of time logic
• Rendering code contains no hard-coded color values

This project is provided as-is for personal and educational use.

You are free to:

• Fork the project
• Modify the source code
• Adapt it to your own hardware

No warranty is provided.

This project intentionally prioritizes:

• Stability over clever hacks
• Explicit logic over implicit behavior
• Debuggability over abstraction

If you can understand this code months later without rereading long issue threads,
the design has achieved its goal.