An ATtiny microcontroller decodes the DCF77 time signal from Germany's 77.5 kHz VLF transmitter and displays accurate time on an I2C screen. Developer kwolf1909 published the full project on GitHub. The receiver hardware is separate, but a single-frequency longwave receiver is straightforward to build.
The interface is one button: short press cycles display modes, long press resets the DCF77 decoder. The firmware also supports an optional external temperature sensor and RTC module, constrained only by available pins on your chosen ATtiny variant. No WiFi, no GPS, no bloat.
The reason to read the full project is the implementation detail: how a chip this small handles bitstream parsing from a low-speed time signal broadcast across Europe. Hackaday's 2012 deep-dive into DCF77 signal filtering pairs well with kwolf1909's code as a complete picture of the stack.
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