DFC77 ESP01

Local DCF77 Transmitter

A personal project to generate a local DCF77-compatible signal for radio-controlled clocks at home.

I live in Slovenia, far from Frankfurt, so my clocks rarely receive the original DCF77 broadcast reliably.

This project solves that by retransmitting a local signal indoors.

Why This Exists

DCF77 reception can be unreliable in my location. Instead of constantly resyncing clocks manually, I built a small local transmitter that gives all clocks in the house a strong, stable signal.

Two Implementations

This repository contains two transmitter approaches:

1. Software-triggered RF
2. Hardware-generated RF using a Si5351A daughter board module from AliExpress

I started with the software implementation first because I was impatient and did not want to wait for shipping.

Results and Iteration

1) Early Software Prototype

• Initial build:
• It worked, but timing jitter was visible:
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• Despite the jitter, it successfully synchronized a clock:
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2) Hardware RF with Si5351A

• Module used:
• Trigger quality improved:
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• RF signal quality improved as well:
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3) Final Tuning and Installation

Final tuning required moving the primary and secondary coils for optimum resonance and power transfer.

• Final tuning was done by observing the induced signal in a nearby DCF77 receiver coil:
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• Best coupling happened when both primary and secondary coils were near the end of the ferrite rod.
• Final practical challenge was finding one orientation that worked for all four clocks in the house.
• The final unit now runs from a cupboard:
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Repository Structure

• Firmware source code: ESP01S/
• KiCAD schematics and PCB: KiCAD/

Notes

• This is a local, experimental low-power setup intended for private indoor use.
• The project is focused on practical reliability rather than laboratory-grade instrumentation.

Legal and Regulatory Note

This project is intended for private, low-power experimentation in a controlled local environment. Radio transmission rules vary by country. Before building or operating a DCF77-like transmitter, check and follow your local telecom/radio regulations.

Status

Working and deployed at home, supports:

• Over-the-Air (OTA) firmware upgrades,
• Transmits 4 times per day for 20 minutes (around 01h00, 02h00, 03h00 and 04h00).
• A tiny web server listens for manual trigger requests..