DIY JJY Simulator with ATOM Lite: Sync a Radio-Controlled Clock Without a Signal

After moving to a new home where my radio-controlled clock could no longer receive the JJY time signal, I built a low-cost JJY simulator using an M5Stack ATOM Lite and a JJY antenna board. This is a record of how I solved the problem for well under JPY 5,000.

The Problem: Radio-Controlled Clock Stopped Syncing After Moving

Radio-controlled clocks are convenient precisely because they set themselves automatically. After moving, however, my clock stopped receiving the JJY signal indoors, and the displayed time slowly drifted out of sync.

The clock could still pick up the signal near a window, but carrying it there and back every time it needed syncing was not a practical long-term solution.

What Is JJY?

JJY is the call sign for Japan’s standard time radio signal, broadcast by the National Institute of Information and Communications Technology (NICT). Two transmitters cover the country: 40 kHz from Fukushima (Ōtakadoya-yama) and 60 kHz from Saga (Haganeyama). Radio-controlled clocks receive this signal to maintain accurate time automatically.

The device described in this article simulates the JJY signal and rebroadcasts it locally, allowing a clock to sync even when it cannot receive the real transmission.

Commercial Retransmitters Are Expensive

There are commercial products designed to solve exactly this problem — they receive accurate time from NTP servers or GPS and retransmit a JJY-compatible signal indoors.

Kyohritsu Denshi AC-Synchronized Time Transmitter P18-NTPAC

Amazonで見る

KEISEEDS GPS Radio Clock Repeater P18-NTPGR

Amazonで見る

Radio Clock Signal Transmitter with Built-in Clock P18-NTPLR

Amazonで見る

These products work well, but they typically cost JPY 10,000–30,000. Hard to justify for a single clock at home.

The DIY Solution: ATOM Lite + JJY Antenna Board

Searching Switch Science (a Japanese electronics parts retailer), I found a JJY antenna board designed to work with M5Stack devices — exactly what I needed.

Required Parts

Note: M5StickC has been discontinued. For new builds, the recommended combination is ATOM Lite with the JJY antenna board for M5Atom. The original article used M5StickC, which is what appears in the photos below.

At the time of purchase, the antenna board and M5 device together came to under JPY 5,000 (prices may vary — check current listings before ordering). That is roughly one-third to one-sixth the cost of a commercial retransmitter.

Setup

Setup follows the instructions in the GitHub repository linked from the Switch Science product page:

• Project page — GitHub: BF-018A
• JJY simulator sketch (.ino)
• Qiita article: Generating a JJY-like signal with M5StickC / M5Atom using Ticker

Flash the firmware following the instructions and there are no unusual steps. It worked on the first attempt without any issues.

Testing

With the firmware flashed, connect the M5 device to the JJY antenna board and place it next to the radio-controlled clock.

The transmitted signal is weak and short-range. The device needs to be within a few centimeters to a few tens of centimeters of the clock to sync reliably. In practice this is not a problem — once the clock has synced, it holds accurate time on its own until the next sync cycle.

Known Limitation

One issue worth noting: if the device is used near a microwave oven, the Wi-Fi connection drops and does not automatically reconnect. A manual restart is required. Either keep the device away from the microwave, or add a periodic auto-reset to the firmware.

Summary

A practical and affordable way to keep a radio-controlled clock synced in a room with poor JJY reception. If you are in a similar situation, this combination is well worth trying.