There are some beautiful radio-controlled watches available these days from Citizen, Seiko, Junghans, and even Casio. These timepieces don’t need fiddling every other month, which is great if you have more than one or two and can never remember what comes after “thirty days hath September…”

In the US, these watches work by receiving a 60-bit 1-Hz signal on a 60-kHz carrier wave broadcast from Fort Collins, Colorado called WWVB. The broadcast is quite strong and generally covers the entire continental US, but some areas of the country can have unreliable reception. I live in the SF Bay Area in an area with high RF noise and my reception can be spotty. My watches sync often enough that it’s not an issue 363 days out of the year, but sometimes they can miss DST shifts for a day or two. The east coast is known to be even more challenging.

Wouldn’t it be great if anyone could set up a little repeater to transmit the time so their watches were always in sync?

WWVB has been around awhile and there have been various other projects (1,2) that have demonstrated the feasibility of making your own WWVB transmitter. But these all had very limited range. I wanted to build something that could cover my whole watch stand and be based on a more familiar toolset for the typical hobbyist, namely USB-based 32-bit microcontroller development boards, WiFi, and Arduino. My goal was to make something approachable, reliable, and attractive enough it could sit with my watch collection.

Is this legal?

The FCC requires a license to transmit, but has an exemption for 60 kHz transmitters as long as the field strength is under 40 μV/m40 μV/m at 300 meters. You will definitely not exceed this limit 💪🏼