No Signal For Your Radio-Controlled Watch? Just Make Your Own Transmitter

You can win any argument about the time when you have a radio controlled watch. Or, at least, you can if there’s any signal. [Henner Zeller] lives in a place where there is no reception of the DCF77 signal that his European wristwatch expects to receive. Consequently, he decided to make his own tiny transmitter, which emulates the DCF77 signal and allows the watch to synchronise.

A Raspberry Pi Zero W is the heart of the transmitter, and [Henner] manages to coax it into generating 77500.003Hz on a GPIO pin – close enough to the 77.5kHz carrier that DCF77 uses. The signal is AM, and transmits one bit/s, repeating every minute. A second GPIO performs the required attenuation, and a few loops of wire are sufficient for an antenna which only needs to work over a few inches. The Raspberry Pi syncs with NTP Stratum 1 servers, which gives the system time an accuracy of about ±50ms. The whole thing sits in a slick 3D printed case, which provides a stand for the watch to rest on at night; this means that every morning it’s synchronised and ready to go.

[Henner] also kindly took the time to implement the protocols for WWVB (US), MSF (UK) and JJY (Japan). This might be just as well, given that we recently wrote about the possibility of WWVB being switched off. Be sure to check the rules in your area before giving this a try.

We’ve seen WWVB emulators before, like this ATtiny45 build, but we love that this solution is an easy command line tool which supports many geographical locations.

26 thoughts on “No Signal For Your Radio-Controlled Watch? Just Make Your Own Transmitter

  1. Are these units legal? As someone who occasionally uses the WWV stations I would be quite unhappy to find out my neighbor was transmitting at the ‘same’ frequency and screwing up my measurements. As far as I can tell they are operating under the guise of ‘as long as no one notices I can transmit at any frequency I want’

    1. I’d say that given the antenna length and signal power (as I see no RF amp) the range can’t be more than a few feet.. so it would probably fall in the range of “acceptable” RF, I would think

    2. If you’re subject to the US FCC, you’re welcome (nay, *required*) to study 47CFR part 15, which details in glorious detail exactly what you may do with specific frequencies. It’s fascinating reading.

      However, interestingly, Part §15.205 “Restricted bands of operation.” makes no mention at all about operation at 60 kHz.
      And §15.5 “General conditions of operation.” says: “(b) Operation of an intentional, unintentional, or incidental radiator is subject to the conditions that no harmful interference is caused”

      Which implies that if you keep it quiet, it won’t be an issue. §15.107 and 109 give limits on how loud you can be, but only at frequencies above 150 kHz.

      But §15.5 also says “(c) The operator of a radio frequency device shall be required to cease operating the device upon notification by a Commission representative that the device is causing harmful interference. “, and you REALLY do NOT want to get one of those letters, especially if you currently hold any kind of radio licence (ham, general operator, boat, aviation, commercial…)

      1. what kind of harm can transmitting on a a watch setting frequency cause?

        it isnt like you can fowl up everyone’s clocks as that frequency is probably reserved just for the watch and if the government is going to turn off the signal it is to save money by no longer supporting obsolete technology?

        however 77 khz is in maritime mobile and radio location and fixed bands so you could cause problems with ship based sat or cell phone or radios and navigation beacons

        1. 77KHz is barely radio, I’d be amazed if anyone sent speech over it. A quick search tells me some fish finders use 77KHz audio as ultrasound sonar, which gives a good idea of how low a frequency it is. Satellite comms are usually up in the GHz. EPIRB beacons are 406MHz. All miles away from this.

          Low frequency bands like this are useful cos they travel a long way. But you’re very limited to how much data you can send. 1 bit / second is a fair bit below the limit, but I suppose they use that because it needs to be reliably received by low-power wristwatch receivers with simple circuitry, and because the correct time isn’t a huge lot of data to send.

          If this gadget really does cause interference, he can stick a resistor in to reduce the power, but I really doubt it’s necessary. A Pi’s GPIO can’t supply much power.

          The transmitting frequency is coming from a computer so it can be extremely stable. And around that sort of frequency, there isn’t much else transmitting because there’s almost no room on the spectrum. Almost no spectrum!

          Presumably he’s only going to switch it on every now and then, to sync his watch. And in fact, the most important thing, it says in the article that there’s no DCF77 signal where Henner lives. So there’s nothing to interfere with! There’s a small chance he might actually be doing a fellow watch-owning neighbour a favour.

    3. It depends on the country. It is possible the far-field signal generated by this unit is so low that it fall under regulation for unintentional radiators. The main method of coupling will be directly inductive between the transmitter and receiver coil. There probably is almost no radiated far-field energy.

      1. You may find it funny to set your watch to the time you want, but your neighbours heating control may not.
        Without professional measurement equipment, you can’t judge what the range of the transmission really is.
        Your watch might not pick it up in the next room, but the outdoor ferrite rod antenna of the alarm system from the bank across the street might.

        Should they shut down the transmitter some day, and the frequency isn’t assigned to any other service, then probably nobody will care.
        But as long as it still runs, you shouldn’t interfere with it.
        Even if YOU can’t get a signal in your area, doesn’t mean that anybody else can’t.

          1. #jh3141

            Ok, show me a heating controller commonly sold in SF which supports DCF77 and WWVB, without a dedicated switch, v-ref switch, RC lock, time-zone lock, or just specified home city which pretty much every multi-band clock has.

          2. Odd indeed. Even if such a device does exist it would be a pretty poor design to just accept random data that just happened to be on a carrier of the same frequency. Without using framing or some sort of checksum and letting bits stack up until there are enough to form a command who knows what would happen.

  2. But why? My Casio Waveceptor watch can be set by hand. If it can’t receive a time signal, it’s like any other watch. I assume it keeps reasonable time, but after ten years there’s only been a few days, never in a group, when it hasn’t sync’d up.

    Unless someone bas a great time source, you might as well let the clock or watch run by itself.
    And yes, unless it is really accurate, I don’t want the neighbor controlling my clock or watch.

    One might just try a big tuned loop, like in the old days, and place the clock or watch within its field, to boost reception in weak locations.

    Michael

    1. NTP is probably going to be more reliable than the watch’s own quartz crystal, over any sort of time period. And this is nice cos it’s automatic and clever! Since the range is probably only a couple of feet he’s presumably only going to turn it on when the watch is on the stand. A bit of shell script could have it only come on once an hour, if it were actually ever a problem to anyone. Maybe even fit a button to it to activate.

  3. For a magnetic loop like this, with the intended target very close inverse square law means there will be negligible field at any significant distance if the power is adjusted correctly. At 77KHz you’d need a huge antenna to get any serious range

    1. Signal is 3.3v going over a resistance of roughly 5k, so ~2mW. You’d need something seriously sensitive if you wanted to detect it outside the room it’s operated in. For reference, DCF77 is a 50kW transmitter and has a 2,000km detectable range. Based on those stats, this transmitter should have a range <1m.

    2. First, as many ham will tell you there are often, even if rare propagation modes that will allow low power signals to go much further than one would expect. If you look at hams and their qrp work you will see what I am talking about. According to wikipedia the current record for a QRP connection is 1 µW for 2640 kilometers (1650 miles) on 10-meter band (28-29.7 MHz). If other people depend on the signal, please just set your watch by hand.

      There may be acceptable low power devices that the authorities would condone, but my guess is compliance testing would cost many orders of magnitude more than the device itself. I doubt you would get caught using an uncertified device, but IMHO doing so is it just rude.

  4. I wonder if something similar would drive a Seiko Message Watch, my dad had one back in the day but they shut the service off in the late 90s/early 00s (Although it was still active in Taiwan, whenever he’d go to Taiwan it’d re-set it self)

    1. There is a paper out there written by the guys that designed the Seiko system, search for “High Speed Subcarrier Data System (HSDS) Gary Gaskill and Ken Gray”. The paper has more info including data structure info but the short answer for how the seiko watch worked is “The HSDS data rate is 19,000 bits per second in a bandwidth of 19kHz, centered at 66.5kHz. The HSDS signal is modulated as a subcarrier ranging from 5% to 20% injection, but typically at 10% on a commercial FM radio station’s carriers in the frequency range of 87.5 to 108kHz.”

  5. Hm, “generating 77500.003Hz on a GPIO pin – close enough to the 77.5kHz carrier that DCF77 uses.”
    I would say that THAT IS the exact frequency. You can’t get any closer, you will always be a bit off. This is actually even better then some cheap VCOs output.

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