I am living in a country where there is no DCF77 sender nearby for my European radio controlled wristwatch to get its time. This vintage Junghans Mega doesn't have any buttons to set the time, so to bring it back to life, I built my own 'transmitter', taking the NTP time of a Raspberry Pi and generating a modulated signal via GPIO pins to then magnetically couple it into the watch ferrite.
Since many other long-wave time stations around the world use similar concepts of sending amplitude modulated time, other time services have been added.
This program is useful if you have a clock that otherwise does not get any reception. This magnetical coupling is very low power and only works over a few centimeters, but before running this program, make sure you follow your local laws with regard to restrictions on radio transmissions.
This runs on a Raspberry Pi. So far, it has been tested on a Pi3 and a Pi Zero W. There has been a report of different frequencies generated with an older Pi (Bug #1), so until we have a definitive list of available clock sources inside these, check out that bug for a workaround.
The DCF77 (Germany) signal is a 77.5kHz carrier, that is amplitude modulated with attenuations every second of the minute except the 59th to synchronize. The length of the attenuation (100ms and 200ms) denotes bit values 0 and 1 respectively so in each minute, 59 bits can be transferred, containing date and time information.
The Raspberry Pi has ways to create frequencies by integer division and
fractional jitter around that, which allows us to generate a frequency
of 77500.003Hz, which is close enough. Can be chosen with -s DCF77 option.
The WWVB (USA) is on a 60kHz carrier, and also transmits one bit per second
with different attenuation times (200ms zero, 500ms one; 800ms sync) and
multiple synchronization bits. Use -s WWVB option for this one.
The MSF (United Kingdom) has yet another encoding, transferring two bits
per second. Carrier is 60kHz. Option is -s MSF.
The JJY (Japan) is similar to WWVB, with same timings of carrier switches,
but reversed power levels. Some bits are different. Two senders exist in Japan
with 40kHz and 60kHz carrier; their simulations can be chosen
with command line options -s JJY40 and -s JJY60.
(Not tested with an actual radio clock yet. Please report if it works for you!)
The external hardware is simple: we use the frequency output on one pin and another pin to pull the signal to a lower level for the regular attenuation.
To operate, you need three resistors: 2x4.7kΩ and one 560Ω, wired to GPIO4 and GPIO17 like so:
| Schematic | Real world |
|---|---|
 |
 |
GPIO4 and 17 are on the inner row of the Header pin, three pins inwards on the Raspberry Pi GPIO-Header.
You don't need GPIO17 and the 560Ω resistor for MSF, as that works with
switching the signal (on-off keying) instead of attenuating. In that case, you
can replace the sequence of two 4.7kΩ resistors with a single 10kΩ.
Now, wire a loop of wire between the open end of the one 4.7kΩ and ground - this loop acts as coupling coil to the watch ferrite. The signal is very weak, so bring this wire-loop close to your radio watch/clock. In the following image it is wrapped around the antenna, but it is not strictly needed: anything within a few centimeters should work. Being too close to the antenna can confuse a sensitive receiver, so you might need to experiment with the distance.
(If you want to go fancy, you can improve the signal quality with a couple hundred picofarad low-pass capacitor between GND and where all resistors meet).
sudo apt-get install git build-essential -y
git clone https://github.com/hzeller/txtempus.git
cd txtempus
make
sudo ./txtempus -v -s DCF77
With -s, you set the type of time signal you want to transmit.
There are a few options you can set. The -r option is useful to have the
program run only for the few minutes it might take for a clock to synchronize.
By default, the current system time is transmitted. The -t option allows
different times for testing.
usage: ./txtempus [options]
Options:
-s <service> : Service; one of 'DCF77', 'WWVB', 'JJY40', 'JJY60', 'MSF'
-r <minutes> : Run for limited number of minutes. (default: no limit)
-t 'YYYY-MM-DD HH:MM' : Transmit the given local time (default: now)
-z <minutes> : Transmit the time offset from local (default: 0 minutes)
-v : Verbose.
-n : Dryrun, only showing modulation envelope.
-h : This help.
In the video below, you can see how a watch is set with this set-up. After it is manually reset, it waits until it sees the end-of-minute mark (which does not have any amplitude modulation) and then starts to count on from second 59, then gathering the data that is following.
An interesting observation: you see that the watch already gets into fully set mode after about 50 seconds, even though there is the year data after that. This particular watch never shows the year, so it just ignores that.
Mostly for understanding the protocol, the -n option allows to observe how
the amplitude modulation of each second looks like.
Unlike the regular transmission, don't need to be root or run on the
Raspberry Pi to use this option.
Underscores (_) show low power carrier, hashes (#) high power:
$ ./txtempus -n -s wwvb
2018-08-17 13:22:00 -> tx-modulation
:00 [________##]
:01 [__########]
:02 [_____#####]
:03 [__########]
:04 [__########]
:05 [__########]
:06 [__########]
:07 [_____#####]
:08 [__########]
:09 [________##]
:10 [__########]
:11 [__########]
... and so on for the whole minute ...
In some of these protocols, there are additional bits that contain information about upcoming daylight saving times, leap seconds or difference to astronomic time. These are currently not set, but usually clocks are fine with it.
Some time stations also phase-modulate their carrier, txtempus does not.
Each set-up will be different. In my case, I need my DCF77 radio
watch getting set over night. So I built this watch holder that presents the
watch upright while the antenna (in the wristband) is close to the
'transmission coil' that is lying flat on the Pi. The bottom of the 3D printed
case is filled with lead shot in epoxy to provide a stable base.
The Raspberry Pi Zero W runs ntpd, PLL locking the system time to various
stratum 1 NTP servers keeping it at atomic time within ±50ms.
This particular watch only checks the radio twice a day at 2am and 3am, so
there is a cron-job that runs txtempus around these times for a few minutes.
| watch holder | ... with watch |
|---|---|
 |
 |
tx common telecommunication abbreviation for 'transmit'
tempus, n Latin. Time; period; age
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