Like the MSF atomic clock project described elsewhere, the installation in Mainflingen near Frankfurt is a highly accurate atomic clock. The transmission frequency is 77.5kHz  and the output power of the Mainflingen clock is considerably higher than the one in Anthorn. Hence this clock can be received over most of Europe

The finished project must be connected to a GPIO line of the Raspberry Pi. I used my 5 in 16 out interface and connected the output of the clok to the GPIO line 29. If you use a different line, don't forget to change the relevant code in the program. The antenna should be aligned with Frankfurt in Germany though I have found that I get the best results pointing the thing in an entirely different direction!

I have always had tremendous problems with the reception of the Mainflingen signal, and I always blamed my location - the far North of Northern Ireland. I now blame the receivers. Our local Lidls shop sold a radio controlled clock the other day (this is written on the 18th of April 2012). I bought one on spec because I wanted the 77.5kHz crystal and the clock was only £5. It turns out that it contained one of the best atomic clock receivers I have come across. The receiver is easily removed and is a self-contained unit (shown on the right". The output is rock steady. It can also easily be converted to a MSF receiver by swapping the crystal and using a 60kHz antenna. Not bad for a fiver!

I designed a small circuit to adapt the radio to the Raspberry Pi. The two diodes drop the 5V supply voltage to make it suitable for the receiver. It operates at 3V, though for all I know it can cope with 5V as well. As I like to have a flashing LED and the original - now useless - clock came with a nice blue one, I fitted a 7404 logic chip as a driver. This gave me plenty of NOT gates which I used to give a normal and inverted output to both the LED and the output pin. Two links select your choice. Anybody interested in the PCB layout, please drop me a line.

The receiver works a treat. It is stationed about 3 meters away from my computer and shows no interference whatever. The signal comes in at a steady 100ms  for 0 and 200ms for a one and is quite predictable. The output is connected to the GPIO 29 line using my 5in 16 out interface board.

If you have bad reception and you run your receiver via a mains power supply try using batteries. This can make a huge difference.

The Program

The DCF clock takes exactly one minute to transmit the date and time. The timing diagram of three seconds is shown below.   The decoding scheme is different from the one used by the MSF clock, though there are similarities.

Every second carries one bit. To show a 0 the carrier amplitude is reduced to 30% for 100ms and for a one for 200ms. Second 0 of the sequence is high for the full second.

Some translations may be of use here. Starting anti clockwise from the top:

Kalenderjahr  -  year

Kalendermonat  -  month

Wochentag  - day of the week. Monday being 1 and Sunday 7

Stunde  - hour.

I leave it to the imagination of the reader to figure out what Minute stands for.

The outside numbers indicate the binary coded decimal place values of the set bits, the inside numbers are second indicators.. An example will make things clearer. All time values are transmitted via bits 21 to 57. The data for the year, for example,  are represented by bits 50 to 57. This article was written in 2011, so bit 51 and bit 54 will be high. This is because bit 54 is worth 10 and bit 51 is worth 1. Add these two numbers and you get - you've guessed it - 11. The 2000 is assumed.

Say the year is 2087. 87 =  80 + 4 + 2 +1, hence bits 57,52,51 and 50 would be set.

The program recognises the beginning of the cycle by measuring the length of the high bit. This is always larger than 130ms at the beginning of the sequence, which is a unique value.

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Here is the program and if you don't like typing, you can download it via this link

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