The DCF77 interface

The circuit interfaces a commercial DCF77 receiver (available for about $15) with a PC using the RS232 serial port. By rewriting the program and/or modifying the circuit it's possible to use also different systems or different I/O ports.
The power is driven from three lines of the RS232 port (that is full protected against short circuits and therefore there is no risk of damaging it). The DTR and RTS lines are set high (+12 V on the port) and the TXD is kept low (-12 V) for the power supply of the receiver.
The RTS line gives positive power to the circuit and the DTR powers only the monitor LED that is kept separated from the rest of the circuit because of its high current peaks.
All the 1N4148 diodes connected to the port and to the BC547 transistor protect the circuit when the port is wrong programmed or not initialized.
The BF244 JFET, the 1.5 kOhm resistor and the second LED compose a voltage reference of about 1.8V needed for powering the commercial receiver. Only a very low current flows through this LED won't be particularly bright.

The TL081 is used as comparator and line driver; its reference voltage is set to 0.6 V on its non-inverting input by a 1N4148 diode.
The TL081 sends the DCF77 signal (RS232 compatible) on the RXD pin. This pin must always be high (-12 V since this pin is inverted on the RS232) and go low one per second (when a bit is received).
Some commercial receivers have an inverted output, and to use them just swap pins 2 and 3 of the TL081.
The receiver should stay away from the PC and from the comparator circuit because switching noise inhibits normal reception.
The DCF77 standard imposes a pulse time of 200 ms for high bit and a of 100 ms for low bit, but, since receivers usually use about 40ms for a safe reception these times are reduced to 160 and 60 ms.
When receiving this pulses with the PC UART 8250 (or better) at the speed of about 40 bit per second N81 a 160 ms pulse will produce a $80 or a $C byte and a 60 ms a $FE or $FC.

This is a picture of the interface: the connector on the left is the RS232 port and the cable on the right comes from the antenna.