INTRODUCTION
------------
This library is for decoding the DCF77 time signal broadcasted
by the german PTB (Physikalisch-Technische Bundesanstalt) in Mainflingen.

I used the sample code written by Matthias Dalheimer ([email protected]) and 
adapted it so that it can be used as a library in other Arduino projects.

To be able to use this code you have to hook up a special DCF77 receiver 
to your Arduino. I used the one sold be german electronics retailer Reichelt.

Unfortunately the Reichelt module has a very weak push-pull output which 
provides only around +/- 5uA. 

To get usable readings out of it I used the following circuit:
                                 ____ 
                            +---|____|---+
                            |     R3     |
         ____     |/c-------+------------|---o PIN2
SIG o---|____|---b|                      | 
          R1      |\e->-----+            |
                            |    ____    |
VDD o---+-------+-------+---|---|____|---+---o +5V
       _|_     _|_     _|_  |     R2           
    D1 /_\  C1 ___  C2 ___  |                
        |       |       |   |                
GND o---+-------+-------+---+----------------o GND

D1: 3.9 Z-Diode 
C1: 100n Ceramic Capacitor
C2: 100u Electrolytic Capacitor
R1: 120k Resistor
R2: 2.2k Resistor
R3: 10k Resistor

To amplify the signal there is a 2N2222 Transistor between R1 and PIN2.


INSTALLATION 
------------
For a tutorial on how to install new libraries for use with the Arduino
development environment please refer to the following website:
http://www.arduino.cc/en/Reference/Libraries


USAGE
-----
This library assumes that you have SIG hooked up to Arduino's PIN2.
The reason for this is that most Arduino boards have two external interrupts: 
numbers 0 (on digital pin 2) and 1 (on digital pin 3). 
If you want to use PIN3 you would have to change the constant at the top of
Funkuhr.cpp and change the interrupt number in attachInterrupt() from 0 to 1.