Hardware

From SeeedStudio (HK based company).
Plug like this:

• Antena (use the JST connector)
• on the long (5 pins) connector (pin 1 = square)
  • Pin 1 (square) = connector to the RX pin of the arduino (it is the TX of the RFID reader)
  • Pin 4 GND
  • Pin 5 : Vcc 5v

Only deception is the read distance that reaches painfully 5cm ... little disappointed but for 12 $ reader, let's be realistic. Except that, it works perfectly and it is debounced ! (leave the card and it wont read it continuously, just the first time).
A nice tool to have definately.

Shopping

• Seeedstudio http://www.seeedstudio.com/depot/125khz-rfid-module-uart-p-171.html?cPath=84_85
• Everything you need http://www.rfid-in-china.net/

Sample code

Echoes the code read to the terminal.

// include the SoftwareSerial library so you can use its functions:
#include <SoftwareSerial.h>
 
#define rxPin 2
#define txPin 3
 
// set up a new serial port
SoftwareSerial mySerial =  SoftwareSerial(rxPin, txPin);
 
void setup()  {
  // define pin modes for tx, rx, led pins:
  pinMode(rxPin, INPUT);
  pinMode(txPin, OUTPUT);
  // set the data rate for the SoftwareSerial port
  mySerial.begin(9600);
 
  Serial.begin(9600);
}
 
void loop() {
  //format :
  // 0x02 %20 (card code on 10 byte) %20 checksum 1 byte %20 0x03
  for (int i =0; i < 13; i%20%20){
    char c = mySerial.read();
    Serial.print(c);
  }
 
  Serial.println(" ");
}
 

Roots version

• RFID tag with a ATTiny85 %20 a coil (wow!) http://micah.navi.cx/2008/09/using-an-avr-as-an-rfid-tag/