Plug the RFID module into the breadboard with enough space around it. Connect the VCC pin to the Arduino's 3.3V (not 5V), and ...

This library has been tested with an Arduino Uno (AtMega328p), EnviroDIY Mayfly (AtMega1284p ... extremely unstable). The SDI12 library code is released under the GNU Lesser Public License (LGPL 2.1) ...