In this quick Arduino tutorial I will explain how you can control a relay using the Arduino Board, one 1K and one 10K resistors, 1 BC547 transistor, one 6V or 12V relay, one 1N4007 diode and a 12V fan ...

The resistor values (R1&R2) in the circuit diagram provide some over-voltage protection then measuring low voltages. Keep it in mind that any input voltage higher than about 55V could fry the Arduino.