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 ...

“Building a Lower-power Linear Actuator with Arduino ... I’m using a motor with an encoder and an Arduino Uno to control position and velocity. “ For more information on more projects ...

These are small rugged devices that can do simple things like monitor switches and control actuators. Being ruggedized, they are typically reasonably expensive, especially compared to an Arduino.