Here, a very simple circuit for controlling a DC actuator (e.g. motor, solenoid) with a MOSFET.
This allows your microcontroller to drive actuators that need much more power than what the microcontroller itself can provide.
Our MOSFET of choice is the IRF540. Find its datasheet here.
Note that this circuit provides NO opto-isolation for your microcontroller; in-other words, if you make mistakes, you could potentially hurt your microcontroller or even your computer! For a more secure solutions consider a solution like Futurlec's DC Opto-Isolated output board.