I am working on a project with the arduino uno and I need it to open and close a 24 volt valve, but the board only puts out 5Vdc and isnt strong enough to open the valve. How would I make the arduino put out more power?
solid state relay is the easiest, one with a 5 volt input and an output that handles 24vdc and the required current (its quite common as many industrial machines use this setup)
The problem is not so much the volts but the current!
An Arduino pin can only source an absolute maximum of 40 mA, far less than your valve will probably draw.
You need to have an external powersupply and control the valve inderectly via a powertransitor or a solid state relay.
Some good reading:
http://www.opencircuits.com/Driving_Large_Loads_with_the_Arduino
http://bildr.org/2011/03/high-power-control-with-arduino-and-tip120/
http://arduino.cc/playground/Learning/SolenoidTutorial