I need a variable current output that is responding to sensor data and actuating some muscle wire. Can I get the digital out pin to output a range of currents according to incoming data? I want a range and not just HIGH and LOW.
An arduino output pin is a source or sink for current, but it can not directly control the amount of current, that is determined by the load resistance. A PWM output can approximate a variable voltage/current output but again the load resistance determines the maximum current that will flow when a 100% PWM output.
Lefty
Note that muscle wire takes a large amount of current, the arduino output can only supply 30mA safely, anything over 40mA damages it so you need transistors or FETs to boost the current.
You need filtered PWM to drive the FETs and as you are driving them proportionately they will get very hot as they burn up the power the muscle wire is not burning.
See:-
http://www.thebox.myzen.co.uk/Tutorial/PWM.html