There are a lot of knobs that can be turned by controling the voltage on an input in real time. In my field, people usually who want to do this just buy whatever National Instruments is selling for $1500. It's expensive and involves a mess of wires, but it works.<p>Does anyone here know of a cheap way or two to wirelessly control a voltage (say from a netbook or phone)? Cheap here means in the $1-$25 range.
Not the cheapest but probably the simplest way would be to use an inexpensive router. The dd-wrt capable variety give you linux and a serial port for under 100$. There are tons of ways you could glue a potentiometer to a serial port.
It might help to be more specific - what kind of voltage are you looking to control? AC, DC, or both? High voltages or low voltages? High current or low current? Is the load reactive or resistive?