Is there a type of resistor that senses the potential applied to it, and sets it's value accordingly? I know there's a part that does this, but I can't for the life of me think of what it's called. Sorry, I'm getting old and the gray cells aren't doing what they used to for me. Wanting something that raises it's value as the potential drops, and it would need to be "programmable" so I can "tune" it to my particular needs. Seems like a standard transistor could be made to do this, but again I can't figure out how. Thanks.