Here is a sample circuit I designed into a piece of test gear where an LM317 was used to supply voltage to a multi-voltage-output with the level selected by a microcontroller. This circuit took 5 outputs to control it. You could lower the processor pin count by either reducing the number of voltages supported or using a decoder type IC to convert 3 bit binary select from the micrcontroller pins to the linear select that drives the transistors shown.



Michael Karas