Here is (I think) the circuit that forms part of my project:
The value of the resistor (2490 Ohms) is fixed. Vi is a variable voltage (driven by the Arduino PWM output - might be filtered but let's just assume it's a nice, clean DC voltage which will never be greater than 5V). What will Vo be or is it not possible to tell? If it were possible to disconnect the resistor, then I'd do it, but it's inside some other electronics to which I don't have access (a Megasquirt EFI controller).
Also tempted to put a diode in between the Arduino (Vo) output and the node but not sure if this is necessary?