A circuit that outputs one input voltage by default, or another if it is present
You could just connect the "internal" voltage to the same node as the "external" input via a high value resistor.

If the input is open-circuit, that node 'A' will take the voltage of the internal voltage source.  If the input is connected, unless its impedance is very high then node A will assume the voltage of the input.

An op amp configured as a voltage follower (unity-gain buffer) takes the result, isolates it from whatever you're connecting the output to and presents a low-impedance source to the output.  If the load is sufficiently high-impedance, the buffer could be omitted.







simulate this circuit &ndash; Schematic created using CircuitLab