Isolate a digital signal that changes its amplitude
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. (a) Simplest option. (b) Constant current sink option.

      0 ----- 6 ----- 12 ----- 18 V
Min   ==================
Max           ===================
                  _______________
Output __________|


Figure 2. The voltage ranges of interest overlap so a mid-overlap switching point might work.

By addition of D2 with the appropriate rating you may be able to get the opto-LED to turn on at > 9 V and this would work across the range of voltages of interest. The design problem is that the current through the LED will vary greatly between your low-voltage and high-voltage. You would need to do your calculations to see if this can be made to work. If you do this then don't forget to look at the current-transfer-ratio of the opto-isolator and see if you can get the required output voltage swing at low currents by increasing the value of R4.

Figure 1b adds in current limiting at 10 mA. That solves some of the problems of 1a but at the expense of complexity which you wish to avoid. You could replace the CC sink with a constant current diode (but watch the heat dissipation calculations).