I have considered using an opto isolator , but wouldn't that require a fixed voltage to light the led consistently without blowing it at the higher voltages?
The point with a linear opto isolator is the output current depends on the input current,
you choose a limiting resistor so that the maximum input voltage is full scale.
There are sophisticated linear opto isolators using two receivers and feedback to
improve linearity, but thats not what you want, a simple opto isolator with output
current roughly proportional to input current ought to be feasible. You only need