mutron/neutron LDR question

[igor12]

Anybody know why the LED is placed in the feedback loop of the opamp?  I mean, I understand how this current controls the LED but does it make it more sensitive or something as opposed to having the LED at the output of the opamp with a current limiting resistor.

[R.G.]

Any impedance placed in the feedback loop of an opamp is effectively driven by a constant current source.

The difference is that the opamp's feedback operation makes the LED current be proportional to the input voltage, instead of having a "dead zone" of the forward drop of the LED before it begins conducting.

It's a more linear transfer of signal level to LED current, and presumably also its light output.