[Xringer]

I used the humidity input on the CAI board to read the optical detector with the circuit above.

The range using a mirror was about 2.25". When I inserted foggy plastic
between the mirror and sensor, I only got change of about 30%.. (90% to 60%).

But when I didn't use the mirror, it displayed 0% until I placed an ice cube in front of the sensor.
The ice is very reflective. At 1/2" it displayed 30% and at 1/4" it was 90%.
I checked the ice in both white and clear parts of the cube, the reflectivity was good in both cases.

It seems like setting up the sensor so it points right at the HX coil
at a distance of 1/2" would detect ice/frost building up.
I would use a shot of flat-black spray paint on the coil, for near zero reflection. (When the coil was ice-free)..

Using a standard analog input would have worked better, but the humidity sensor input has screw Euro-terminals.

With a control board, it should be pretty easy to use this type of IR detector to control defrost cycles.