I wasn't able to find much info on the Debron 1052 Optical Sensor,
But, it's a good example of an emitter-detector open-gap sensor.
These sensors are commonly used to detect solid material across an air gap.
Kinda like the object sensors used on a garage door opener.
It would be easy to try out a 99 cent reflector sensor, and glue a small mirror on the HX coil.
http://www.vishay.com/docs/83760/tcrt5000.pdf
(Has a Daylight blocking filter • Emitter wavelength: 950 nm)
New 10 Pcs TCRT5000L TCRT5000 Reflective Optical Sensor Infrared 950mm 5V 3A | eBay
Have the sensor 10-12 mm away from the mirror on the coil.
When frost weakened the signal, the transistor would signal your defrost alarm line.
But, not if the ice acted as a mirror!! Have to try it to be sure..
I'll order some to try. It's takes a few weeks to get them,
and it will take a couple of days to set up a test rig and post some results.
I'm thinking of trying to make the mirror-to-sensor spacing adjustable,
so I can widen the space until signal loss causes signal switch.
Then, move it in a bit and do the frost testing.. Maybe CO2 spaying?
