Slick! You could also write some false triggering software to recognize a evaporator blockage from other than frost ie dust, mold, bent fins, errant plastic bags - if its defrosting in non frosting humidity/temps it would alert you.
Be sure to put some kind of foam or aquarium filter on the end of your sensor tubes - mud dauber wasps, spiders etc love to plug up little open tube ends.
Early heat pumps used simple spring loaded flaps in the air stream which, when the frost blocked enough airflow, tripped a limit switch to trigger the defrost cycle - not sure why the industry moved away from that.
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