A system for controlling a defrost cycle of an evaporator having a sensor module and a control module. The sensor module includes a light source configured to emit light toward the evaporator when activated and to deactivate in response to a lockout signal. The sensor module also includes a light sensor configured to determine an amount of the emitted light reflected by the evaporator and to generate a detected light signal that corresponds to the amount of the emitted light reflected by the evaporator. The control module is configured to receive the detected light signal from the light sensor and to compare the detected light signal to a preset threshold. The control module is also configured to generate a termination signal when the detected light signal is less than the preset threshold and to generate the lockout signal when the detected light signal is greater than the preset threshold.