You can refer to this circuit, but first know the chip information of ST 178 photoelectric sensor and LM3 1 1 comparator. There are many on this website. Circuit principle: Photodiode continuously emits light. When the illuminated object is black, the photocell can't receive the reflection and the circuit is disconnected. The positive input of the voltage comparator is high. After being reshaped by LM3 1 1 comparator, the output high level is transmitted to the single chip microcomputer. It outputs a low level when it is irradiated to white. The potentiometer can adjust the mosfet of LM3 1 1.