Interface circuit composed of photocoupler

As shown in the circuit, the photocoupler 4N25 plays the role of coupling pulse signals and isolating the single-chip computer 89C51 system and the output part, making the two parts of the current independent of each other. The ground wire of the output part is connected to the chassis or the earth, while the power ground wire of the 89C51 system is floating and not connected to the ground wire of the AC power supply. This can avoid the impact of the output part power supply change on the single-chip computer power supply, reduce the interference to the system, and improve the system reliability.