The photocoupler has a diode tube-a diode tube, a diode tube-a tertiary tube type, a diode tube-a Darlington tube type and a diode tube-available For control and other types, according to the system requirements, this design uses a photocoupler 4N35, the forward current is: 60mA, the current transfer ratio is 100%, the drive current of the solenoid valve is 1.5 ~~ 3A, and the current amplification factor of the transistor 2SD880 60 ~~ 300, the current limiting resistor at the input end of the photocoupler, the resistance value is as follows