Application of AC optocoupler
When you have signal at the R85 input generaly is PWM the transistor at OK2 is conductive and the capacitor C12 is charged through R32 and 3.3V when the capacitor is charged the transistor T5 switch from blocked to condictive and when the capacitor is discharged through R83 the transistor T5 is blocked. The resistors R32 and R85 are used to define the charge and discharge time where the resitor R33 and R85 are used to limite the input current.