Determining the resistor values and assuring performance of the circuit associated with a photocoupler (also called an optocoupler or optoisolator) is simple in principle, but there are some points to consider when creating a robust, trouble-free design. Using an example based on the Renesas PS8902 (a high-speed, analog-output photocoupler with a GaAlAs LED on the input side, and a PIN photodiode and high-speed amplifier transistor on the output side), it’s illustrative to do the calculations for resistor selection and drive, The schematic “fragment” of the figure summarizes the analysis scenario.
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