The FOD817A3S is a single-channel optoisolator from onsemi with a DC-input LED driving a phototransistor output, rated for 5000Vrms isolation between primary and secondary sides. It comes in a 4-SMD Gull Wing package (surface-mount, tube delivery) — the same footprint as the through-hole DIP-4 but reflow-ready for automated assembly.
Current transfer ratio is guaranteed between 80% and 160% at a forward current of 5 mA — that means with 5 mA through the LED, the output transistor can sink at least 4 mA and at most 8 mA. The 80% min CTR gives you headroom to drop the LED current below 5 mA if your MCU pin is limited, but you lose margin on the output side — check the load current against the 50 mA absolute max output. Typical Vce saturation is 200 mV max — the transistor pulls close to ground, so you can drive a logic input directly without a pull-up resistor on the collector.
The base product number is FOD817, which covers a family of CTR-binned and package variants — the A3S suffix specifies the 80-160% CTR bin in the SMD Gull Wing package.