The 6N139S is a single-channel optoisolator from onsemi with a Darlington output transistor and a base pin for external bias or feedback. It accepts DC input and delivers a minimum current transfer ratio of 500% at only 1.6 mA forward current — meaning you can drive the LED side from a low-power logic output or a GPIO pin and still get a solid logic-level signal on the output side. The Darlington-with-base topology gives you a third terminal on the output side (pin 6 on the 8-SMD Gull Wing package) that lets you adjust switching threshold or add a pull-down for noise immunity. Typical turn-on time is 1.5 µs, turn-off is 7 µs — fast enough for PLC input modules, relay drivers, and isolated status feedback, but not for high-speed serial links.
Obsolete — sourcing and replacement planning
The 6N139S is marked obsolete by onsemi. Any remaining inventory is limited to what distributors or brokers hold on the shelf. (2) evaluate a pin-compatible replacement from the same 6N139 family or a similar Darlington-output optoisolator from another manufacturer. The 8-SMD Gull Wing footprint is common, but always verify the CTR, switching speed, and isolation rating against your design margin.
The 500% minimum CTR at 1.6 mA If is the headline parameter. It means the output transistor can sink at least 8 mA when the LED is driven at 1.6 mA — enough to pull a 5 V logic input low through a 620 Ω pull-up. If you need more output current, the Darlington can deliver up to 60 mA continuous, but the LED drive must increase proportionally (max 20 mA If). Forward voltage is 1.3 V typical at rated If — a standard LED drop. The output side is rated to 18 V maximum, which covers 5 V and 12 V logic rails but not 24 V industrial supplies without a series resistor or zener clamp. The 8-SMD Gull Wing package is surface-mount — pick-and-place friendly.