6N137 High-Speed Optoisolator Link
Send 10 Mbit/s digital data across a 3 kV isolation barrier using photons instead of wires.
Difficulty: Through-hole. Estimated build time: about 30 minutes. Estimated parts cost: about US$2.50. 4-line bill of materials. Compare supplier offers when available. A galvanically-isolated digital link built from the Lite-On 6N137, a single DIP-8 that pairs an infrared LED with a photodiode and a comparator output stage.
Wire the input LED
Pin 2 is anode, pin 3 is cathode. Drive pin 2 through a current-limiting resistor (560 Ω from 5 V gives ~9 mA, ample for full speed). Pin 3 goes to the input-side GND. The LED is forward-biased when the driving pin is HIGH.
Wire the output side with a pull-up
Pin 5 is the output-side V_CC — connect it to the clean side’s 5 V. Pin 8 is the output enable, tie to V_CC. Pin 6 is the open-collector output; add a 4.7 kΩ pull-up to pin 5’s V_CC. Pin 5 is also output-side GND — separate from input GND. The output now follows the input at up to 10 Mbit/s.
Test with two batteries
Power the input side with a 9 V battery and a 5 V regulator. Power the output side with a separate 9 V battery and another regulator. Touch one battery’s negative to your finger — it should be safe even if you also touch the other battery’s positive. That’s the point of isolation: no shared current path between the two sides.
Send real data across
Wire an Arduino on the input side with Serial1.begin(115200); printing characters. Wire another Arduino on the output side reading the 6N137 output on its RX pin. Bytes appear identically on both serial monitors. The link is bidirectional with two 6N137s.