An optocoupler bridges two electrically isolated circuits using a photon as the messenger. Inside the package: an infrared LED shines across a tiny air gap onto a phototransistor (or photodiode, or photo-Darlington). No copper crosses the gap, so kilovolts on one side can't reach the other. Typical isolation ratings: 2.5 kV to 5 kV RMS for standard parts, up to 10 kV for reinforced safety-rated units.

The defining spec is Current Transfer Ratio (CTR) — the ratio of output collector current to input LED current, expressed as a percentage. A 4N35 has CTR of 100% minimum: drive 10 mA into the LED, get at least 10 mA out of the transistor. But CTR varies wildly: it drops 50% over the part's lifetime as the LED ages, falls with temperature, and varies 3:1 between units of the same part number. Always design for worst-case CTR, typically 20-50% of the datasheet typical.

Real example: isolating a microcontroller from 240 VAC mains sensing. You want to detect when the AC line is live without any galvanic connection. Drop the AC through a 47 kΩ resistor and a reverse-protection diode into the LED of a 4N35. At 240 VAC peak (~340 V), peak LED current is ~7 mA — comfortable. On the output side, tie the collector to 5 V through a 10 kΩ pull-up; the emitter goes to MCU ground. When mains is high, the phototransistor pulls the collector low. The MCU sees a clean 5 V logic signal with zero risk of mains touching its pins.

Rule of thumb for LED drive current: Pick I_LED so that I_LED × CTR_min ≥ 3× the required output current. If you need 1 mA at the output and worst-case CTR is 20%, drive at least 15 mA into the LED. Then size the input resistor: R = (V_drive − V_F) / I_LED, where V_F ≈ 1.2 V for the IR LED.

Watch the bandwidth. Standard phototransistor optos (4N35, PC817) max out around 10 kHz — fine for logic-speed signals, useless for fast data. For high-speed digital isolation use digital optocouplers like the 6N137 (10 Mbps) or modern magnetic/capacitive isolators (Si86xx, ADuM series) that hit 150 Mbps with better lifetime and tighter CTR-equivalents.

Common applications: triac gate driving (MOC3041 zero-cross opto-triac), feedback in offline flyback supplies (PC817 across the isolation barrier), industrial digital inputs, and safe USB-to-RS485 converters.