The PC817 is a widely used 4-pin optocoupler (also known as an opto-isolator). It consists of an infrared emitting diode (IRED) optically coupled to a phototransistor. Its primary function is to provide electrical isolation between two circuits, allowing a low-voltage signal to control a higher-voltage circuit without any physical electrical connection, which protects sensitive components from high-voltage spikes and noise.

Specifications

• Isolation Voltage (Viso): 5.0kV (RMS) — Provides a strong safety barrier.
• Collector-Emitter Voltage (Vceo): Up to 80V.
• Forward Current (IF): 50mA (Maximum continuous).
• Collector Current (IC): 50mA (Maximum).
• Current Transfer Ratio (CTR): 50% To 600% (varies by rank/binning).
• Response Time (tr): 4us (Typical) - Suitable for low-to-medium speed switching.
• Package: DIP-4 (Through-hole).

• Input Resistor: Always include a series resistor with the input (Pin 1) to limit the current through the IR LED. For a 5V logic signal, a 220 Ohm to 470 Ohm resistor is typical.
• Output Switching: Since the output is a transistor, you generally need a pull-up resistor on the Collector (Pin 4) if you want to read a logic signal on the output side.
• Orientation: Look for the small "dot" or notch on the package body; this indicates Pin 1 (Anode).
• Speed Limits: While great for DC switching and low-speed signals (like PWM or status signals), it may struggle with high-speed data protocols like high-speed SPI or I2C.

Datasheet