4N35 IC Optocoupler Phototransistor IC

Prototyping and Engineering Tips from Robogears

• The "Input Resistor" Rule: CRITICAL: Just like a standard LED, the internal IR LED (Pins 1 & 2) requires a current-limiting resistor. If you connect Pin 1 directly to a 5V Arduino pin, the internal LED will burn out instantly. Use a 220-ohm to 470-ohm resistor for a 5V signal.

• Wiring the Output: In most digital applications, the 4N35 is used in a "Common Emitter" configuration. Connect Pin 5 (Collector) to your target VCC (e.g., 5V) through a 10k ohm Pull-Up Resistor, and connect Pin 4 (Emitter) to the target Ground. Your isolated signal is then taken from Pin 5.

• Ground Isolation: To achieve true "Optical Isolation," the ground of your input circuit (Arduino) must NOT be connected to the ground of your output circuit (Motor/Relay). If the grounds are joined, the optocoupler's safety benefit is bypassed.

• Using Pin 6 (Base): For standard digital switching, Pin 6 is usually left unconnected. However, if you find the optocoupler is "too sensitive" or switching due to noise, you can connect a high-value resistor (100k - 1M ohm) from Pin 6 to Pin 4 (Ground) to stabilize the phototransistor.

• AC Zero-Cross Detection: Because the 4N35 uses a single IR LED, it is commonly used in zero-cross detection circuits for AC dimmers. When the AC sine wave passes through zero, the IR LED turns off, creating a pulse that the microcontroller can use to time its firing pulses for a TRIAC.

Package Includes

• 1 x 4N35 IC Optocoupler Phototransistor IC

Disclaimer: Do not exceed the maximum collector-emitter voltage of 30V. This IC is intended for signal isolation; for switching high-power AC loads directly, use a Triac-output optoisolator (like the MOC3021) or a Solid State Relay. Always verify the pinout (Pin 1 is marked by a small dot/notch) before soldering to prevent damage to the internal IR diode.