4N35: Optocoupler (Opto-Isolator) Explained

The 4N35 is an optocoupler (opto-isolator) that provides electrical isolation between two circuits using light. It is commonly used in signal isolation, microcontroller interfacing, and switching applications.

🔹 1. What is the 4N35?

• The 4N35 is an optocoupler with an infrared LED and a phototransistor inside a single 6-pin package.
• It electrically isolates one circuit from another while allowing signal transfer using light.
• Used for protecting microcontrollers from high voltages and noise-sensitive circuits.

📌 Why use it?

• Prevents electrical damage between circuits.
• Reduces interference (noise isolation).
• Allows communication between different voltage levels.

🔹 2. 4N35 Pinout & Functions

The 4N35 has 6 pins:

📌 Key Features:

• Optical isolation between input and output.
• DC signal transfer with low power consumption.
• Input Voltage: 1.2V (LED Forward Voltage).
• Output Voltage: Up to 70V (Collector-Emitter Breakdown Voltage).

🔹 3. How 4N35 Works

• The internal LED lights up when voltage is applied to pins 1 & 2.
• The light activates the internal phototransistor (pins 4 & 5).
• The transistor switches ON or OFF, depending on the LED state.

📌 This allows a low-voltage microcontroller to control high-voltage circuits safely.

🔹 4. Using 4N35 for Microcontroller Isolation

🛠 Required Components

• 1x 4N35 Optocoupler
• 1x 220Ω Resistor (LED Current Limit)
• 1x 10kΩ Resistor (Pull-down for Transistor Output)
• 1x Arduino (or Any Microcontroller)
• 1x External Circuit (e.g., Motor, Relay, or High-Voltage Load)

🛠 Wiring Diagram

🔹 5. Arduino Code for 4N35

This example turns ON/OFF a high-voltage device using an Arduino and 4N35.

#define OPTOCOUPLER 7  // Connects to 4N35 Anode (Pin 1)

void setup() {
    pinMode(OPTOCOUPLER, OUTPUT);
}

void loop() {
    digitalWrite(OPTOCOUPLER, HIGH);  // Turn ON the optocoupler (Activates output)
    delay(2000);  // Wait 2 seconds
    digitalWrite(OPTOCOUPLER, LOW);   // Turn OFF the optocoupler
    delay(2000);
}

📌 What happens?

• The Arduino controls the optocoupler LED.
• When the LED is ON, the transistor allows current flow.
• When the LED is OFF, the transistor is OFF (isolates the circuit).

🔹 6. Using 4N35 for AC Mains Isolation (Safe Switching)

📌 4N35 can be used to switch AC appliances (e.g., lamps, fans) via a relay.

🛠 Circuit Steps

1. The Arduino controls the 4N35 LED.
2. The 4N35 transistor switches ON a relay.
3. The relay controls the AC device.

📌 Using 4N35 prevents high-voltage AC from reaching the microcontroller!

🔹 7. Applications of 4N35

✅ Microcontroller Isolation – Protects Arduino from high voltage circuits.
✅ Switching High-Voltage Devices – Controls relays, motors, and power electronics.
✅ Noise Reduction – Used in industrial automation and signal isolation.
✅ AC/DC Power Control – Safe switching of mains appliances.
✅ RS232 & Serial Communication Isolation – Protects data lines from voltage spikes.

🔹 8. 4N35 vs Other Optocouplers

📌 Verdict:

• 4N35 is best for general-purpose switching.
• PC817 is a smaller, lower-power alternative.
• MOC3021 is used for AC load switching (Triac control).

🎯 Conclusion

• 4N35 is an optocoupler that provides electrical isolation between circuits.
• Used for switching high-power devices, protecting microcontrollers, and noise reduction.
• Works with Arduino, ESP32, STM32, and other microcontrollers.
• Prevents high-voltage damage and interference in sensitive circuits.