PC817: Optocoupler (Optoisolator) Explained

The PC817 is a phototransistor-based optocoupler (also called an optoisolator) used to provide electrical isolation between two circuits. It is widely used in signal isolation, microcontroller interfacing, power electronics, and switching applications.

🔹 1. What is the PC817?

• The PC817 consists of an infrared LED and a phototransistor in a 4-pin package.
• It isolates low-voltage control circuits (e.g., Arduino, ESP32, STM32) from high-voltage power circuits.
• Prevents electrical damage and reduces noise interference.

📌 Key Features:

• Provides electrical isolation up to 5kV.
• Can switch DC signals with low power consumption.
• Maximum Collector-Emitter Voltage (Vce): 35V.
• Current Transfer Ratio (CTR): 50-600% (Efficiency of signal transfer).
• Used in industrial automation, power supply feedback, and microcontroller protection.

📌 Why use it?

• Prevents damage between different voltage circuits.
• Reduces interference (eliminates ground loops).
• Allows communication between different voltage levels.

🔹 2. PC817 Pinout & Functions

The PC817 comes in a 4-pin DIP package.

📌 How it works:

• Pins 1 & 2 → Control the LED inside the optocoupler.
• Pins 3 & 4 → The phototransistor switches ON/OFF based on the LED state.

🔹 3. How PC817 Works

• When current flows through the internal LED (Pins 1 & 2), it emits infrared light.
• This light activates the phototransistor (Pins 3 & 4), allowing current to pass.
• When the LED is OFF, the phototransistor is OFF, isolating the circuit.

📌 This allows a low-voltage microcontroller (e.g., Arduino) to safely switch high-voltage circuits.

🔹 4. Using PC817 for Microcontroller Isolation

🛠 Required Components

• 1x PC817 Optocoupler
• 1x 220Ω Resistor (For LED Current Limit)
• 1x 10kΩ Resistor (Pull-down for Transistor Output)
• 1x Arduino (or Any Microcontroller)
• 1x External Circuit (e.g., Relay, Motor, or High-Voltage Load)

🛠 Wiring Diagram

🔹 5. Arduino Code for PC817

This example turns ON/OFF a high-voltage device using an Arduino and PC817.

#define OPTOCOUPLER 7  // Connects to PC817 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 phototransistor allows current flow.
• When the LED is OFF, the phototransistor is OFF (isolates the circuit).

🔹 6. Using PC817 for AC Mains Isolation (Safe Switching)

📌 PC817 can be used to switch AC appliances (e.g., lamps, fans) via a relay.

🛠 Circuit Steps

1. The Arduino controls the PC817 LED.
2. The PC817 phototransistor switches ON a relay.
3. The relay controls the AC device.

📌 Using PC817 prevents high-voltage AC from reaching the microcontroller!

🔹 7. Applications of PC817

✅ 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. PC817 vs Other Optocouplers

📌 Verdict:

• PC817 is best for low-power microcontroller isolation.
• 4N35 is a higher voltage alternative.
• MOC3021 is used for AC load switching (Triac control).

🎯 Conclusion

• PC817 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.