How to Use Optocoupler 4n35: Examples, Pinouts, and Specs

The Optocoupler 4N35 is an optoelectronic device that uses light to transfer electrical signals between two isolated circuits. It consists of an infrared LED and a phototransistor housed in a single package. This design allows for electrical isolation between the input and output, making it ideal for protecting sensitive components from high voltages or noisy signals.

• Signal isolation in microcontroller circuits
• Protection of low-voltage circuits from high-voltage systems
• Noise suppression in industrial control systems
• Data communication between systems with different ground potentials
• Motor control and power supply circuits

Below are the key technical details of the 4N35 optocoupler:

The 4N35 optocoupler has a 6-pin Dual Inline Package (DIP). The pinout is as follows:

1. Input Side (LED):

   • Connect the anode (Pin 1) to the positive side of the input signal through a current-limiting resistor.
   • Connect the cathode (Pin 2) to ground or the return path of the input circuit.
   • Calculate the resistor value to limit the LED current to 10mA (typical). Use the formula: [ R = \frac{V_{in} - V_f}{I_f} ] where ( V_{in} ) is the input voltage, ( V_f ) is the forward voltage of the LED (1.2V), and ( I_f ) is the desired forward current (e.g., 10mA).

2. Output Side (Phototransistor):

   • Connect the collector (Pin 5) to the positive supply voltage of the output circuit through a pull-up resistor.
   • Connect the emitter (Pin 4) to ground.
   • The pull-up resistor value depends on the desired output current and voltage levels.

3. Isolation:

   • Ensure that the input and output circuits share no direct electrical connection to maintain isolation.

Below is an example of how to use the 4N35 optocoupler to isolate a digital input signal for an Arduino UNO:

• Input Side:
  • Connect a 5V signal to the anode (Pin 1) through a 470Ω resistor.
  • Connect the cathode (Pin 2) to ground.
• Output Side:
  • Connect the collector (Pin 5) to the Arduino's 5V pin through a 10kΩ pull-up resistor.
  • Connect the emitter (Pin 4) to the Arduino's ground.
  • Connect the collector (Pin 5) to a digital input pin (e.g., D2) on the Arduino.

// Example code for using the 4N35 optocoupler with Arduino UNO

const int optoInputPin = 2; // Digital pin connected to the optocoupler output

void setup() {
  pinMode(optoInputPin, INPUT); // Set the optocoupler output pin as input
  Serial.begin(9600);          // Initialize serial communication
}

void loop() {
  int optoState = digitalRead(optoInputPin); // Read the optocoupler output state

  // Print the state to the Serial Monitor
  if (optoState == HIGH) {
    Serial.println("Input signal is HIGH");
  } else {
    Serial.println("Input signal is LOW");
  }

  delay(500); // Wait for 500ms before reading again
}

• Always use a current-limiting resistor on the LED side to prevent damage.
• Ensure the pull-up resistor on the phototransistor side is appropriately sized for your circuit.
• Avoid exceeding the maximum voltage and current ratings to prevent permanent damage.
• For high-speed applications, consider the switching speed of the optocoupler, as it may introduce delays.

1. No Output Signal:

   • Check the input LED circuit. Ensure the current-limiting resistor is correctly calculated and the LED is forward-biased.
   • Verify the pull-up resistor on the output side is connected properly.

2. Output Signal is Always HIGH or LOW:

   • Ensure the input signal is within the operating range of the LED.
   • Check for loose connections or incorrect wiring.

3. Slow Response Time:

   • The 4N35 is not designed for high-speed applications. If faster switching is required, consider using a high-speed optocoupler.

4. Excessive Heat:

   • Ensure the input and output currents do not exceed the maximum ratings.
   • Verify that the circuit design includes appropriate resistors to limit current.

Q: Can I use the 4N35 for AC signal isolation?
A: Yes, the 4N35 can be used for AC signal isolation, but you will need to use a rectifier circuit on the input side to drive the LED.

Q: What is the purpose of the base pin (Pin 6)?
A: The base pin is typically left unconnected in most applications. However, it can be used to control the phototransistor's gain if needed.

Q: Can the 4N35 handle high voltages?
A: The 4N35 provides isolation up to 5000V RMS, but the phototransistor side is limited to a maximum collector-emitter voltage of 30V.

By following this documentation, you can effectively use the 4N35 optocoupler in your electronic projects for signal isolation and protection.