Optocoupler Explained: How It Works, Pinout and Circuit Example

An optocoupler, also called an opto isolator, passes a signal between two circuits using light, so the circuits stay electrically isolated. Inside the package, an LED on the input side faces a light sensor on the output side. The sensor is usually a phototransistor. When current flows through the LED, it emits light, the sensor conducts, and the output switches. No electrical connection crosses the gap.

This guide explains how an optocoupler works, how to read its symbol and pinout, the main types, the specs to check, a worked circuit example, and PCB layout tips.

What Is an Optocoupler?

An optocoupler is a component that transfers a signal with light across an isolation barrier. It is used when two circuits must share a signal but must not share a ground or a direct electrical path, for example a low-voltage microcontroller and a higher-voltage circuit.

The names optocoupler, opto isolator, optoisolator and photocoupler mean the same thing. This guide uses "optocoupler".

How an Optocoupler Works

1. Input current: A current flows through the internal LED. A resistor sets this current.
2. Light: The LED emits light, usually infrared, across a small transparent gap inside the package.
3. Detection: The light reaches a light sensor, for example a phototransistor, and makes it conduct.
4. Output: The output side switches on. With the LED off, the output switches off.

Because only light crosses the gap, high voltage spikes and ground noise on one side do not travel to the other side, up to the isolation rating of the part.

Optocoupler Symbol and Pinout

The optocoupler symbol shows an LED on one side and a light sensor on the other, separated by a dashed line or a gap. The LED uses the normal diode triangle and bar, with arrows pointing toward the sensor. See our diode symbols guide for how to read the LED symbol. On the output side, a phototransistor looks like a transistor with arrows pointing in, and our transistor symbols guide explains the collector and emitter pins.

Many common 4-pin optocouplers use this pin order. Six-pin parts add a base pin. Always confirm the pinout in the datasheet of your exact part.

Pin	4-pin optocoupler	6-pin optocoupler (with base)
1	Anode (LED +)	Anode (LED +)
2	Cathode (LED -)	Cathode (LED -)
3	Emitter	Not connected on many parts
4	Collector	Emitter
5	-	Collector
6	-	Base

Types of Optocouplers

Type	Output side	Typical use
Phototransistor	Phototransistor	General signal isolation, such as microcontroller inputs and outputs
Photodarlington	Two transistors in a Darlington pair	Higher gain from a small LED current, but slower
Photo-triac	Triac driver	Switching AC loads from a low-voltage control circuit
Logic-gate (high-speed)	Photodiode with a logic gate	Fast digital communication lines
Linear	Photodiode pair	Isolating analog signals or feedback in a power supply

Key Specifications to Check

Spec	What it means
Current transfer ratio (CTR)	Output current divided by LED input current, shown as a percentage. For example, a CTR of 50% at 10 mA of LED current gives up to 5 mA of output current. CTR changes with LED current, temperature and age.
Forward current and voltage of the LED	The current the LED needs and the voltage across it. You need these to size the input resistor.
Isolation voltage	The voltage the barrier is rated to withstand between input and output. The datasheet states the test conditions.
Collector-emitter voltage	The maximum voltage the output transistor can take.
Switching speed	Rise and fall time. Phototransistor types are slower than logic-gate types.
Operating temperature	The temperature range for which the ratings apply.

Optocoupler Circuit Example

This example isolates a 5 V logic signal from a microcontroller that runs on 3.3 V. The values are examples, so check the datasheet of your part.

Step 1: Size the LED resistor

The input side is a normal LED circuit. Use this formula: R = (Vin - Vf) / If. With Vin = 5 V, an LED forward voltage Vf of about 1.2 V and a forward current If of 10 mA, R = (5 - 1.2) / 0.01 = 380 ohms. The nearest standard value is 390 ohms, which gives about 9.7 mA. For more on sizing, see our LED resistor guide.

Step 2: Add a pull-up on the output

Connect the emitter to the microcontroller ground and the collector to a 3.3 V pull-up resistor of 10 kilohms. The collector is also the signal line to the microcontroller input.

Step 3: Check that the output switches

With the LED on, the output transistor can pass up to CTR x If. At a CTR of 50% and 9.7 mA, that is up to about 4.8 mA. The pull-up only needs about 0.3 mA (3.3 V / 10 kilohms), so the transistor saturates and pulls the collector low. With the LED off, the pull-up holds the collector high.

Part	Example value	Role
LED resistor	390 ohms	Sets the LED current to about 9.7 mA
Pull-up resistor	10 kilohms	Holds the output high when the LED is off
Output logic	Inverted	Output is low when the LED is on

Note that the output is inverted: the signal is low when the input is on. Invert it in software, or add a second stage if you need the same polarity. A larger pull-up saves current but slows the switching.

Common Applications

• Microcontroller inputs and outputs: isolating signals from a different ground or voltage.
• Power supply feedback: sending a feedback signal across the isolation barrier of a switching supply.
• Relay, motor and triac drivers: keeping noisy or high-voltage loads away from control logic.
• Communication lines: isolating data lines to cut ground noise.
• High-voltage systems: isolation in designs such as the EV battery management system.

PCB Layout Tips for Optocouplers

• Keep the gap clear: Do not run copper, traces or vias across the gap under the package between the input and output pins.
• Respect creepage and clearance: Space the input and output copper as the datasheet and your safety standard require. See IPC-2221 for general conductor spacing guidance.
• Separate the grounds: Keep the input ground and the output ground apart. A shared ground defeats the isolation.
• Place the resistors close: Keep the LED resistor and the pull-up near the pins they serve.
• Check pin 1: Match the footprint pin 1 to the part, and mark it on the silkscreen so that assembly places the part the right way.

Common Mistakes

• No LED resistor: The LED can burn out without a current-limiting resistor.
• Ignoring CTR drift: CTR falls with temperature and age, so design with margin.
• Using the wrong pinout: Pin order differs between parts and packages.
• Sharing a ground: A common ground between both sides removes the isolation.
• Expecting high speed from a slow part: A phototransistor type is not suited to fast digital lines.

For other symbols you will meet on a schematic, see our PCB schematic symbols visual reference and electronic component symbols guides.

Optocoupler FAQ

What is an optocoupler used for?

An optocoupler passes a signal between two circuits while keeping them electrically isolated. It protects low-voltage logic from high voltage, noise and ground differences.

Is an optocoupler the same as an opto isolator?

Yes. Optocoupler, opto isolator, optoisolator and photocoupler are different names for the same type of component.

What does CTR mean?

CTR is the current transfer ratio. It is the output current divided by the LED input current, shown as a percentage.

What resistor does an optocoupler LED need?

Use R = (Vin - Vf) / If. Take Vf and If from the datasheet. For example, 5 V, 1.2 V and 10 mA give 380 ohms, so use 390 ohms.

How do I test an optocoupler?

Test the input side like a diode: in diode mode on a multimeter, the LED shows a forward voltage drop in one direction. For the output side, power the LED through a resistor and check that the output transistor conducts. Make sure you do not exceed the LED current rating.

Can an optocoupler pass an analog signal?

Standard phototransistor types are not linear enough for analog signals. Linear optocouplers are made for that job.

Conclusion

An optocoupler isolates two circuits with light. Size the LED resistor from the datasheet values, check CTR and the isolation rating, keep the gap clear on the PCB, and confirm the pinout before you finish the footprint.

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