Introduction to Optocouplers. principle, Application

What are Optocouplers?

Optoisolator or better known as an optocoupler is an electric photo switch consisting of a LED emitting infrared light and a phototransistor as shown below:

The LEDs and phototransistor are contained in a pack with two input connections and two output connections.

For this component, there is one parameter that needs to be considered besides the individual for the LED and phototransistor as described before which is a combination of led and phototransistor namely current transfer ratio (CTR) which is a ratio of the collector current and diode current to a certain condition. Where CTR can be used to find the number of holes in a field to press as small as possible a calculation error.

Working principle of Optocouplers:

The optocoupler consists of two parts, namely:

  1. The transmitter is built from an infrared LED. When compared to using ordinary LEDs, infrared LEDs have better resistance to visible signals. The light emitted by infrared LEDs is not visible to the naked eye.

  2. The receiver section is built on the basis of a phototransistor component. Phototransistor is a transistor that is sensitive to light power. A light source produces heat energy, as does the infrared spectrum. Because the infrared spectrum has a greater thermal effect than visible light, the phototransistor is more sensitive to capture radiation from infrared light.


  • If the phototransistor and LED are blocked, the phototransistor will be off so that the output from the collector will logic high.

  • Conversely, if the phototransistor and LED are not blocked then the phototransistor and LED will be on so that the output will be logic low.

Usually in the market optocoupler available with type 4N25 / 4N35 has an isolation voltage of 7500 volts with the maximum ability of the LED to be fed by a forward current of 3A.

The physical optocoupler can be of various forms. If only used to isolate the voltage level or data on the transmitter side and receiver side, then this optocoupler is usually made in solid form (there is no space between the LED and phototransistor). So that the electrical signal at the input and output will be isolated. In other words, this optocoupler is used as an IC optoisolator.

Application of Optocoupler:

  • Photoresistor based optoisolators are slowest types of optocouplers, but also the most linear isolators and are used in the audio and music industry.

  • Most optoisolators use available use bipolar silicon phototransistor sensors and reach medium data transfer speed which is enough for applications like electro-encephalography.