A Complete Guide on OptoCoupler Relay

The optocoupler Relay circuits are used in various electronic projects. In this, we discuss the basics of Optocoupler Relay in detail

What is Optocoupler Relay?

A relay works on the opto-isolation principle, which means there is no direct or hardwire connection between the controller and the relay. Also, since there is no hardware, the controller will be protected from excessive voltages that could harm it.

Now, coming to the actual optocoupler relay, let’s get started with the parts used in an optocoupler relay. Following are the parts of the OptoCoupler

  • Relay
  • Opto-isolator
  1.  Relay

Relays are electrically operated devices that may open or close contacts electronically. It can be used to turn on or off (make or break) circuits, control multiple circuits with a single signal or control high-power circuits using low-power signals. This peformed with complete electrical isolation between control and controlled circuits, or several circuits that should be controlled by one signal. Relays are mainly used for circuit components preferred in telecom applications, gas detector circuits, RF control applications, Infrared control applications, small volume circuits (gum relay), etc.

  •  Relay pinout and Construction-

A Relay has an electromagnet, armature (movable contact or COM terminal) and two fixed contacts (NC, NO). In the initial stage, the COM terminal will be close to the NO terminal. When we activate an electromagnet, it emits magnetic flux, which attracts the armature towards it, changing the armature’s position from COM to NC.

1) NO (Normally open):

When the electromagnet is powered, the flux created by the electromagnet attracts the armature towards the NC terminal, resulting in a normally closed connection.

2) NC (Normally closed):

When the electromagnet is not active, the armature will be close to this terminal, making this a Normally Open connection.

3) Common terminal


  • Used for ON/OFF applications
  • Capable of switching multiple circuits
  • Coil relays are used for the protection of circuits
  • Controls high power circuit with a low power signal
  • For home appliances such as refrigerators, washing machines
  • For molding equipment, packing machinery, vending machines
  • They are used in Motor & lighting control
  • Used in Aerospace, Defense, and automotive industries
  • Used in the traffic signal controllers, temperature controllers, heaters

2. Opto-Isolator

Opto Isolators are also known as Optocouplers. Optocouplers are electronic components that use light to transfer electrical signals between two isolated circuits They prevent high voltages from affecting the circuit receiving the signal. An optocoupler is made up of two electrically isolated circuits. The first circuit has an infrared emitting diode, whereas the second circuit contains an infrared sensor device, such as a photodiode, phototransistor, photo TRAIC, or photo SCR.

First, the current is applied to the optocoupler, which causes the LED to generate infrared light proportional to the current flowing through it. When light strikes the photosensor, it conducts a current and turns on. The IR beam is switched off when the current running through the LED is disrupted, forcing the photosensor to stop conducting. The photosensor is the output circuit that detects light, and the output is either AC or DC depending on the type of output circuit.

Applications of Optocoupler:

  • Used for ground isolation
  • Used in high voltage monitoring circuits
  • Used in lighting control circuits
  • Used in dimmer circuits

Opto-coupler Relay Circuit with Arduino/STM

Optocoupler Relay

Sample Arduino Code

int D1=11; //initialize the pin to which the relay is connected

void Setup()


pinMode(D1,OUTPUT); //declared as output


void loop()


digitalWrite(D1,HIGH); //set D1 to high


digitalWrite(D1,LOW); //set D1 to low



Hope this blog helps you to understand what a relay is? Its construction, working, configurations, advantages and disadvantages, and applications. We, MATHAELECTRONICS  will come back with more informative blogs.

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