The Relay is an electrically operated switch. In projects, I used Electromagnetic relay type mostly. In this section, we shall cover this electromagnetic type. It is exciting when current flows through the coil. Let’s explore the internal circuit diagrams.

Internal Components

It has 5 terminals – Vcc, GND, NO, NC, COMMON.

  • Vcc – From MCU – 5 V/9 V/12 V.
  • GND – From MCU.
  • NO – To output device – Normally opened.
  • NC – To output device – Normally closed.
  • Common – To output device (GND).

It has a movable switch key and the coil, the heart of this component. Movable switch moves over NO and NC terminals with one end fixed at COMMON. An Electromagnetic Coil placed near to the moving switch. It kept such that when coil excited it pulls key and rests NO terminal and it rests NC terminal in case of no supply.


A relay is an application of Inductor. Relay contains Coils, Contacts, Movable arm and Power source. If you provide supply to the Coil, it acts as an Electro Magnet. This will produce a Magnetic Field. This field, in turn, attracts the Contacts inside the relay.  The two different contacts used, namely Normally Open and Normally Closed.

Normally Open Contact:

This Switch normally open. If you apply power supply or an external source, this will act as a Closed Circuit. In the Relay, NO  excited when the electromagnet produces Magnetic Field. The magnetic field attracts the movable armature and rests on NO contact. This is an important part of the Relay. In this pin, the load connected. This will act as a controlling part.

Normally Closed Contact:

This Switch normally closed. If you provide power supply or an external source, this will act as an Open Circuit. In the Relay, this opened when the electromagnet produces Magnetic Field. Initially, the armature has attached with NO contact only.

When the coil excites, the coil behaves as an Electro Magnet. This produces Magnetic Field which attracts the armature or Movable coil. They lay on NO open contact. The movable arm will be on NC contact, it makes contact on NO. The loads connected to NO contact. If the relay is de-energized when it is in contact with NC means, it becomes as an Open Circuit.  The movable arm will return back to NC when the supply stopped with the help of Spring.


  • For the Switching process.
  • To control a device from Low-power source.
  • Used for Protection purpose.
  • Used to prevent Voltage drop

If you want to buy a relay, get it from below link.


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