A relay is an electrically operated switch which will be turned on or off, letting the present undergo or not, and may be controlled with low voltages, just like the 5V provided by the Arduino pins.

Controlling a relay module with the Arduino is as simple as controlling the other output as we’ll see afterward .


The relay is that the device that opens or closes the contacts to cause the operation of the opposite electric control. It detects the undesirable condition with an assigned area and provides the commands to the breaker to disconnect the affected area through ON or OFF.

Types of Relay Based on the principle of operation

1. Electrothermal relay.
2. Hybrid relay.
3. Solid-State relay.
4. Electromechanical relay.


It works on the principle of an electromagnetic attraction. When the circuit of the relay senses the fault current, it energizes the electromagnetic field which produces the temporary magnetic flux. This magnetic flux moves the relay armature for opening or closing the connections. the tiny power relay has just one contact, and therefore the high power relay has two contacts for opening the switch.

The inner section of the relay is shown in the figure below. it’s an iron core that is wound by an impact coil. the facility supply is given to the coil through the contacts of the load and therefore the control switch. the present flows through the coil produce the magnetic flux around it.

Pinout of Relay

01. COM: common pin.

02. NO (Normally open): there’s no contact between the common pin and therefore the normally open pin. So, once you trigger the relay, it connects to the COM pin and power is provided to the load.

03. NC (Normally closed): there’s contact between the common pin and therefore the normally closed pin. there’s always a connection between the COM and NC pins, even when the relay is turned off. once you trigger the relay, the circuit is opened and there’s no supply provided to the load.


01. They can be used for both ac and dc systems for the protection of ac and dc equipment’s

02. Electromagnetic relays operating speeds that have the ability to operate in milliseconds are also can be possible.

03. They have properties such as simple, robust, compact and most reliable.

04. These relays are almost instantaneous. Though instantaneous the operating time of the relay varies with the current. With extra arrangements like dashpot, copper rings.

05. Electromagnetic relays have a fast operation and fast reset