Single Channel Relay Module, What is a relay module?, Specification of relay module, Working principle of relay
Single channel relay module  
Single Channel Relay Module circuit

What is a relay module?

The Single Channel Relay Module is a convenient board which can be used to control high voltage, high current load such as motor, solenoid valves, lamps and AC load. It is designed to interface with microcontroller such as Arduino, NodeMCU, etc. The relay's terminal (COM, NO and NC) is being brought out with screw terminal. It also comes with a LED to indicate the status of relay.

The relay is the device that opens or closes the contacts to switch ON/OFF other appliances operating at high voltages. It is also used in safety circuits where it detects the undesirable condition with an assigned area and gives the commands to the circuit breaker to disconnect the affected area through ON or OFF.

Every electromechanical relay consists of:
1. Electromagnet
2. Mechanically movable contact
3. Switching points
4. Spring

COM: Common Pin

NO: Normally Open – There is no contact between the common pin and the normally open pin. So, when you trigger the relay, it connects to the COM pin and power is provided to the load.

NC: Normally Closed – There is contact between the common pin and the normally closed pin. There is always connection between the COM and NC pins, even when the relay is turned off. When you trigger the relay, the circuit is opened and there is no supply provided to the load.


  • Digital output controllable
  • Compatible with any 5V microcontroller such as Arduino.
  • Rated through-current: 10A (NO) 5A (NC)
  • Control signal: TTL level
  • switching voltage 250VAC/30VDC
  • switching current 10A
  • Size: 43mm x 17mm x 17mm

Working principle of relay

It works on the principle of an electromagnetic attraction. When power flows through the first circuit (1), it activates the electromagnet (brown), generating a magnetic field (blue) that attracts a contact (red) and activates the second circuit (2). When the power is switched off, a spring pulls the contact back up to its original position, switching the second circuit off again.

This is an example of a "normally open" (NO) relay: the contacts in the second circuit are not connected by default, and switch on only when a current flows through the magnet. Other relays are "normally closed" (NC; the contacts are connected so a current flows through them by default) and switch off only when the magnet is activated, pulling or pushing the contacts apart. Normally open relays are the most common.

Working of relay during absence of current in the coilWorking of relay during presence of current in the coil

Taking another example of how relay links two circuit together. It’s essentially the same thing drawn in a slightly different way. On the left side, there’s an input circuit powered by a switch or sensor of some kind. When this circuit is activated, it feeds current to an electromagnet that pulls a metal switch close to itself and activates the second output circuit (on the right side). The relatively small current in the input circuit thus activates the larger current in the output circuit.

  • The input circuit (blue loop) is switched off and no current flows through it until something (either a sensor or a switch closing) turns it on. The output circuit (red loop) is also switched off.
  • When a small current flows in the input circuit, it activates the electromagnet (shown here as a dark blue coil), which produces a magnetic field all around it.
  • The energized electromagnet pulls the metal bar in the output circuit toward it, closing the switch and allowing a much bigger current to flow through the output circuit.
  • The output circuit operates a high-current appliance such as a lamp or an electric motor.


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