Let now understand the working principle and relay working with some circuitry. In this tutorial, we connected two LEDs on two switching positions of relay that are NO and NC see in pin configuration.
One led (LED-Yellow) is connected on NC terminal and other led (LED-Red) is connected to NO terminal.
When the coil is not energized, the COM terminal is connected to the NC terminal. This is the normal condition of relay.
So what is happening here is (see diagram), because the switch (SW1) is open circuited there is no electrical signal provided to the relay for now, so relay remains in its normal position. A source of +5V is connected (source that is required to drive the load (LED)) on COM terminal that I required to drive a led and COM is short circuited to NC terminal this makes the path complete and yellow led glows.
The Coil terminals control the switching. When voltage is applied across the coil it becomes an electromagnet (rating of relay depends on coil and that much input must required across the coil to work properly). Its core attracts the switch armature (lever) and activates the switch (switching it to NO contact point).
When the coil is energized, the COM terminal will connect to the NO terminal, as shown in the image.
Now as we press the switch and makes the circuit short circuited an electrical signal attracts the lever of COM towards the coil and now COM will short circuited with NO terminal and the supply from the COM is directly received by the Red led which glows red led and yellow led will turned off.
Few Examples of relay circuits:
If I do not need yellow led in circuit, I can simply remove it from the circuit then if relay coil is not-energized nothing will happen to the circuit means no any led glows and if relay coil is energized then red led will be turn on.
And if I remove red led from the circuit then if relay coil is energized then yellow led will be turn off.
The Normally Open Terminal (NO), Normally Closed Terminal (NC) and Common Terminal (COM), make up the switch contacts. How they’ll be connected depends on the application.