As you can see on our wireless relay boards like SR-RCS-202U and SR-RCS-204U, the terminals of relay output is always marked as N/O N/C COM etc, they are abbreviations that are commonly used with relays, here are what they means.
N/O means Normally Open ( to COM output )
N/C means Normally Closed ( to COM output )
COM means Common output.
Since we’re using SPDT relay ( single pole double throw ) – so there are 3 terminals in total – N/O N/C and COM ( N/O and N/C are connected to respective throw, while COM is connected to the pole )
In electronics, a open circuit means the target is not connected or powered, while a closed circuit means target is connected or powered.
So N/O – normally open means the terminal is by default not connected to COM terminal, while N/C – normally closed means the terminal is by default connected to COM terminal.
As you all know, relay acts like a switch in circuit, in most circumstances, we can connect N/O and COM to your circuit, so the switch is by default cut off, and when relay energized -> N/O connects COM -> circuit is connected -> light on or door open.
While N/C and COM are connected if you need something be powered by default, and when there is a signal -> relay energized -> N/C and COM disconnected.