The Relay Module is a convenient board which can be used to control high voltage, high current loads such as lamps and AC load. It is designed to interface with a microcontroller such as Arduino, ESP8266 and etc. It comes with a LED to indicate the status of the relay. The relay modules generally have the following terminals:
1. NC(Normally closed)=The normally closed configuration is used when you want the relay to be closed by default, meaning the current is flowing unless you send a signal from the Arduino to the relay module to open the circuit and stop the current
2. NO(Normally Open)=The normally open configuration works the other way around: the relay is always open, so the circuit is broken unless you send a signal from the Arduino to close the circuit.
3. Common pin=It is generally in the middle
4 Input pins=It is the pin through which relay is connected to the controlling device.
If not mentioned on the module we can find out the NC pin using a multimeter in continuity setting by connecting the common with the other terminal through multimeter probes if it creates a beep sound then the pin is NC.