This is a LOW Level 5V 2-channel relay interface board, and each channel needs a 15-20mA driver current. It can be used to control various appliances and equipment with large current. It is equiped with high-current relays that work under AC250V 10A or DC30V 10A. It has a standard interface that can be controlled directly by microcontroller.
Relay Maximum output: DC 30V/10A, AC 250V/10A 2 Channel Relay Module with Optocoupler LOW Level Triger expansion board, which is compatible with arduino Standard interface that can be controlled directly by microcontroller ( 8051, AVR, *PIC, DSP, ARM, ARM, MSP430, TTL logic) Relay of high quality loose music relays SPDT. A common terminal, a normally open, one normally closed terminal optocoupler
See the picture below: A is an electromagnet, B armature, C spring, D moving contact, and E fixed contacts. There are two fixed contacts, a normally closed one and a normally open one. When the coil is not energized, the normally open contact is the one that is off, while the normally closed one is the other that is on.
Add a certain voltage to the coil and some currents will pass through the coil thus generating the electromagnetic effect. So the armature overcomes the tension of the spring and is attracted to the core, thus closing the moving contact of the armature and the normally open contact (or you may say releasing the former and the normally closed contact). After the coil is de-energized, the electromagnetic force disappears and the armature moves back to the original position, releasing the moving contact and normally closed contact. The closing and releasing of the contacts results in power on and off of the circuit.
VCC : Connected to positive supply voltage (supply power according to relay voltage)
GND : Connected to negative supply voltage
IN1: Signal triggering terminal 1 of relay module
IN2: Signal triggering terminal 2 of relay module
Each submodular of the relay has one NC(nomalclose), one NO(nomalopen) and one COM(Common). So there are 2 NC, 2 NO and 2 COM of the channel relay in total. NC stands for the normal close port contact and the state without power; No stands for the normal open port contact and the state with power. COM means the common port. You can choose NC port or NO port according to whether power or not.