KY-019 5V relay module

The 5V Relay Module KY-019 (or KF-301) is perfectly made for Arduino application, is part of the "37 In 1 Sensor Module Board Set Kit For Arduino".
It has three pins, the VCC, GND and Signal. It can act as switch if the circuit and the load circuit have different supply voltage.
It is commonly use if the load circuit is AC. It is a switch used to connect isolated connection from the circuit using a circuit signal. It has green LED that turns on every time the coil is energized or the signal pin has a high input. A red LED indicates power.
For further information see https://win.adrirobot.it/sensori/37_in_1/KY-019-5V_relay_module.htm