When the button is pressed the Arduino board will put pin 2 in HIGH state ... the Arduino board and the transistor + relay. The load uses its own power supply, for instance if you use a light bulb ...

This is a low-level 5VDC 2-channel relay module with a standard interface that ... 5V to JD-VCC for powering the relay section. If so, remember to connect the VCC pin to 5V of the Arduino board to ...