For switching a relay, we only need to use one pin on the Raspberry Pi and have its mode set to OUTPUT and turn this high or low to open or close a relay. Most relays need 5V to be able to switch, so we will be using a 2N3904 NPN transistor. With this transistor, we are able to switch a 5V lane with 3.3V so we can switch a relay.
In our schematic, we will only be switching the relay. This is done for safety reasons (please read A word of caution). When you are more experienced, loads can be added to the relay or a solid-state relay, which suits AC loads better. I would advise that you...