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Relays are used when devices of different voltage or amperage are being used together. One common example of this is relays that are used to control headlights on a car. The switch inside the vehicle has a lower amperage signal that is ran through it to be used as a signal. When the switch is turned on, a relay is closed which then connects a different circuit to turn on and off the headlights.
In the Traffic Pi project, relays are used in a similar manner. The Raspberry Pi outputs 3.3DCV from the GPIO pins. Those pins are connected to a relay board. The relay board is also connected to a 120V which is needed by each of the signals. When the Pi sends an "on" signal to the relay board, the relay activates, thus sending 120V from the source (in this case an outlet) to the signal and then corresponding signal then turns on.
Wire Color Standards
The color of each wire always denotes its purpose. When using different color wires, you don't need to have a tag on each wire. Instead, you create a service manual or other documentation that lists which color wire each is responsible for.
For example, if you look a wiring harness on a car, you will notice that it has a number of wires that are bundled together. Each of those wires has a purpose. The vehicle manufacturers create a document, often referred to as a service manual, that lists each of the colors of the wire in the harness and their purpose.
With an AC circuit, there are commonly 3 wires. Black, white, and bare or green. Black wire in AC circuit represents the "hot" or supply voltage. White wire in AC Circuit represents the neutral. The neutral
With a DC Circuit, there are two wires. Red and black. The current in a DC system only flows in one direction. Red wire in DC circuit represents the positive. This is the voltage that comes from the power source. Black wire in DC circuits represents the negative. This is the voltage that goes back to the power source.
AC vs DC
Alternating Current (AC)
The current in an AC system alternates between the hot and neutral wires. The frequency at which this occurs varies. In the North American (including the United States), the alternation takes place at 60 Hertz or 60 times per second.
AC circuits are commonly used for items that require a lot of power to operate. These include large motors, like the ones used in AC refrigeration and industrial machines.
Direct Current (DC)
DC circuits are commonly used for battery powered systems and devices that do not require a lot of power to operate.