5-pin SPDT (single pole, double throw) automotive relay.

Relays are essentially two parts in one package. The first part is an inductive coil (pins 85 and 86). This coil is the “trigger” function of the relay. By applying both a positive (+) and ground (-) to the contacts, current passes through the coil. When current passes through the coil, a magnetic field is generated and moves the switch (the arrow in the diagram). This is where the second part of the relay comes into play.

The second part of the relay is the “switch.” The switch has three interface contacts—common, normally closed, and normally open. “Normally” refers to the state of the relay not being energized.

While relays can be used for virtually any task, there are some very useful, simple applications for adding LED lighting to a vehicle. Please note that this wiring only pertains to the relay—not the wires in the vehicle. Before interfacing with a wire in a vehicle, always ensure you test the wire with a digital multimeter first to confirm circuit function.

Positive Switching

Positive switching is used for applications where the “trigger” for your switch or circuit is positive. This could be an ignition, brake, or reverse light wire for example.  This could be used for applications such as off-road light bars, work lights, additional brake lights, and other exterior lighting.

Negative Switching

Negative switching is used for applications where the “trigger” for your switch or circuit is negative. Negative triggers are often found in door, hatch, and trunk triggers as well as many interior switches. This could be used for applications such as footwell lighting, truck bed lighting, under-hood lighting, and other interior lighting.

Switch Activated With Ignition Override

This is useful for applications where you want to turn your light on or off with a switch but only want the light to remain on when the car is running or key is in the “ignition” position. The main advantage to this is—even if the switch is left on—the light (or output circuit) will shut off when the key is turned off so that the battery will not be run down.

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