How to Use a Relay in Car Electronics


A relay is an electromagnetic switch. When power or ground is applied to a coil, the relay clicks on or off. The result is that a power wire to a function is either closed (connected) or opened (disconnected). When the line is connected, the function is turned on and works. When the line is disconnected, the function is turned off and will not work. A relay can be used on alarm systems to disable the starter, to flash the parking lights, and more.

Using a Relay to Flash Your Car's Lights

  • Read instructions that come with the alarm and relay.

  • Apply constant 12 volts to poles 85 and 87.

  • Put the lead from the alarm's output to pole 86. When negative pulse throws pole 86, poles 85 and 87 are already getting 12 volts. The switch is clicked and 12 volts are going from pole 87 to pole 30.

Using a Relay to Turn On an Amplifier

Using a Relay to Set Up Starter Kill With a Hidden Toggle Switch

  • Connect starter solenoid line to pole 87.

  • Connect poles 30 and 85 to 12 volt accessory line.

  • Connect toggle switch to pole 86 and ground the toggle switch.

  • Connect the positive from the battery through a fuse and flashing LED to the toggle switch.

  • Ground off of the battery.

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