How to set up a polarity reversing relay

A polarity reversing relay is used commonly when you have a circuit that is triggered by a ground and you would like to activate one that is triggered by +12 volts or a positive trigger. Commonly dome lights in automobiles or on occasion door switches may be wired up with a positive or negative trigger and you need the opposite.

What you'll need
*Assorted lengths of wire in several colors to make tracing easier
*ONE relay (either four or five pole is fine we only need four terminals)
*Crimp On Terminals

For this example lets say we are installing a security system on a car or boat and would like our interior lights to come on when we disarm the system.. Our alarm has a variety of outputs and for the sake of this example lets say the dome light output on the alarm is a negative however our boat/car is wired so that the lights come on when they receive a positive signal.

How it works:

+12 Volts is connected to terminals #87 and #85.
Terminal #86 is connected to our negative output.
Terminal #30 is connected to our light circuit.
Terminal #87a is not used.

When the negative (ground) signal is sent to the relay from our security system the relay is activated and closes the connection between terminals #87 and #30 this sends +12 volts out through terminal #30 turning on our lights.

If you need a negative output with a positive input you can simply connect the red wire on the diagram to ground instead, the black wire would of course then be connected to a +12 volt positive trigger sending a ground when the relay was activated.