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Connections

Relays are available for around $5 in some automotive stores, electronics stores such as Radio Shack, or at your nearest car stereo shop.Here is how a relay looks from the bottom:

Contacts:       30: Common       87a: Normally Closed       87: Normally Open       85 & 86: Power

  Operation

Relays do exactly the same thing a regular switch.   Instead of being moved by a finger, they are moved by an electromagnet that attracts a metal switch.  When the "switch" is inactive (no power through electromagnet) the common and normally closed contacts are tied together.

  

When the relay is activated (12 volts across electromagnet), the switch is pulled, disconnecting normally closed contact, and connecting the normally open contact with common terminal.  As soon a power is removed from the coil, the contacts go back to their original position.