relay basics

Relays serve 2 main purposes, The provide isolation and they are used when you want a small current circuit to control a heavy current load. Isolation means that the control circuit (the circuit or device that activates the relay or turns it on) is not electrically connected to the power circuit (the circuit your device draws power from that you are wanting to switch on or off). Basically, the relay is used when you want something that has low outputs (such as outputs of alarms, keyless entries, etc.) to control something that draws large amounts of power. Lets use your fog lights for an example. If you did not use a relay, then you would have to run wires that can carry the power your lights use (generally 15-30 amps) to a switch, then back to the lights. The switch would have to be rated to handle the power your fog lights use. Now, if you use a relay, you still need the heavy power wires, but you can control the lights using a less costly, lower power rated switch, becuase the only thing the switch will need to turn on is the coil in the relay. The relay handles the heavy current instead of the switch. For a load such as your cameras, since each one uses 100mA, the total load current for them is 200mA. Most toggle switches can handle that, so I dont think you need relays for that. I would use a relay for the fog lights. Now, about the connections. Most relays are 5 post. 2 posts are connected to the coil, and the other 3 are the switch. The posts or terminals should be labeled 85, 86, 87, 87a, and 30. (85 and 86 are the coil, 87, 87a, and 30 are the switch) So for fog lights, you would connect 85 to ground, 86 to a +12 volt switched source (run a wire from a +12 volt source to a toggle switch, then to the 86 terminal). At this point, when you turn the toggle switch on, you should hear the relay click. If not, double check your connections. Now run a power wire (rated to your fog light ampere draw) to the relay and connect it to terminal 30. Run another wire of the same rating from terminal 87 to your fog lights. Now when you turn on the toggle switch, it activates the relay, which closes the switch contacts inside the relay, turning on your fog lights.

Thats a basic summary of relays. They are pretty easy to install. Relays can do many other things. I hope this helps you.

http://www.bcae1.com/relays.htm

they have a nice little flash demo about half way down that shows you exactly how to use a relay (wiring wise)

hope that helps