When I was designing my lighting setup, there were certain "wants" that I designed into my lighting package:

to have the stock 'chime' remind me that my auxiliary lights are still on (when used with stock lights)

to have my fog lights on with my low-beams

to have my fog lights on with only my parking (marker) lights turned on

to have my driving lights used as daytime driving lights with just my parking lights on

to have my driving lights tied to my high beams so I can toggle both on/off together

to have my long range driving lights (off-road pencil beams) tied to my high beams [like the driving lights]

to be able to turn my long range driving lights on whenever I want (giant flashlights!)

to have my auxiliary backup lights tied to my reverse lights so they come on together

to be able to turn my auxiliary backup lights on whenever I want (for loading, etc.)

Although some of these "wants" may seem to be contradictory, they can be accomplished by using a different kind of switch from the normal on/off (SPST) switch we are all familiar with.

In a "normal" complex circuit using a relay, you have an SPST switch that "controls" the relay. The power that flows thru this switch comes from a single source. In a way, it is the power from this source that determines when a circuit can be turned on.

Remember that the control circuit of a complex circuit arrangement using a relay, does not have to have the same source as the power circuit. The control circuit requires less than 1/10th an amp to energize the coil in the relay. The power circuit is what actually has enough 'juice' to power the accessory and is often hooked up to the battery or PDT (power distribution terminal).

Here's an illustration:

Let's say for example that we are adding a set of fog lights and the power circuit source will be the PDT. In a 'normal' complex circuit, we can either use the PDT or another source to power the control circuit for the relay. Let's say that we decide to use the low-beams to control the fog lights. We tap the low-beam lamp at a point just behind the headlight assembly. We then run the wiring from this tap thru an SPST on/off switch to control the relay for the new fog lights. This means that even if we turn the switch on (close the circuit), the fog lights will not come on until the low-beams are also on! In this manner, an existing circuit is used to control your new control circuit. Pretty straight forward, huh? A corollary to this setup is that the fogs cannot come on when just the parking (marker) lights are on. They also cannot come on when your high beams are on (they'll go off as you toggle the multi-function stalk to high beams and then the fogs will come back on when you go back to low beams).

Most people would never consider having fogs on with the high-beams anyway (kinda defeats the purpose doesn't it?). But, let's say that besides having the fogs on with the low-beams, we also want the choice of turning the fog lights on with just the marker lights on (for an explanation refer to footnote 1). In a 'normal' complex circuit, this flexibility would be impossible. But, by using an SPDT switch, you can decide which existing lighting circuit you want to use to control the input to the fog lights relay! You do this by tying one pole of the SPDT to a tap of the low-beam feed, while tying the other pole of the switch to a tap of the marker lights. This is the reverse of a "normal" hookup for an SPDT but the important thing to remember is that a switch can be 'reversed'. It'll work both ways!!!

This is not rocket science. Once you stop and think about it, it's pretty darn simple. For those who may be confused by my terminology (or lack thereof) the diagrams should be self-evident.

Think of it as a switch in a railroad track. 'Normally' a train would come down a main line and when it gets to the switch it can go one way or the other (much like a 'Y' in a road). If you turn the switch around, it would be like a train from Chicago and a train from New York that use the switch to get onto the same main line. The switch determines which one you will let onto the main line at a particular time.

But, instead of using a standard SPDT switch which contact one pole or the other, I used a center-off SPDT. The center-off type switch gives you a 'neutral/OFF center position'. If I had used a standard SPDT switch, the fog lights will always turn on when one or the other tap is also on. Sometimes you may not want the fog lights to come on with either tap! The center-off feature allows you to do this.