First thing first, I KNOW there are other instractables for this, I just wanted to show my version. PLEASE don't flame because it's already been done!

Second, this is my first instructable. I have writen another but do not have a camera to take pictures with so cannot yet post it.

Third, this uses the simplest Double Pole Double Throw switch. This requires nothing more than the switch and some creative wiring. Please note that the "off" position only works if you have a "center off" switch! A DPDT relay can easily be substituted if you have one. If you want a 'struct' on this, leave me comments.

[EDIT, 3-28-'09] I have added some new information, please see the new step 4.

Step 1: Selecting Your Switch

You need to decide if you want to purchase a switch or salvage one from something else. if you purchase one, you have more options as there are many out there. if you salvage one you are limited to what you can find.

Make sure the switch you use can handle the amount of current you need
Decide if you want an off position or not.
Decide if you want a sliding switch, a toggle switch, rocker switch, or even a spring loaded switch that returns to off when released.
If there is a junk car around, check it for power window switches or electric seat adjusters. Both are most often DPDT switches, some are already set up to be inverting switches. Don't forget to steal the motors or sometimes linear actuators out of the seat adjusters, if you can!!!!
Broken stereos commonly have one or two switches in them

If you salvage a switch, the first thing I would do is test ACROSS the switch to be sure it is double pole. The switch has two rows of contacts with three pins per row. NO pin in one row should have continuity to ANY pin in the OTHER row. In the "center off" position, if equiped, NO TWO PINS should conduct.

In the case of a sliding switch: You should find that the center pin in each row conducts to the pin at the same end that the slider is on, but will not conduct to any other pin in the same row or to any pin in the other row.

In the case of a toggle switch: You should find that the center pin of each row conducts to the pin at the end OPPOSITE to the toggle lever, but will not conduct to any other pin in the same row or to any pin in the other row.

In the case of a rocker switch: You should find that the center pin in each row conducts to the pin at the same end of the switch as the RAISED side of the rocker, but will not conduct to any other pin in the same row or to any pin in the other row.

Step 2: Wiring the Switch

Wiring for this switch is deadly simple.

To make my instructions a little more easy to follow, hold your switch in such a way that you are looking at the pins and they are arranged 2 pins wide and three pins tall. Imagine that the pins are numbered as below:

1 4
2 5
3 6

Connect a wire from 3 to 4, and another wire from 1 to 6. These wires will form an "X" across the center of the switch. Only one of these wires needs to be insulated as long as both are too tight or too rigid to move and touch other pins!

In my diagram the source wires are the red and black wires connected to pins 3 and 6 and the load wires are connected to pins 2 and 5. there are many acceptable wiring methods though. Since there is nothing to restrict the flow through the switch, load and source can be swapped wihtout any problems. If one pair uses pins 2 and 5, the other pair can use 1 and 3, 1 and 4, 3 and 6, or 4 and 6, but not 1 and 6 or 3 and 4! I hope that was clear :P

Once your wiring is done, setting the switch will cut off power, allow it to pass through, or invert the polarity, respectively as shown in the diagrams below

Step 3: ENJOY, and Leave Me Comments

I hope you have enjoyed, and gotten something useful out of reading my first Instructable. Please leave comments if there was anything that was unclear or anything I should change as I will not know unless you tell me. Flames or non-constructive criticism WILL BE DELETED (if I have that option.. ignored otherwise).

Thanks for reading,
DieCastoms.

Step 4: Premade Switches

In household wiring, there is something called a 3-way switch. 3-way switches are used in pairs. You might have a pair in your house. If you have a switch at each end of a hallway that both control the same light in the hall, that's a 3 way switch set. Either switch will operate the light.

There is also a "4-Way" switch. If you needed 3 or more light switches for the same light, you would start with a 3-way pair and add a 4-way switch (as many as needed) in series between the 3'way switches.

After doing a little research, it has come to my attention that the individual 4-way switch does exactly what my 'ible shows. I would assume that being designed for 110-120 volts and capable of handling 500 or more watts, this switch would be useful in most DIY projects needing polarity reversal, and where physical size would not be a restriction.

If anyone can confirm or contest this, please do, as I have not had the money to go buy a 4 way switch to test it.

DC.