Converting Holiday Lights for 12V Operation  

  Tools You'll Need  

• Power supply, 12V, for testing.  (I used an old computer power supply.)
• Xacto knife, for probing wires without stripping them
• Wire cutters/strippers
• Soldering iron and solder
• Multimeter
• A couple of alligator clip-ended jumpers (or Pomonas if you have those)

  Get the Right Lights  

This type of chain has three wires going along it: the two power wires going straight through, and the actual string of lights.  Basically, it's an extension cord plus a string of lights.  The first and last lights are wired "specially" and attached to one or the other power wire. 

Schematic of chain as it comes out of the box. Not all the bulbs are shown:

  The General Idea  

Schematic of modified chain.  Note alternating connections.

An added bonus is that if a bulb comes loose, only the small section it's in will go out -- all the rest are independent and will stay lit.

By the way, the bulbs don't seem to care if they're getting AC or DC power.

  Procedure  

1. Figure out the voltage of your lights.  The chains I used have 50 lights apiece.  I've checked that all the lights are all along the same wire.  So, since the string uses 120V total, which means each light bulb is using (dropping) 120/50 = 2.4V. 
2. Figure out how many light bulbs are needed to drop 12V.  In my case, 12/2.4 = 5.  If I want to have my lights burning at "normal" intensity, I need to have strings of 5 of them at a time connected across 12V.
3. Hack off the plug and strip the wires.  This is so you can hook it to your power supply (go ahead -- not much will happen, the chain is expecting 120V and you're only giving it 12) but also so you don't go and accidentally plug it into the wall, later -- which would be a Bad Thing.
4. This is also a good time to adjust your wire strippers (if you have the adjustable kind) to cut only through the plastic, not into the wire.  This will allow you to bare a section of wire "in the middle" without cutting it through.  If you don't have the adjustable kind of wire strippers, figure out which "notch" is the right one to use.
5. Make the first cross-connection.  The trick here is, you need to connect between the light string wire and the power wire the light strings are not connected to at the first lamp.  Follow the same procedure as for intermediate cross-connections (below), only attach your "last connection" probe to the power wire that goes to the first light.
   
6. Make intermediate connections.  Each cross-connection is to the opposite power wire from the one before.  The easiest way to figure it out is:
   
   • Connect one probe of your multimeter to the last cross-connection you made.  I use an alligator clip-lead or Pomona clip-lead for this.
   • Strip a small section of the light-chain wire midway between the bulbs (in my case, between the 5th and 6th ones from the last cross-connection),
   • Use another alligator clip-lead to connect the X-Acto knife to the other probe of your multimeter, and
   • Probe the two unstripped wires (i.e., the power wires) to find which has the lowest resistance to your previous cross-connection.  It should be very close to zero.
   • You want the other wire, the one with more resistance.
7. Don't forget to wrap the previous connection with electrical tape.  Leave the current one unwrapped to connect the meter to for the next one.
8. Fix up the end of the chain.  In my case I had to rewire the final bulb's connection to the opposite power wire from where it was connected.  This meant I had to cut two wires to preserve the power wire connection, and then connect the bulb to the other power wire.
   Before: 
   After: 
   
9. Hook up the wires from where you hacked off the plug to something more useful, like a Radio Shack 12V quick-disconnect.  I've previously wired one into my motorcycle for accessories such as radar detectors, so that was the obvious choice for me.  For automobile use, you might want a cigarette lighter plug.