Now it's time to modify the small end of the 12V adaptor. Making sure the adaptor is not connected to anything electrical (like the car), simply cut off the small end and remove the insulation as shown below. I put on some shrink tubing for later.
The tip of the large end of the cigarette lighter is 12V and the contacts on the side are ground. Simply solder the wire going to the tip of the cigarette lighter plug to the two center pins of the 4-pin header and the other wire to the outer pins. I can't recall how to identify the two wires from the markings alone. One has a solid gray line and the other has a dashed gray line. You'll just have to buzz it out with a voltmeter to find out.
When you're done soldering the wires and covering it with some electrical tape or shrink tubing you can connect it to the board. It will look like:
Make sure that nothing's shorted by checking the impedance (ohms) between the tip and side contacts of the cigarette lighter end of the adapter. If it reads close to zero ohms you've got a problem.
You'll definitely want to secure the cable so it doesn't get yanked out. I decided to reuse the grommet that was used to secure the original power cord. You can use a screwdriver to unlatch the grommet from the original power cord. There are two pieces that sandwich the cord that will come undone when you pry it with a screwdriver. When it comes apart it will look like:
The 12V adaptor cable is much thinner than the original 120V AC power cord. In order to have the cable held securely in place by the grommet you'll want to wrap it with some material to give it a larger diameter. You can use some electrical tape or some of the shrink wrap tubing material. Once the grommet is snapped into place over the thickened cable, it can be reinserted into the original opening on the back of the unit:
I decided to forgo the extra credit assignment and left the cut away notch open. For a cleaner setup you can come up with a way to fill it.
