I wanted to add a Microcontroller to my Technic lego, but I can't afford Mindstorms or NXT right now, so why not add my Arduino?
I wanted a solution to adding motors that is non invasive, does not destroy any "prime" parts, and simple to use. This is what I came up with.
You will need for each interface:
1 2x2 tall Lego brick, any color
1 2 position screw terminal block
about 2" solid core wire, 22-24 AWG
Small drill bit or finishing nail
Diagonal cutting pliers, smaller is better
Fine tip sharpie
Step 1: Mark and Drill holes
Measure up the holes on one side of the brick by holding the screw terminal and mark holes as shown. Drill holes with small nail or drill bit. Drill holes in top 2 corresponding studs as well. You want these to line up as close as possible, but centered in the studs is good.
Step 2: Add conductors and solder
Strip 1" of wire for each stud/terminal. Bend an 1/8" down and insert through the hole in the stud. Insert the terminal strip on the side of the brick and tape in place. I found it easiest to put the wire in the hole, and then attach a power bus brick on the top, so I can test to make sure it makes connection. Solder the wire to the screw terminal contact and repeat for both studs. Make sure the wire points out at a 45 degree angle to the outside to ensure it makes contact with the electric bricks. Careful not to muck up the inside of the brick with the hot iron, if you do, just clean out the excess with the exacto knife.
Step 3: Use it!
Plug the bricks topside into the bottom of a power brick, and insert stripped stranded wire into the screw terminals. Insert the screw terminals into your Motor Shield, or other motor control unit. You can also use the brick to get external power from the Lego battery pack in the same fashion.
This method prevents you from cutting wires that are to precious to cut, or damaging bricks that you only have a few of.