Felting is the craft of transforming fibers into fabric. New methods of processing metals can create conductive fibers that can also be felted. This instructable will show you how to felt together a 3 Volt battery holder which can also act as a simple force-sensitive resistor, opening up possibilities for interactive soft-circuit felted fabrics.

You will need:

Materials

2 Different colors of wool fibers in roving form
Conductive fibers
1 3 Volt Battery
1 LED

Tools

Felting Needles
A solid piece of foam
2 Crocodile-clip jumper wires
A multimeter

I got the conductive fibers from Sparkfun. If you can't find any but have access to conductive yarn, just take a piece of conductive yarn and unravel it until it turns into fibers. Or keep it in its yarn form and felt it into the piece of fabric you'll be making.

Wool fibers and felting needles may be difficult to find, but are often available in craft stores or online.

Step 1: Felting Your Wool Base

Felting occurs when protein fibers are agitated and pushed up against each other, causing the microscopic scales of the fibers to interlock. There are several methods to felt fabrics together. This project will utilize the needle felting method, using needles with barbed ends to force the protein fibers to mat together. Although this method is a little repetitive and tiresome, it does have three major advantages for implementing soft-circuit materials:

It's a dry method, which is great because water and electricity isn't the best combination.
The fabric doesn't shrink unexpectedly in the end.
It can be sculpted into 3D shapes, not just a flat piece of fabric.

The steps for needle felting are simple, but as mentioned before can be a little tedious.

Pull out a tuft of fiber from your wool roving. Don't cut the roving using scissors as this will create blunt ends that won't felt very well. Just hold your wool roving gently and tug at one end.
Lay out your tufts of fiber on a piece of foam width-wise. I'm using a soft foam material used for the packaging of electronic equipment. Some people like to use styrofoam.
Lay out a second layer of fibers, this time length-wise to create a cross-hatch for the fabric. For thicker fabric, you can continue to layer fibers, alternating directions.
Once the fibers are placed on the foam, take a felting needle and stab the fibers into place at the corners.
Continue stabbing the fibers all over with the felting needle. The fibers should become less and less fluffy, matting themselves to each other creating a more flat material. This step might take a while.
When the fibers look like they've started to mat together into one whole piece, flip the felt upside down and continue stabbing.
If you see any holes appearing in the felt, take a tuft of fiber, lay it on top of the hole and felt it closed.
Create two pieces of felted fabric in different colors. I like to use red for positive and white for negative.

Step 2: Adding Conductive Fibers

Once the wool base is ready, it's time to add conductive fibers to the fabric.

Place the battery on the pieces of felted fabric and cut them into the appropriate sizes for a battery holder. Remember to measure twice, cut once.
Take a tuft of conductive fibers - use the same pulling method as treating wool fibers - and lay it out across the piece of felted fabric. Leave some fiber hanging over the fabric so it can be folded over and felted into the other side of the fabric.
Take a felting needle and stab one end of the conductive fibers into place. Continue felting the rest into place. This material felts with the wool but it is much more fine and more likely to clump together. Take your time while felting the conductive materials with wool. It's easy to get discouraged, but you should continue felting until the conductive material becomes a piece of the fabric.
Test the conductive material with a multimeter, making sure that it is conductive from one end to the other.
Repeat the above steps with the second piece of fabric.

Step 3: Felt Together a Battery Holder

Once the fabrics are felted with conductive materials, you're ready to felt them together to create a battery holder.

Sandwich the 3 volt battery with the felted fabrics. I like to leave a little overlap so I can see the two colors from either side of the battery holder.
After you figure out the placement of your battery, insulate the inside of the battery holder so that only the center of the battery touches the conductive fibers by placing more of the non-conductive wool fibers in between the two fabrics you created. This method is hard to explain, and you can potentially mess up by covering too much of the battery or not enough of it. Use the multimeter often to check whether the battery connects with the fabric or not, and that the two pieces of fabric are not short circuiting each other.
Use the felting needle to felt the two pieces of fabric together. You can felt the battery right into the holder, or leave an opening up top. Keep testing with the multimeter to make sure the conductive materials are still connected, and just as importantly that the two sides of the fabrics are not connected to each other.
Because of the nature of felted conductive materials, you can tightly felt the battery holder together to create a permanent connection. Or you can loosely felt the battery holder to create a simple switch. With the second method, there won't be a connection unless the battery holder is squeezed.

Felting conductive materials onto wool felt has gotten much easier and opens up many possibilities for soft circuit electronics. I hope this has been helpful, and if you have any comments, suggestions, or make any projects based on this instructable please show them off and send them my way!