Knit Touchpad

This knit touchpad is possible due to the properties of the anti-static gloves, which are sold for handling sensitive electronic components. The glove is knit from stretchy fibers that also include resistive yarn (poorly conductive). Because of the way the fibers in knits travel from one side to the other, from top to bottom, the resistive yarn travels it doubles back on itself in a way that maintains the resistive connection even when the knit is cut (interrupted). The surface thus has a unique resistance at any point when measuring from at least three points. In this demonstration I chose to measure from all four corners.

The following updated video shows the data visualization from all four inputs. And calculates the position of the nail. As you can see it is not perfect, but you can also see that it does kind of work >>



This is the original video using the finger cap >>



Instead of attaching the Vdd or Vcc to the finger, one could also have it as a continuous conductive layer mounted underneath or above the knit, separated by perforated foam. Although this minimizes resolution to the number of perforations, it does free the user from the additional step having to wear the finger cap. But in this case I consciously wanted to test this method for a future project idea.
The knit does not need to be kept on a flat surface, I just did this for demonstration purposes and the next Instructable will show how this method can be used to turn the whole glove into a touchpad. And then hopefully the whole body, though the problem I have to solve here is how to isolate against the knit interfering with the measurable continuous resistance by touching back on itself.

MATERIALS:
- Conductive acrylonitrile stretch glove from http://www.all-spec.com
also see http://cnmat.berkeley.edu/resource/conductive_gloves
- Conductive copper tape with conductive adhesive by 3M http://www.3m.com/
also see http://cnmat.berkeley.edu/resource/conductive_tape_conductive_adhesive
- Ni/Cu Fabric Tape with conductive adhesive from LessEMF http://www.lessemf.com/fabric.html
also see http://cnmat.berkeley.edu/resource/ni_cu_fabric_tape
- Solderable perfboard with copper line pattern from All Electronics http://www.allelectronics.com/
- Male and female headers from Sparkfun http://www.sparkfun.com/
- Ribbon cable
- 4x20K resistors
- Thin, flexible wire
- Double-sided sticky tape
- Stretch conductive fabric from http://www.lessemf.com
also see http://cnmat.berkeley.edu/resource/stretch_conductive_fabric
- Fusible interfacing from local fabric store or
also see http://www.shoppellon.com
- Stretch fabric (jersey)
- Thread
- Cardboard
- Arduino and USB cable from Sparkfun http://www.sparkfun.com/
- Computer
- Arduino software free for download from http://www.arduino.cc/
- Processing software free for download from http://processing.org/

TOOLS:
- Sewing needle
- Scissors
- Wire clippers
- Wire strippers
- Soldering iron and solder
- Iron

For demonstration purposes I wanted to get the largest rectangle possible out of the glove and cut this from the palm section. I cut it so that the knit does not run diagonally, but from side to side and from top to bottom.
To mount this piece of knit, which likes to roll up on itself, I cut a piece of cardboard, about 2cm larger than the knit all around and mounted it on some strips of squishy double-sided sticky tape. The reason I chose to use squishy tape was only because it was the only kind I had that day, but I also thought it could be nice than a hard surface and to some extent the surface is even pressure sensitive, since the harder one presses, the better the electrical connection becomes. Increasing electricity flow equally to all measuring points.
Before sticking the knit to the tape I connected 4 wires to the corners of the cardboard by stripping their ends and simply fastening them down underneath a small corner of copper tape that has conductive adhesive.

After sticking down the knit I connected its corners to the copper tape at the corners by simply sticking another small squares of conductive fabric tape with conductive adhesive. Be sure to check that the fabric tape is sticking well to both the knit and the copper.

Trace both sides of your index finger onto a piece of stretch fabric and add a bit for the seam and cut out. Iron-on (fuse) a strip of stretch conductive fabric along the center length of one side. Fold in half (right sides together, conductive strip inside) and sew around the edges. Turn inside out and affix a wire to the conductive strip at the open end of the finger cap.

I actually made a silly mistake here in my circuit. Not that it doesn't work, it just does not plug in directly to the Arduino. So you can either follow my faulty version which is also what the pictures are of, or you can follow the text bellow that offers a more ideal way of connecting.

MY WAY
Cut a 6 x 7 hole piece of perfboard (copper lines running the shorter, 6 hole, length). Cut a row of 7 female headers and solder them to the first row of 7 holes. Solder 20K resistors between the third hole and each of the 4 holes next to it. Also see photos for instruction.
Solder the ends of all four (red) cables coming from the corners of your knit touchpad to the other end of the perfboard so that each input runs to the ground through a resistor. Solder the (red) wire coming form the finger cap to one of the two leftover holes on the perfboard that does not pass through a resistor and goes directly to the header. To connect this version to your Arduino you will need to use a piece of rainbow wire that connects things in the right way. The inputs from the four corners should go to your first four analog inputs. The header that is connected to the other end of all the resistors should go to your GND and the header that goes directly to your finger cap should connect to the +5V. See photos!

BETTER WAY
Cut a 6 x 9 hole piece of perfboard (copper lines running the shorter, 6 hole, length). Cut a row of 4 male headers and solder them to the first four holes in the row of 9 holes. Cut 2 male headers and solder them to the last two holes in the same row of 9 holes. Solder 20K resistors between each of the 4 male headers and the closest of the two male headers - these match up with your Arduino board. Solder the ends of all four (red) cables coming from the corners of your knit touchpad to the other end of the perfboard so that each input runs into a resistor. Solder the (red) wire coming form the finger cap to the left over header in the corner. Now this should fit directly on top of your Arduino and should be able to plug the male headers sticking out of your perfboard into the first 4 analog inputs and the two headers should plug into your GND and +5V. See sketch!

For Arduino microcontroller code and Processing visualization code please look here >> http://www.kobakant.at/DIY/?cat=347

Enjoy!

I recently modified the touchpad slightly. Instead of the finger cap I attached a nail to the +5V wire and instead of the square pieces of conductive fabric tape on each corner, I stuck on a longer strip that runs further down the sides, starting at each corner and connecting to the copper tape. See pictures >>