Knit Touchpad

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About: My work combines conductive materials and craft techniques to develop new styles of building electronics that emphasize materiality and process. I create working prototypes to demonstrate the kinds of electr...
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.

Step 1: Materials and Tools

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

Step 2: Preparing Touchpad Surface

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.

Step 3: Connecting the Touchpad

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.

Step 4: Sewing Conductive Finger Cap

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.

Step 5: Soldering Pulldown Resistors

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!

Step 6: Testing, Testing

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

Enjoy!

Step 7: Updates

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 >>

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    34 Discussions

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    sli7

    5 years ago on Introduction

    it is awesome and very nice project, but I can not find the code. Could you give me any information about the code? I will appreciate it. Thank you very much.

    1 reply
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    gtkid_1990Asli7

    Reply 1 year ago

    yeah same here is it possible if you could upload the code?

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    Barsoomian

    7 years ago on Step 5

    Hi, I was wondering if you remembered which corners were attached to which analog input port on the arduino?

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    NoSuchNick

    8 years ago on Introduction

    Hi,
    i was wondering if this sensor has a varying resistance relative to the amount of pressure applied.
    If it has, how do you correct for it? Do you first calculated the total pressure out of the 4 individual inputs, adjust the values and then calculate the position?

    Bye,
    NsN

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    Radobot

    9 years ago on Step 7

    This is a great project! But can you change the Conductive acrylonitrile stretch glove to something different (like some metal)? And can you use instead Arduino some circuit (what don't need program and have USB)?

    1 reply
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    PluseaRadobot

    Reply 9 years ago on Step 7

    if you use any resistive material you can measure from multiple points and calculate the position. and i'm not quite sure if i get the next part of your question. you want to use another analog/digital converter that is not the arduino? of course you can do this, but since i mainly work with the arduino, i don't really have any other recommendations. what are you trying to do?

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    avibank911

    9 years ago on Step 7

    wow nice project!! can you use something besides Arduino , like usb or summin?

    1 reply
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    Pluseaavibank911

    Reply 9 years ago on Step 7

    Don't quite understand your question? Arduino is translating the analog signals into a serial signal, which is what is sent via USB to the computer. You can use other A/D converters, it does not need to be an Arduino.

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    chrischavez

    9 years ago on Introduction

    I don't have any access to conductive fabrics or anything of the sort. would there be anything that i could make. i have access to a multitude of hardware stores and a walmart. maybe a conductive glue that is spread on a mesh. maybe just maybe a few items from the Internet.

    2 replies
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    Pluseachrischavez

    Reply 9 years ago on Introduction

    well you can order conductive fabrics super easily online from >> www.lessemf.com

    from a hardware store you might be able to get some velostat in form of antistatic plastic bags that are used for storing/packaging sensitive electronic components.

    and it would be very cool if you could try making conductive glue and coating textiles with it. there is a good instructable for conductive glue, though it hardens and can not handle the stretchiness or flexibility of textiles >>
    https://www.instructables.com/id/Make-Conductive-Glue-and-Glue-a-Circuit/

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    chrischavezPlusea

    Reply 9 years ago on Introduction

    Thanks what I will try to do is make a drawing pad. maybe not that accurate but its something.

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    leahbuechley

    10 years ago on Introduction

    love this & all of your instructables plusea! so glad you're out there doing all of this cool stuff & sharing your projects!

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    macsimski

    10 years ago on Introduction

    you can also use the antistatic bags Farnell uses to send you chips in. it's black plastic and also has a decent resistance. i've made one measuring 20x20cm to be used as a life midi interface, with a teaspoon as a controller.

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    Pluseamacsimski

    Reply 10 years ago on Introduction

    Yes, i've been working with Velostat (black anti-static plastic) for making bend sensors and combinations with layering and grids might make for a nice pressure sensitive, wearable touchpad.

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    archipeepmacsimski

    Reply 10 years ago on Introduction

    I would love to know a little more about your 20x20 midi interface! Perhaps you are thinking of posting your own instructable on this? ;) How well does it work, how accurate, etc.?

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    muffin1

    10 years ago on Introduction

    what if you had a carpet made out this stuff and you incorporated it in a video game, so you could walk around the room (with a peace of metal on your shoes) in the game. you could wear your suit thingie and it would be like a virtual reality game.

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    srhadaham

    10 years ago on Introduction

    Any reason why you would use this over the touch pad on your laptop or is this just something to have fun playing around with?