Sphere Dual Pressure Sensor

I have found a way to control the resistance of using conductive felt. The trick is to build the conductive felt around a stable resistor. The was discovered by a suggestion from Eric Rosenthal and some tests I did by making a small ball of conductive felt and putting a 2.2K hard resistor in the middle. When I hooked up my multimeter to the two ends of the resistor using them as my leads, I was able to change the resistance by squeezing the conductive felt around it, the resistance went considerable down.

resistance of cond. felt 2.2K resistor leads from lara grant on Vimeo.

resistance of cond. felt ball with 2.2K resistor leads from lara grant on Vimeo.

After I established this, I questioned whether or not I could make the stable resistor soft, while still keeping the range of values fixed. I remember seeing the suggestion of making a resistor out of stitching conductive thread on fabric using a sewing machine. I sat down at my machine and tried it out.

Straight stitched resistor from lara grant on Vimeo.

Stitched resistor from lara grant on Vimeo.

To my excitement, this worked really well. Only thing is, I would not be able to get a very large range unless I had the space to stitch on. The longer the stitch, the more resistive it would be. There is some resistive thread out there which I need to get my hands on, it boasts <1000K Ohms per 10 cm. This could work really well… all I need is the cash to get it…

With this discovery I decided it was time to construct a larger, more sophisticated sensor. My first was a sphere constructed with two analog sensors using two stitched resistors and conductive felt.

First, I needed to make a pattern for the shape in which to stitch the resistors in. The question was how to create a 3d sphere out of a 2d map. This lead me to Buckminster Fuller’s Dymaxion map.

This wasn’t quite right and would not work to create a smooth shape. It then occurred to me that I should look up the pattern for a tennis ball/baseball. (click on About Penn, then The Virtual Factory)

Each resistor had a different value from each other.
one was 420 Ohm
the other 360 Ohms
put in series 780 Ohms.

Testing two resistors in parallel from lara grant on Vimeo.

Using a ball I previously needle felted, I began to wrap and needle felt my two resistors around the sphere.

Stitched resistors used in dual pressure sensor from lara grant on Vimeo.

The piece of conductive felt gets wrapped around the two resistors, I pinned the corners to hold them while I needle felt them place.
The ends of my threads on each resistor (2 for each one, 4 in total) became my leads. When wrapping the conductive felt around them, I needed to make sure to pull these leads out before securing it.

After I was done, I tested it, works well!
As of now, I am able to read one resistor, or both in series.

Dual pressure sensor – 1st Draft from lara grant on Vimeo.

Dual pressure sensor (in series) – 1st Draft from lara grant on Vimeo.