Electronic Kalimba

This simple controller is inspired by the south African kalimba. The kalimba lends itself to rapid assembly because of its use of a single central bar held down by two screws to trap the array of tines between two pivot points.

Wooden tines are used in this prototype because they are faster to shape than the traditional metal and this controller doesn’t require the tines to be tuned. The flexibility of copper tape is exploited as strips follow the contour of the flat base around the curve of a half-round pressure pivot. Each tine is covered in conductive copper tape. Trapped between this copper strip and the base strip is a piece of peizoresistive fabric. The rear pivot of the tines also has a copper strip which is a grounding bus for the tines. The 18f2550 on the controller board has 10 ADC’s and sends an OSC-encoded estimate of the voltage formed by a pull-up resistor and the variable resistance pressure sensor of each tine. Notice that the length of each base copper strip is trimmed to simplify the wiring flow of the conductors to the microcontroller.

The tines can be used in the traditional fashion to define the size of a “pluck”. Also they can be used as bend sensors after the pluck.

Conductive Paper and Conductive Plastic Slide Potentiometer

These variations on the slide potentiometer theme employ conductive carbon paper (from Pasco) or conductive plastic (velostat) and a giant paper clip. Like pots the value of this arrangements is the physical memory of the value. Note at the back how the potentiometer wiper is connected to a conductor. This is typically achieved in a conventional pot using a conductive strip next to the resistive strip. That approach is also possible. I found that the conductive tape (Laird) added friction which helps lock the slide in position.

Conductive Rubber Band Dual Touch Position Sensor

The rubber band is shorted to the copper tape conductor. This provides separate variable resistive paths between the copper and the bulldog clips at each end of the band. To save wiring pull up resistors I simply clamp with another bulldog clip in the middle of the band on the other side of the plexiglass.

Dual Touch Pad: multipoint touch with pressure

A pair of SlideWide sensors (http://infusionsystems.com) are stuck to each other at right angles.

Instead of measuring single touch position for each axis using the well-known potential divider method I modified the pads and wire them to ground the “wiper” contact and measure the two end point resistances to this ground node to estimate the position of the outer most touch point pair. This example also includes a sheet of piezoresistive fabric to measure a single pressure estimate. The SlideWide sensors flex sufficiently for a useful touch pressure range.

This duo pad was part of our public demonstration at Maker Faire 2007:

Dual Touch Position and Pressure Sensor

This design uses four layers: flexible conductive fabric, air gap, piezoresistive felt and conductive tape to implement a dual position sensor with pressure. The two dark areas next to the sensor are paper resistors for expediency.

Fabric Touch RGB LED Color Synthesizer

This color synthesizer uses finger pressure to control brightness of each color of an RGB Led array. It was designed to illustrate that fabric and organic conductors can handle high current levels with modulations of low resistance values and some geometric variations that exploit switching materials.

Piezoresistive Fabric and Conductive Fabric Tape Pressure Sensor Array

Eeontex fabric and Laird conductive tape with conductive adhesive arranged to create 3 independent pressure sensing points.

Tablo, new multitouch keyboard/drum controller integrating displacement and pressure

This controller with no direct antecedents was inspired by recently available conductive stretchable fabric and a curve known as the witch [sic] of Agnesi.

The fabric is stretched in an embroidery hoop and draped over an inverted circular bowl. A piece of conductive plastic cut in a special shape forms a corolla on the surface of the bowl. The tips of each petal are folded inside the bowl and taped with conducting adhesive copper tape. The microcontroller board measures the electrical resistances of these petals from their tip to a common center established with a conductor at the flat of the bowl. As the conductive stretchable fabric (the “calyx” to complete the flower analogy) is displaced towards the bowl it shorts out different lengths of each conductive plastic petal. The result is a circular array of nearly mass-less displacement sensors. The gesture-to-displacement relationship changes according to distance from the center of the bowl (variable “gearing)”. This allows for several different playing styles. One style, similar to hand drum technique, involves tapping the fabric surface directly onto the bowl with the fingers of one hand and leaning towards the other side of the bowl with the palm.

Another style involves both hands interacting from the outer hoop towards and around the base of the bowl.

You can put it on the ground and use it as multiple foot “pedals”. I might make it wireless and put it on a cake decorating stand.

Adding Pressure Sensitivity to Monome Keys

By cutting a small disk of piezoresistive fabric with a central hole I retrofitted a higher resistance range pressure sensor into the Sparkfun clone of a Monome pad.

THe Monome (http://monome.org) interfaces are square arrays of illuminated switches interfaced over USB using OSC messaging.

A large part of the desirability of this interface is the tactile quality of the buttons which comes from careful design of the silicone molding. Each button has a ring of conductive rubber attached to connect with a circular array of interdigitated contacts. The conductivity of this connection does change with pressure but the conductivity is so good it is hard to measure the change accurately.

Bobbin: Conductive-Thread Touch Controller

This bobbin is laced with conductive thread. The thread is pushed against the conductive tape on the interior. Up to two string depressions can be sensed independently. Thanks to Natalie Freed for constructing this prototype.

Touch Fabric

Fabric touch sensor demonstration with capacitive sensing on fabric conductors behind felt.

Uses a Qprox development board