Crafting Material Interfaces

Sam Jacoby

WHAT

Passive capacitive sensors can be made out of just about anything that holds charge. Paper, for it’s versatility, lightness, strength, and accessibility, is a natural space to explore. A capacitive paper sensor consists only of a conductive substrate, sandwiched between layers of conventional paperpulp–in this case, cotton.

MATERIALS

The sheet is formed from interwoven layers of cotton paper pulp and .25” chopped graphite fibers. The fibers are highly conductive and form a reliable capacitive surface. A lead, either of stainless-steel thread or adhesive copper tape is embedded in the carbon, to allow for convenient connections.



Processes

Paper is usually made out of plant fibers. Most need to be extensively processed before use, but pre-processed pulps are widely available. A vat of conditioned cotton pulp is agitated, suspending the pulp in water. A mould and deckle–traditional paper-making supplies–is dipped in the water, forming a thin layer of woven fiber over a fine screen. Once the water is drained, the resulting sheet is ‘couched’ (rhymes with smooched), on an absorbent surface. The conductive layer is formed from .25″ carbon fibers, which when wet and agitated, interface with each other in a way very similar to paper. A conductive strand, either steel thread or adhesive copper, is sandwiched between the layers, completing the sensor.

To create multiple capacitive panels, I made a divided deckle, shown here after immersion in the carbon-fiber pulp.

Four conductive carbon-fiber panels.

Finally, a conductive lead was embedded within the carbon, to provide for easy hook-ups to circuits and other sensors. This step is not strictly necessary, and naturally, modifies the overall capacitance of the panel, but I thought it would be useful (and it was!)

Adhesive copper tape embedded in carbon fiber.

TESTING

The capacitors were used in a simple circuit, connected to an Arduino. The CapSense library generated a signal, and the response was observed visually in a Processing script. The capacitance of several sheets was measured independently, using specialized lab gear for the task. For active capacitance, it spat out the happy values for a 4 x 5.5” square sheet:

• Side-by-side, 6pF
• Face-to-face, 40pF

Passive capacitance was not independently measured, which just seemed trick, though the effects of the changing capacitance in a relaxation oscillator circuit was observed on an oscilliscope.

Carbon-Fiber Capacitive Sensor Presentation

Carbon-Fiber Capacitive Sensor Datasheet