Conductive Fabric & Thread as Input
by Kate Hartman
Rather than looking at an existing sensor, I chose to explore some materials that can be used to create homemade sensors. Conductive fabric and thread is particularly exciting in the world of electronics because it allows us to move from hard interfaces to soft. Soft is inherently human - our bodies have no hard surfaces or sharp corners. Working with soft materials as input opens up a world of possibilities.
GENERAL COMMERCIAL & INDUSTRIAL APPLICATIONS:
EMF shielding - often used for creating homemade solutions for protecting against electromagnetic fields
Anti-static - used in carpeting and other commerical products to prevent static
Medical - certain types of silver conductive fabric are used for dressing wounds because of their minimal adhering and antimicrobial properties.
A LOOK AT SPECIFIC MATERIALS:
|Lame Lifesaver||lame repair||approximately 40 ohm/ft||$15 CDN / 200-yard spool|
|Bekitex BK50 (Bekaert)||originally designed for conductive backing of |
carpeting for computer rooms and offices
|approximately 500 ohm/ft||$30/1-lb cone|
|Bekinox VN (Bekaert)||Anti-static textiles|
Heat resistant sewing
|?||$36 - $244 / 1-lb cone|
Note: Berkaert is often willing to send samples. More info about their products can be found here.
|Shieldex Fabrics |
available from Fine Silver Products
|Bremen||Silver plated nylon fabric||Conductive fabric for general use||$20.97/yd|
|Kassel||Conductive fabric for general use||$32.07/yd|
|Koln||Conductive gasket skin||$23.90/yd|
|Nora Dell||Conductive fabric for general use||$35.62/yd|
|Tulle||Polyamid monofilament||theatrical drapery|
Sauquoit Industries is another producer of conductive fabrics. Also, Less EMF offers a variety of fabrics for sale. They are less expensive, but many of their more conductive fabrics are noted as being inappropriate for skin contact, so I've avoided using them.
USING FABRIC & THREAD AS SENSORS
DIGITAL: Here are some examples of soft switches. Below you can see a simple, independant system where switches trigger LEDs. In this situation, the two layers of conductive fabric are separated by mesh. This is a fairly resilient option. The switches require significant touch in order to be triggered. They are also not triggered by simple bending og the quilted piece.
Here we have a quilt which is intended for use in an assistive technology environment. The touch of the various patches trigger audio playback. Because the quilt is intended for someone with limited motor control, the switches are designed to be rather sensitive. Rather than separating the conductive fabric with mesh, a customized batting form is used. More information about this project can be found at www.qquilt.com.
ANALOG: As an analog sensor, I decided to follow the model of a resistor ladder. A line of thread zigzags across the sheet creating a total of 35 lines. Each line has a resistance of approximately 220 ohms. The sewn resistor has a total resistance of approximately 8K ohms.
A line of stiff, conductive fabric runs down the length of the resistor. It is sewn in such a way that it is suspended above the thread. When you press on it, the fabric lowers and creates a connection with the thread. Where you press determines the total resistance.
I powered it with a 3V coin cell battery and used it as a simple LED dimmer. However, the analog values coming out of this system could be used as a variety of applications. I envision a wall hanging that could adjust the lighting of room.
BRIDGING WORLDS: Techniques for moving from hard to soft
SNAPS: A great way to move from thread to your hard components is through the use of snaps. Sew one side of the snap to your soft surface with conductive thread. Then solder wire to the other side. This is also really handy because it makes your hard components easily removable, thus making your soft elements washable. But be sure not to use colored or coated snaps - this will obviously prevent the flow of electricity.
LOOPING: An effective way to make LEDS and resistors easier to sew is create loops out of their legs. Simply bend the leg so that it touches itself, solder it in place, and then sew.
FABRIC GLUE: Some types of conductive fabrics have a tendency to fray. This can be very problematic, as the tiniest thread connecting one side of the switch to the other can ruin the integrity of your switch. Painting the edges of your material with fabric glue is a very effective solution. Also, putting a dab of fabric glue on the knots of conductive thread prevent it from unraveling, making your circuit more durable in the long term.
BOBBIN LOADING: Some types of conductive thread are rather thick. Loading from the bobbin prevents unnecessary stress on the needle of your sewing machine.