Digital and Analog#

e-textiles

I. About#

One of my personal ideals is that the best design is invisible. This is particularly true to technology – the best interfaces are intuitive. This week was about either a adding electronics to an already familiar object or adding the already familiar into electronics. My intention was to augment both the novel and familiar without imposing a new interaction or process on either medium.

II. Digital brooch#

Materials#

Process#

I used a heavy gray felt material as the base for this brooch. By cutting a small hole in the center I was able to insert the tip of an LED. The LED legs were bent into loops to allow conductive thread to bind. I then sewed on a battery holder and cut through a pin fastener. Pinning the pin, closes the fastener and completes the loop.

Results#

As a result, the brooch lights up when the fastener is closed.

III. Embroidered NFC antenna#

Materials#

Context#

I wanted to create a textile inductor that could be incorporated into an NFC tag. The idea was appealing to me because NFC tags are powered by the Radio Frequency of the device in their range. They do not require an additional power supply and NFC is a technology with abundant affordance – nearly all smart phones have the ability to read tags for example. Therefore NFC tags appealing for wearables.

While the tags themselves are pretty small, I wanted to produce the antenna in thread or integrate it with a textile more cohesively.

To generate an inductor with the right properties, I referenced this data sheet.

Depending on the shape of the inductor, the equations for the inductance changes. Here are some examples from the data sheet.

There’s a lot that goes into designing antennas – electrical engineers refer to antenna design as black magic. However in the case of an NFC tag, I’m only making an inductor. With that said it can still be complicated, depending on the application, target inductance would vary. There are some trade offs associated with the number or turns of the inductor, NFC chip type, and the distance over which the coupling should be successful. I won’t go into that but here are some resources: -Design of a Simple Structured NFC Loop Antenna for Mobile Phones Applications - NXP website has some videos on the topic: Antenna Design 1 and Antenna Design 2 For my coil I decided to aim for an inductance of 4 mH.

Production files#

Inductor spiral#

Inductor square#

Process#

Using Illustrator to generate spirals#

To generate the outlines for the coils, I used Illustrator that has a nifty spiral tool.

Because sewing squares is easier you may wish to make your inductor square. Here’s another quick video on how to do that

Sewing thread and wire onto cloth#

The gauge of the stainless steel thread I was using was too thick for my machine to use on top but it was fine when passed through the lower bobbin. The wire samples were produced by binding the wire to the cloth using a narrow boxy stitch.

LCR meter results#

In total I produced 4 spiral and 4 square inductors

Spiral samples The maximum frequency for the LCR meter I was using was 10kHz

Sample name c(mm) L (@ 10kHz) Q R (Ω)
thread #1 38 2.26 mH .0103 13.79
thread #2 18 .86 mH .009 6.02
Cu #1 38 1.31 mH .181 .46
Cu #2 18 .87 mH .167 .33

Square samples

Sample name d(mm) L (@ 10kHz) Q R (Ω)
wire #1 60 1.95 mH .32 .38
Cu 60 1.76 mH .01 .23
thread 60 1.34 mH 0.008 10.72
wire #2 70 4.55 mH 0.77 0.37

The main drawback of using thread for this application is that it has a higher resistance and thus a lower Q factor. This means that the inductor will not work as well as its Cu counterparts.

Soldering* After achieving a sample with a high enough inductance (Square – wire #2) I cut up an NFC tag and tried to solder the chip to my textile sample.

Results#

Unfortunately my integrated circuit didn’t work well. I am not sure whether it is due to poor connection. I also think my target inductance goal may have been off. In the Antenna Design 1](https://www.nxp.com/video/nfc-antenna-design-1-which-antenna-for-what-purpose:WHICH-ANTENNA-PURPOSE-VIDEO) video the narrator specified that the inductance goal should be somewhere between 1-3 mH.