Knit a Working Circuit Board

For this instructable, I'll demonstrate how to knit circuitry, using a simple LED circuit as an example. The instructions assume you already know how to knit, solder, and wire up LEDs. (If you need to brush up on those skills, there are great tutorials all over the web.)

Introduction

After discovering e-textiles through websites like Kobakant's How To Get What You Want, I became fascinated with textile methods for electronics, and started experimenting with the circuit possibilities in knitting and knitting machines.

Eventually I developed a method to "print" circuit boards on my knitting machine, with materials that are inexpensive, easily available, and solderable. The method works with both traditional electronic components and with e-textile components. And while I use a knitting machine for rapid production, the materials should work fine for hand knitters.

Disclaimer #1: Wearables made with this method are not intended for rough handling. Prom, yes. Soccer, no.

Disclaimer #2: Solder is toxic. Don't wear the circuit against bare skin. Wash your hands after handling. Wear goggles when working.

Disclaimer #3: Some readers have expressed concern about lead from the solder leaching through fabric. I'm not an expert, but I suspect this would be an issue with wet fabric, not dry fabric. So keep your circuits dry -- avoid rain, spills, and contact with sweat. [Added 7/14/14]

Materials

knitting:

2-3 spools of bus wire (1/4lb size)

4-ply cotton yarn (fingering weight)

circuit:

one battery with case

LEDs

Tools

Your regular toolkits for knitting and for soldering

Plus....

Wire Dispensing Rack - A horizontal dowel that can hold the spools of wire. I use a craft paper dispenser. Whatever it is, the dowel should fit through the holes in the wire spools.

Nylon Stocking - a spare you can cut-up. Child-sized fits the wire spools well.

Bus Wire

I had experimented with magnet wire, but soldering once it's knitted is difficult (even the "solderable" kind). Then I realized I could get bus wire in a knittable gauge-- perfect!

Bus wire is what makes circuit knitting fast and easy. It's uninsulated copper wire that is tin-coated and ready to solder. A 1/4 spool of 36AWG holds several thousand feet and costs about $11. Bus wire is designed for use in circuits, so it's an ideal conductor- it has negligible resistance and soaks up solder like a sponge. Knitting multiple strands adds strength and means that even if one strand does break, the other strands will conduct around the break. For knitting, 1/4 lb spools are preferable. Larger spools are heavy; it's hard to pull the wire off without breaking it. I suggest knitting with 2-3 strands held-together-as-one of 34-36 AWG bus wire.

Cotton Yarn

Knitted circuits require a yarn that can tolerate soldering heat (over 300C). I tested a variety of fibers and found 100% cotton works well. Synthetic yarns like acrylic will simply melt. (And soldering wool creates the smell of burning hair.) Soldering may leave scorch marks on light-colored yarns. If this happens, try wetting the yarn before soldering, then wait for the yarn to dry completely before powering the circuit.

I use fingering-weight mercerized cotton yarn, with a gauge of 8+ stitches per inch.

A big challenge of knitting with wire is maintaining tension on the wire, to keep it free of kinks.

You'll need to put the wire on a dispenser you can pull it from. This should consist of a horizontal rod that the spools can spin easily on (I use a kids' tabletop craft paper dispenser). The spools should both be wound the same direction. Cut a few bands of nylon from the stocking. Place one around each spool, threading the end of the wire through the nylon. This will act as a brake on the wire unspooling.

(For machine knitting, I stretch the nylon over the tension mast take-up unit. With this setup, the wire would occasionally unspool and get wrapped around the rod. I solved the problem by cutting out cardboard circles and taping them on the rod at the ends of each spool.)

The circuit pattern is knitted as a 2-strand multi-color (aka fairisle knitting).

Here's an image of a basic pattern (created with Tricksy Knitting's free chart generator)

gray = bus wire stitches

white = cotton yarn stitches

Each row with wire acts as a single electrical node (like a single row on a breadboard or stripboard). Thus, it's very important that each row of wire does not touch another row of wire. I use the pattern of alternating cotton stitches with wire stitches to add strength to the wire rows. The pattern also makes it easy to line up components neatly.

If you want to add more cotton stitches between the wire stitches, keep in mind that the wire "floats" that form behind the cotton stitches will get longer and could potentially short out by touching another row. The same applies to the number of cotton rows between the wire/cotton rows. I find the two-row gap of cotton stitches to be great for LEDs, but if I'm working with larger components, like transistors, I need a 4-row gap.

The best number of cotton rows and stitches will depend on your gauge and your circuit, of course, so experiment. The pattern can be embedded in any knitted object you like. I suggest embedding it in a place where the fabric will not be sharply bent back and forth, repeatedly. Once you've blocked the knitting and bound off open stitches, you're ready to do the electronics.

For soldering I use a Hakko 888 iron with a chisel tip. I highly recommend a variable temperature soldering iron for this project. If you find soldering difficult, you probably need higher-quality tools. (Using my Hakko is like painting with silver. And so much fun!) Don't forget to wear safety goggles.

Lay the knitted piece flat, right side up. With the soldering iron, heat up the wire stitches, and apply solder. These will serve as pads for connecting the components. Bus wire is designed for soldering, so this should be very easy. Make sure you tin the pads on the ends of each row. For these end pads, the solder should hold the wire stitches firmly in place.

Next, flip the piece over (purl side up) and use flush cutters to cut the "floats" of wire running between each row. Also cut any extra loose wires that may be sticking out. Be thorough. A single wire touching another row can short the whole circuit.

Flip the piece right-side up and lay an LED down on top of the pads.
Make sure the positive lead of each LED goes to the top row, and the negative goes to the bottom row.

I like axial lead SMD LEDs. They're already flat, they have a low profile, and their small size (less than 2mm) makes them perfect for wearables.

If you use the more common 5mm LEDs, you'll need to bend the leads. Use round nose pliers and gently bend each lead out so the LED can sit flat on the fabric.

With one hand, hold the LED in place with the pliers.

With the other, use the soldering iron tip to heat up the LED lead. After a second or two, it should melt the solder underneath and sink into the pad. You may need to push the lead down with a tool while the solder cools and hardens. Remember that too much heat will fry your LED, so practice soldering first, and learn to work fast.

**Good solder technique -- clean your tip frequently, and be sure to wet it each time with a bit of solder.**

***Round nose pliers (the kind used for jewelry making) are fantastic for bending your component leads into shape.

After you solder each LED on, check to make sure it works in the circuit: attach the battery to the rows with test leads (positive side goes to the top row, negative to bottom).

If the LED won't turn on:

• Reverse the battery leads-- if it turns on then, you've connected the LED backwards. De-solder and try again.
• You have a cold joint -- the solder didn't melt into the wire/LED. Try reheating.
• Still doesn't work? It may be dead. Try a new LED.

Once all of your LEDs are soldered in, cut the leads short with the flush cutters.

Now it's time to add power.

You can solder your battery leads directly to the two rows, pop in your battery, and presto! Light!

However, it's very useful to add a switch. So I changed the pattern to make it easier.

The gap creates a long float of bus wire, which is cut. A sliding switch is then laid on top and soldered in place. Now, when the switch is closed, it bridges the gap.

The coin cell battery leads are bent outwards and soldered to the knitted wire. The positive lead goes to the top row and the negative lead goes to the bottom. If the battery case is wider than the gap between the two bus rows, solder it diagonally.

**My example circuit uses 5 yellow axial leds, wired in parallel. Since I used LEDs with a forward voltage of about 3V, and a 3V battery, I didn't need to add resistors to this circuit.**

You shouldn't wear the circuit directly against your skin-- wires on the back can scratch, and solder contains poisonous lead. For a lining, you could sew the knit to a felt backing or use an iron-on backing.

In the first picture, I applied my iron-on backing before I soldered, but I didn't cover the floats, so I could cut them later.

I also used masking tape as a "temporary" backing for one piece.

Once you get the hang of it, it's fun to turn "traditional" circuit designs into knitted circuit designs.

I use this sheet to plan my circuits.

The knitted fabric is like stripboard, and each row is a track. Planning out the circuit isn't so different from planning a stripboard circuit.

Once I've drawn in where the components will go and marked the "track" breaks, it's easy to use image software to create a finished pattern.

That sketch became the basis for this circuit. (In the first 3 pictures, I put in the components for part of the circuit, and photographed the back where I snipped the long floats for that part.)

I hope this gives you some ideas. I'm working on patterns to share and would love to try out your designs as well!

(Check out the videos for extra-blinky action!)

To see more of my knitted circuit projects, visit my blog.