Color Mixing LED Bracelet

This project can be a good introduction to electronic textiles (e-textiles) and how to build a basic circuit. It uses conductive thread and CNC cut conductive fabric and resistive plastic also known as Velostat. An RGB LED is sewn to the bird design as its eye and changes color when the three homemade pressure sensors are pressed.

After building this project you will hopefully have a better understanding of what a RGB LED and a basic circuit is. As well as picking up some essential techniques for building e-textile projects.

The bracelet uses a basic circuit which consists of a LED, power source, and a resistor. Let's go through and talk about how each of these components work and how they translate into a soft circuit.

This project swaps hard metal parts for soft metallized fabric and thread. Typically copper wires or metal traces on a circuit board connect all the components together. Metal is conductive, so it allows electricity to flow from the battery to the LED, causing it to light up. Instead of wire, this project uses conductive fabric. Connections on a circuit board between the wire and the components are made by soldering with metal alloys. Instead of using solder, we will be making these connections by sewing with conductive thread.

Before we talk about the resistor in the circuit, let's go over what it does and why it's needed. A battery provides a specified amount of voltage and current. The LED takes a certain amount of current and voltage to operate. If it gets too little, it won't light, if it gets too much, it will burn out or blow out (a minor explosion, finding an LED's limit can be quite a fun experiment). The resistor limits (resists) the electrical current to a safe level that won't burn the LED out.

The resistor in the above circuit diagram has a value of 220 Ohms, a standard value that works when powering a LED with a low voltage. LEDs usually operate with about 2.2 volts, depending on their color and manufacturer. This bracelet has three resistors, one for each color channel of the LED. Instead of being an off-the-shelf resistor, we will use thin sewable plastic called Velostat. It is impregnated with carbon which makes the plastic electrically conductive. It has a pretty high resistance when left alone, much higher than wire. When the electrical current runs through the plastic and pressure is applied, the amount of electricity let through changes, which is why the brightness of the LED changes.

Want to know more about how current, voltage and resistance relate to one another? I recommend reading more about it over at Sparkfun, they have a great intro to current, voltage, resistance and Ohm's Law.

If you don't already, it's helpful to know how an RGB LED works before making the bracelet.

RGB stands for Red, Green and Blue. There is one LED of each of these colors inside one RGB LED. There are two kinds of RGB LEDs to be aware of, common cathode (ground) and common anode (power). An LED is a diode meaning electricity flows through it one way. It will not light up if it is connected incorrectly. When a LED is common ground it means that all 3 channels share the fourth lead that should be connected to ground ( - side of battery). If it's a common anode (power) LED, the shared lead is connected to power (+ side of the battery). Once you know if you have common ground or power LED, you will also know which way to sew your battery on. To figure out what kind you have, you can do a quick test.

Test

Look for the longest lead, this will be the lead that hooks up to either ground or power. Separate one of the other leads out and touch one to power and the other to ground, if it doesn’t light up, switch the power and ground of the battery. Keep touching the other two leads to the battery to identify which pin is what color. The diagrams shown here don't represent all RGB LEDs, so it's good to identify the leads for yourself.

Common ground is more often found, however I am using a common power LED.

Color Mixing

The color produced by mixing light is called additive color. When the brightness level of one channel is mixed with another you get a different color. For example, if the green channel gets zero power, but an equal amount is going to the red and blue, you get purple. Brighten the red a bit more and you get a fuschia. Yellow is made by giving the red channel power and the green channel a little less. When all channel are on at the same brightness, white is produced.

This step is optional. I have attached the bird design for you to use so you can get to building. Alternatively, you can put a pencil to paper and create a different design to work with. Skip to the next step if you would like to use the bird.

There is a bottom design and a top design of the bracelet. Let’s start with the top.

While designing the top, there are 2 things to make sure of:

1) The final design is made up of 4 separate shapes. Each of these shapes will be connected electrically to different leads of the RGB LED. You do not want them to touch at any time, that will create a short!

A shape will be connected to the red, green, and blue channel of the LED, the fourth will be connected to power (or ground depending on the kind of LED).

2) The three shapes that will connect to the red, green, and blue channels of the LED need to extend towards one end of the bracelet. Put some space between the shapes and straighten them out. This gives you more surface to press on once the bracelet is put together.

Once satisfied with the design, scan it and open it in Illustrator or another CAD program that has draw and trace tools for illustration. Trace the image with the Pen tool in Illustrator so you can clean up and manipulate the path of the design as much as you like.

Bracelet Size

To help scale your design and make the right size bracelet measure your wrist with a soft measuring tape. Take that measurement and draw a rectangle that is your wrist measurement x 2.5”. Use this rectangle to fit the design in. Keep the outer edges of your wrist measurement marked, these two lines is where the bracelet closes. Add 1.25" on both ends for the bracelet closure. To close the bracelet, one end gets pulled through a slot at the opposite end. Mark the slot to be about 1.5" in length on the end where the resistive plastic will be sewn. For the end that goes through the slot, cut .625" from the edges towards the middle.

It helps to print out the bracelet pattern and your design on paper and try it on to make sure it looks as you want it to. This is also perfect for drawing in the battery holder, stitch lines and anything else that is part of your layout.

Download the attached .zip file, it contains all the cut files as jpegs and illustrator (.ai) files so you can alter the size and shape if you like.

Steam out the felt to get rid of wrinkles and creases. Lay the pattern down on the felt. Cut around the edges and mark the slot and tabs with pins or something that can be removed from the felt later on.

The conductive traces will be cut from two materials, paper and conductive fabric. You are welcome to omit the paper step, but it's helpful for placement when ironing on.

Cut Paper Design

Take some paper and smooth it on the Silhouette’s cutting mat. Load it in the machine and connect your computer to it via USB.

If you designed your own pattern, save it as an image file in your CAD software before importing it.

Import the previously downloaded jpegs into Silhouette Studio. Use the trace tool to select and trace the outline of each image. After the tracing is complete, switch to the cut settings and do some tests with some presets. Card stock should work well. When ready, cut the entire top and bottom pattern out. Check it with your bracelet pattern one last time, making sure the battery holder will hit both the top and bottom and the three channels. If it all looks good, move on to conductive fabric!

Cut Conductive Fabric

Cut a piece of fabric big enough for the designs. You can group the bird pattern and bottom trace together in Illustrator to save space if you like.

Smooth the fabric over the cutting mat, load and cut the final designs using the below settings.

Blade : 2

Speed : 8

Thickness : 14

Piece to cut:

- Bird pattern (top four traces)

- Bottom trace

For more details on this method and how to use the Silhouette Cameo, check out my CNC Conductive Fabric Circuit Instructable.

Otherwise known as Velostat.

Follow the previous steps to cut using the silhouette or you can be easily cut these by hand.

Take the paper cut outs and place them on the bracelet, tape them down onto the felt and try the bracelet on. Check to see if the bottom design overlaps and touches the top design along the three separate channels. One tab of the coin cell battery holder will be sewn to a top trace, the other tab will be sewn to the bottom.

If all looks good, start replacing the paper pieces with fabric, with the iron set to Medium heat.

Take the RGB LED, single out the common ground or power pin and bend it to a 90 degree angle. Take the other three pins and do the same. Use rounded needle nose pliers and curl up the ends of the pins towards the LED.

Place the LED on top of the design and arrange the pins so they touch their respective shapes, but remember they can not touch each other!

Sewing

Thread a needle with about 18” of conductive thread and make a knot at the end. Push the needle through the bottom of the bracelet piercing through the conductive fabric on the top. If the needle is not already through the curled looped leg you created, thread it through. Make several tight, small stitches going around the loop and through the conductive fabric and bracelet. Make at least 6 stitches. It’s important to make a good and secure connection between the LED lead and the conductive fabric trace. Finish by making 3 small stitches in the same place and cutting the remaining tail. Do this for the rest of the leads.

When done sewing, turn the bracelet around and clip off any remaining tails of thread. Then take a multimeter, put it on the continuity setting and test the connections. Do this by putting one probe on the an LED lead and the other on the trace it’s sewn to. If it beeps that means electricity is flowing through the connection and it is solid.

Test For Shorts

Next, test that the connections are NOT touching each other. Thread is hairy and one little stray strand that can’t be seen can create a short. Touch one probe to one connection and the other to its neighbor, if you get a beep, that means there is a short happening. You can take a piece of tape and use the sticky side to take off stray strands, or you can scrape between two connections to move away any strands that are hard to see. Continue to test all the connections until the short is gone.

Secure Thread

Use fabric glue or clear nail polish to secure the last stitches once you have confirmed there are no shorts.

Take the battery clip and identify which tab is the positive side and which is the negative. Match the tab with the trace and lay it on top. If you have a common anode (power) LED the positive tab gets sewn on top of the bird trace and the negative gets sewn to the bottom trace. If you have a comment cathode (ground) LED the negative tab gets sewn on top of the bird trace and the positive gets sewn to the bottom trace. Sew the tabs down making as many passes as you can until the needle can’t pass through the hole any longer. Knot off on the bottom and dab some glue on it.

At no time do you want the conductive fabric from the bottom trace to touch the conductive fabric on the top traces. Keeping this in mind, lay the Velostat over the conductive traces, covering the ends of the bird's tail feathers. The electricity must run through the plastic to the second piece of conductive fabric. If one piece of bread of the sandwich touches the other (the two conductive fabric pieces), the electricity will skip the meat (plastic) creating a short circuit.

Thread a needle with non-conductive thread and start sewing the plastic down. Use a basic running stitch and go around the edge once, then a second time, filling in the spaces between the stitches to produce a solid stitch line. You can also use a backstitch.

When building electrical circuits, the moment of truth is always when power is applied. It’s either going to work or not. I always count on it to not work, which seems pessimistic, but is likely true. It’s helpful to be ready with a list of things to troubleshoot if things do not work as expected.

Now is the time for your circuit’s moment of truth. Pop the coin cell into the holder and put the bracelet together. Press the bottom trace pads on top of the resistive channel pads. The LED should light up and change color depending on what color channels are being pressed the hardest.

If it is not working as expected, here are some things to try:

If the LED is not lighting up:

1. Check the connections again with a multimeter
2. Check for shorts with the multimeter
3. See if the LED’s power and ground legs are connected to the power and ground sides of the battery

If the LED is on but the color can not be changed:

1. The bottom and top traces are touching and bypassing the resistive plastic. Recut and/or reposition the plastic.

If only the red channel is lighting up:

1. Each color channel takes different amounts of voltage. The red typically takes around 2V and the blue and the green typically take more. Each battery is a little over it’s voltage, so the 3V battery we use will power the blue and green channels for some time just fine. But, when it gets too low it will only have enough juice to power the red and not the other two. Swap or recharge the battery and the blue and green channels will glow once more!

This bracelet is pretty on it's own but really shines when you stop to show it off by interacting with it. Enjoy your new skills in sewing circuits and feel free to ask me questions on techniques, materials or design in the comment section.