Sew A Circuit! This project includes the design and construction of a LED bracelet. The bracelet is in a “sandwich” structure to safely embed a circuit in the e-textile. The engineering goals of the project are electrical insulation, user comfort, maintainability, and gaining experience with conductive thread.
The bracelet structure: a cotton top layer, felt middle layer containing the sewn LED circuit, and a cotton bottom layer. Felt is used as the circuit’s substrate because it is a natural insulator, prevents conductive thread from shifting, and provides stability while still allowing the circuit to bend. The cotton layers protect the circuit from friction, moisture, and skin, ensuring safety and comfort while worn.
To support easy maintenance, the bracelet is sewn shut on three sides, leaving a short side open to create a pocket. This pocket allows the circuit layer to be inserted/removed for battery change and switching the circuit on and off. A button closure near the pocket connects the bracelet to the user’s wrist and closes the fourth side, preventing the circuit layer from sliding out.
This document covers the construction process and design considerations for the bracelet and circuit.
Circuit
- Sewable battery holder (LilyPad)
- Lilypad LEDs (x3)
- Conductive thread (stainless steel)
- Felt
- Multimeter
- CR2032 battery (3V)
- Jumper wires
- Clear non-gel nail polish
- Sewing pins
Bracelet
- Cotton fabric (x2) (3in x 8.75in) customize size
- Button
- Sewing machine and sewing accessories
- Needle
- Thread
General
- Sewing pins
- Fabric scissors
- Seam ripper
Goal: Test that the components work.
- For testing, I used jumper wires to connect the battery holder to an LED
- The battery holder has an on/off switch
Goal: Find out the number of LEDs that can be used in parallel while having the brightest intensity of light.
- I found that 3 LEDs could be powered and still be very bright
- I hand-sewed the battery pack to Felt material. (Felt is a good insulator)
- I hand-sewed the positive terminals together
- Then sewed the negative terminals together, to ensure the battery pack was stable and not going to shift around during testing
- Used the running stitch
Constraints: I decided to use a yellow cotton fabric for the top layer of the bracelet and a dark blue cotton fabric for the bottom layer.
Thought Process:
- Yellow is a lighter color and will allow more of the LED's emitted light to pass through
- Dark blue will block the emitted light from going through
- I wanted to use 3 different colors
Decision:
- I had a pack of LilyPad LEDs: Pink, Red, White, Blue, Green, Yellow, Orange
- I found the Yellow, Red, and Green LEDs had a ‘bright intensity’ of emitted light shown, didn’t drown each other out while covered by the yellow fabric, and I liked the combination
- Reminded me of a stop light
These were different testing interactions for deciding which LEDs to use. Like the previous step, sewing pins keep the LEDs in place for testing.
Constraints: Battery pack and LEDs to be where the arm is 'flat' to limit poor connections caused by bending of the fabric and thread.
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Threaded path to be relatively short to minimize the added resistance that comes with the wire's length
- The longer the conductive thread is, the more resistance the thread adds to the circuit
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The path of the circuit is a battery pack and 3 LEDs in parallel
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I cut the felt to be long enough to cover my wrist while testing positions of the LEDs
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Used sewing pins to test the position of the LEDs and keep the felt around my wrist
Goal: Gain experience with SEWING A CIRCUIT!!
Circuit: Battery with 3 LEDs in parallel
Implementation:
I used stainless steel conductive thread as the wire between electrical components. I hand-sewed the components on the felt by...
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Running Stitch
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Sewed a path of positive battery terminals to all of the positive terminals of the LEDs
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Then again, with negative battery terminals to each of the LEDs' negative terminals
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I did an LED at a time. Connected + and - terminals of the battery to the green LED
- Tested that the battery could turn on the LED
- Connected green LED terminals to the corresponding yellow LED terminal, and then yellow to red LED terminals
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At a terminal pad, I made 3+ loops to secure the connection
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If the thread was not long enough to make it to the next component and make the loops
- I tied it off and started the new thread by making some more loops at the current LED to be able to pass the signal to the next part
Notes:
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Power should be OFF while sewing the circuit
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Thread needs to be tight! If there is slack in the thread, that could cause issues with the signal
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It’s important to cut the leads short on the knots to prevent short circuits
Improvement:
- Test the quality of connections by seeing how LEDs react to bending the fabric more, and whether LEDs flicker
Goal: Prevent fraying and short circuits
Conductive Thread is a non-insulated wire. To prevent thread fraying and breaking the electrical connection, I coated the thread with clear non-gel nail polish in three layers.
- I tested that the circuit worked as expected after the first and third coats -> no issues
Goal: Gaining experience with a sewing machine
- Tip: Cut the fabric with right sides touching so they are the same size
- Straight stitch with a sewing machine, sew along 3 sides, leaving a small side open
- Right sides of the fabrics touching
- Flip the material inside out
- Right side of the fabric on the outside
- Seams are on the inside
- On the open side, fold inward enough fabric to make a hem (x < 1 in)
- Sew it down with the sewing machine
- (This prevents loose strands from preventing the felt’s movement in and out of the opening)
- Mark with a writing utensil where the button hole and closure should go
- Use the sewing machine’s buttonhole settings to form the buttonhole enclosure over both fabric layers
- Use a seam ripper to make the buttonhole in the enclosure
At first I sewed the bottom bracelet. However, it was not wide enough to easily slide the circuit inside.
- I cut the length and width of the felt, which helped get the circuit through the opening
- When the circuit was inside, the seam allowance blocked some of the emitted light
- (Seam allowance is the fabric between the end of the fabric and stitching)
I decided to resew the bracelet to be bigger (3 in x 8.75 in).
- Tip: You might want to customize the size of the bracelet to be a better fit for you
- The seam allowance was no longer in the way of the emitted light
- Easier to place the circuit inside it
- LEDs were no longer tightly pressed against the yellow fabric when it was around my wrist
Circuit in the Original Bracelet.
Circuit in the Final Bracelet.
The final bracelet was comfortable. It was easy to access the circuit through the opening when the bracelet was unbuttoned. The circuit is protected when in the bracelet. Occasionally, the red LED flickers; tapping it usually fixes the issue. It was a great introduction to wearable electronics.
Personal Goal: Introduction to e-textiles. Making a bracelet with an LED circuit inside. This is my first time sewing a circuit or using LilyPad components. I have done more hand-sewing than machine sewing; I would like to improve my ability to use a sewing machine.
When checked with the multimeter:
- Battery voltage: 2.8V
- LEDs when lighted: ~2.3 - 2.4V
- How to replace a needle on a sewing machine
- Get the sewing machine’s needle unstuck from going too far near the bobbin
- How to sew a buttonhole with the sewing machine
- Experience with conductive thread and problem solving: insulation and ensuring an electronic connection
- Before cutting the conductive thread, do more testing of bending the felt while the circuit is on to be sure the connection is stable. (test for flickering)
- Markdown formatting