Signal Sling


Code @ GitHub

Edited @ 10:30 p.m. to add photos and GitHub link.

The Signal Sling is practical wearable for cyclists that places a pair of illuminated turn signals near the rider’s left shoulder. Using stretch-variable resistant fabrics, the devices tracks flexion at the elbow and armpit to determine which hand signal is being made.

Who is the intended user?

I built this wearable for any cyclist that would like to improve their safety in poor visibility. This came from a recurrent observation that dark roads and trails make hand signals invisible to pedestrians and motorists.

What is the intended application?

To improve the visibility of hand signals at night by mimicking motor vehicle turn signals, thereby improving rider safety.

Where does the interface live?

Primarily on the cyclist’s left arm, which is conventionally used for hand signals. The wearable is affixed over the cyclist’s clothing, and uses adjustable nylon straps to accommodate a range of garment configurations.

What is the scale of the interface?

The interface stretches across the chest and shoulder of the cyclist, descending down to the left arm. Because it leverages the movement of the whole arm, this wearable can be cosnidered an upper-body interface.

What is the material palette?

The harness is primarily constructed from 1″ polypropylene straps, stitched together with all-purpose sewing thread. The shoulder, elbow and wrist cuffs are tightened and loosed with plastic strap adjusters, and the chest strap uses a plastic buckle for affixing and release.

The elbow and shoulder sensors use 1″ by 2″ patches of Eeonyx’s stretch-variable resistant fabric. The illuminated signals use 5mm LEDs, switched with TIP120 transistors and a 9V battery. All electrical connections are made with conductive thread.

img_9495 img_9496
Left-hand and right-hand turn signals.

When the arm is lowered, the stretch sensor relaxes. This makes it more resistant, lowering the signal that enters the microcontroller.


When the arm is raised, the sensor is stretched, changing the signal sent to the microcontroller.


Another stretch sensor on the back of the elbow behaves similarly.

What is the response mechanism?

When the cyclist makes a left-hand turn gesture (left arm stretched straight left), an array of LEDs on the back of the cyclist’s left arm blink on and off. When the cyclist makes a right-hand turn gesture (left arm out with forearm up), an array of LEDs near the cyclist’s left shoulder blink on and off.

How does that affect how the user interacts with it?

In practice, the device should be nearly invisible to the cyclist—it simply leverages actions the rider would generally be doing anyway.

How does sensor data map to the response mechanism?

The stretch resistors trigger a simple binary switch: once the appropriate pin registers signal past a certain threshold, the microcontroller enters a corresponding blink state that powers the relevant turn signal.

How many modes of interaction are there?

Realistically, just signalling turns.

What type of switches and sensors does it use?

The harness does not use any switches, instead relying on two stretch sensors to track the user’s body.

What attitude are you hoping to incite in your users?

I’d like think of this kind of device as one that complements things you were going to do anyway—one that makes no special demands of user, and generally tries to keep out of the way. For this reason, I targeted a factor that would be easy to stow and flexible across clothing configurations.

Findings and discussion

This project taught me several lessons that I hope to apply going forward.

First off, I discovered that transitioning from a breadboard prototype to a textile is something of a big leap. There are so many extra considerations one must make: strain relief on the rigid pins of standard electric components, spacing to prevent shorts between conductive threads, careful routing of wires—in my next project, I may consider adding an extra step between the breadboard and the final deliverable just to iron out these kinks.

I found that choosing a position for the board and battery is really important and needs to be considered very early in the design process. In my case, I ended up hiding my board on the outside of the harness near the armpit, as the relative stability of the chest reduces strain on the board. This proved less than ideal—in a practical application like this, the board would be exposed to dust and moisture.

By chance, I discovered that one could layer several traces on the same area by sandwiching fabric between layers stithed with conductive thread. I hope to employ this technique in future experiments.


Sketches regarding where and how to affix stretch sensors. Before I landed on nylon straps, I also explored using elastic textiles and repurposing a single-use poncho.

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