This a little case that holds a fiber-optic bundle, a standard 5mm LED, and a CR2032 coin cell battery. From Chapter 4: Fiber-Optic Fabric Scarf from our book Make It, Wear It: Wearable Electronics for Makers, Crafters, and Cosplayers. If you make one and post on social meda tag it with #makeitwearitbook so we can see it!