This cap Instructable is a part of our larger Lumenhattio project, in which we're experimenting with ways to make stretchable glowing helmet covers that provides affordable, changeable style by day and safety light by night. We recently presented the Lumenhattio at the World Maker Faire, and a lot of folks suggested that this is also a great project for teaching kids about electricity! The cap requires only 5 inexpensive materials to make, and because of its low voltage power supply and the resistance of the conductive thread, you won't get zapped with electricity while you make this cap!
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Step 1: Materials

• 20 to 30 Piranha LEDs
with identical voltages (check part specs)
we tried:

• 20 feet of Conductive Thread
we tried:

• 1 Spandex Cap
available at sporting or beauty supply stores
we tried: Feel Beauty Supply, Brooklyn

• 1 Sewable Coin Cell Holder
we tried:

• 1 Coin Cell Battery
CR2032 220 mAh Lithium Ion
we tried:

• 1 Mid-Sized Needle

Step 3: (Preliminary) Examine the battery

Study the cell battery. The positive side is marked with a "+" sign, while the negative side is usually unmarked.

Step 4: (Preliminary) Determine LED polarity

Picture of (Preliminary) Determine LED polarity
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Determine the polarity of your LED by holding the battery between the LED’s two poles. Each side of the battery should touch the two prongs of each pole. Once the LED lights up, you’ll know which side of the LED is positive and which side is negative.

Step 5: (Preliminary) Understand the Battery Holder

Picture of (Preliminary) Understand the Battery Holder
Add the battery holder into your setup. Put the battery into the battery holder and use conductive thread to connect the battery holder to the LED. When the LED is correctly connected to the battery holder, the LED will light up.
IMPORTANT: the thread connecting the positive poles must not touch the thread connecting the negative poles.