Instructables
Picture of LED Pattern Hat
This is a (moderately) simple Arduino project that is a spectacular display from LEDs - that you can wear on your head! It also uses a program that is not intricate at all, so you can modify it for different patterns and cycles. It's also a great first or second project for people new to Arduino. And, of course, it looks great in the dark!

(I know the video isn't ideally situated; I'll try to fix that soon)
 
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Step 1: Parts List

Picture of Parts List
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You will need:

A hat (of course)
16 paperclips
24 LEDs for the version pictured - Radioshack sells a bundle of red, yellow and green LEDs, which is what I used x2
Jumper wires
Duct tape
9V battery
9V connector or plug
Electrical Tape
Fishing line or string
Very small breadboard
Arduino Uno, Leonardo, etc.

Step 2: Parallel vs. Series

Picture of Parallel vs. Series
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It might be simpler, but building the circuit with the LEDs in series (with the positive leads connected to the negative ones of the next; 5th picture) requires more voltage than the Arduino can provide. Two LEDs work, but for this design with three LEDs I used a parallel circuit. When connected in this way, like the in first four pictures, the Arduino can power at least four LEDs at once, although they are slightly dimmer.

Step 3: Construct the Ground Circuit

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Since the Arduino Uno has only 3 ground ports, you'll need a circuit that you can plug all 8 LED strips into. With a super-small breadboard (this one is 5x10), plug one wire into the ground pin on the Arduino and a row on the breadboard, and two more wires from that row into two other rows. 
Hackerstore2 months ago

Very nice project, it's is a real tinkering project. Good for dark nights in the winter, also because the over-heated ATMega will keep extra your warm :-)