Super simple, two part, low profile arc reactor made as an under the shirt Halloween costume.
Shows clearly through a black thermal and only gets better as it gets dark out.
Here is what you need:
1 battery pack (6V 4x AA)
4 3x8mm bolts (glue or tape would also work)
1 strap (I cut a road bike tire in half for this)
Tony Stark style facial hair
A pocket for the battery pack
-It's a good idea to test the LEDs first to make sure they all work.
-Bend the leads up and press the LEDs into the front part.
Note: If it is a tight fit with some of them then lightly sanding the edge of the LED makes this much easier.
-Tie every two LEDs together in series so they each get 3V.
-Solder all the pairs together in parallel.
-Attach a wire long enough to reach your pocket.
-Attach the back part and add a strap.
Adding a switch would be nice although not necessary.
I wore it diagonally so it didn't move up/down.