The arc reactor has two strands of 5 hi-brightness leds in 1206 package.
Power comes from two AA cells and a TPS61041 boost converter circuit.
This regulator uses a current sense resistor as the feedback signal so it
becomes a current source of ~30mAmps. The LED chains are paralleled
with two 22ohm resistors, so that each gets about 15mAmps.
Using a doublesided finepitch protoboard (1.27mm spaced holes), the circuit
takes less than a square cm, including the tiny on/off switch.
Truly a nerd, the real challenge was to sew the stuff onto the shirt. Of
course I had to make the mistake of using too small piece of cardboard, and
discover that I accidentally stitched back and front of the shirt together on
one of the loops.
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Scissors and superglue!