This is a replacement for the track clip pro, an actively illuminated headpiece for the trackir head tracking system, commonly used in flight simulations on PCs. BOM: 3xred or optimally ir leds 1xcathode current resistor (select appropriate value for your LEDs, i used a 180 ohm for the prototype) about 12" of wire a 9v battery with clip. some Velcro strips I designed this to be as small, light and simple as possible, it is intended to velcro to the top of a set of koss pro/4aa headphones. The lower led holes are left closed to help you fine tune their position, expect to ream the top hole to fit your particular LED's footprint.