https://www.instructables.com/Magnetic-Rainbow-Tail-light-for-Bike-Helmet/
Magnetic Rainbow Tail-light for Bike Helmet : 18 Steps (with Pictures) - Instructables
Magnetic Rainbow Tail-light for Bike Helmet: I have Giant Compel bike helmet. It has a mysterious plastic piece on the back which I pried off to reveal this...
tail light for bike