Ben's engagement ring is awesome not only because it lights up, but because it lights up when he and his girlfriend hold hands:
Putting a battery of capacitor inside a ring is nigh impossible, so [Ben] decided to power the LEDs with an inductive charging circuit. A coil of wire wound around kapton tape serves as the inductor and a small SMD capacitor powers three very bright and very tiny LEDs.
The inductive charging unit itself is a masterpiece of hackery; [Ben] wanted the ring to light up whenever he and his ladyfriend were holding hands. To do this, [Ben]‘s inductive charger is also a wearable device: a large coil of wire is the charger’s transformer and was would to fit around [Ben]‘s wrist. The entire charging circuit can be easily hidden under a jacket sleeve, making for a nearly magical light-up ring.
You can watch a video of the ring at the link.