Consider that when you use a wireless charger, or touch a keycard to a reader, you’re using induction. This phenomenon allows a primary coil contained in a reader, charger, etc. to transfer power to your device — meaning charging without plugging something in, or buying items by placing your NFC-enabled card or phone near a compatible reader.
While these readers normally give off a minuscule amount of power, with the right antenna setup, it can be enough to power an LED. As shown in this Make: write-up and the video below, the antenna can actually take the form of thin (30 gauge) wire, wrapped in a circle tight enough to fit around your finger. Put the wrapped wire in a mold with epoxy, and you’ve got a secret light-up ring! Perhaps the biggest challenge here will be fitting your LED into a small enough space to fit in the ring. While a 3mm LED is used here, if you’re adventurous enough, you could attempt to use a surface-mount component to further reduce your footprint.
Other tricks shown include adding glitter, or even using a double-coil setup where different colored LEDs light up depending on which direction the ring faces. Seeing what kind of gadgets illuminate would certainly be amusing, and maybe there would even be some practical applications for such a device when troubleshooting.
For another inductive ring hack, check out this similar wearable project that instead of lighting up an LED, lets you make credit card payments!