A keychain style white LED light, that runs off a SuperCapacitor, Lithium cell and micro-Dynamo.
The light itself is a thin Green plastic rectangle that has a membrane switch and a small lanyard chain attached to a strong nylon rope, as well as a small hole for the LED.
Normally the light is powered
from the Lithium watch battery, which should last a few hours with very good regulation.
When that runs out, the secondary power source comes into play:
The Lanyard when pulled will wind up a internal flywheel that will power the micro-generator and charge the super capacitor. Similar to how some flywheel wind up toy cars operate. This mode of operation will provide several minutes of operation. The flywheel may have to be pulled several times to get a full rotation speed, but at that point it will keep the capacitor charged until it slows down. The capacitor itself will supply a few minutes of slowly dimming light.
The casing itself is water resistant (the electronics are sealed against moisture, the mecanical parts are not. The plastic is a glow in the dark type.
The flywheel is really just a large neodymium magnet coated with teflon.