GravityLight

A $5 Light For The Developing World With An Ingenious Fuel: Gravity – provides illumination similar to a kerosene lamp and runs for about half an hour, but it’s safer and free, especially when the alternative is no light.  Simply push the bag of sand up to start another half hour.

The GravityLight gets power from the slow lowering of a weight. All it takes is enough elbow grease to hoist the bag, and you can light a room with nothing but a bag of sand. …

It gets its energy from gravity: A 22-pound bag of sand that gradually cranks a gear-train attached to a D.C. motor.