GravityLight
GravityLight is a gravity-powered lamp designed by the company Deciwatt for use in developing or third-world nations, as a replacement for kerosene lamps. It uses a bag filled with rocks or earth, attached to a cord, which slowly descends similar to the weight drive in a cuckoo clock. This action powers the light for up to thirty minutes.
There are no operating costs after the initial purchase of the appliance. A standard GravityLight kit comes with an adjustable lamp and a ballast bag. The light can be turned on by filling the bag with approximately 20 pounds weight (10 kg) and lifting it up to the base of the device; the weight falls over a period of 25 minutes, pulling a strap that spins gears and drives an electric generator, which continuously powers an LED. This creates enough energy to last 25 minutes whenever it is needed.
GravityLight was called one of The 25 Best Inventions of the Year 2013 by Time Magazine.
Innovator: Martin Riddiford's Gravity-Powered Lamp 14.March.2013
Source: Wikipedia