This post deals with the gravity light. The one illustrated in this blog was invented 2013. It can be used off grid, so it can be used in developing countries and place which lack the supply of electrictiy. Normally they use petroleum lamps instead which has a bad effect on their health. But this product could help them out. It is financed my Shell.
The principle is quite simple and quite simlilar to how a mechanic clock works. A weight is connected to a stripe of plastic. As it is lifted above the ground the weight pulls down the plastic stripe. This movement is transformed with gear in a faster movement, which is used for moving the generator. The picture illustrates the process (a bit simplified)
Similar to the mechanic clock work the weight is at some point back at the ground and can not produce electricity anmore. Currently the time this LED produces light is 25 minutes. After that the weight has to be pulled back up so it can produce electricity again.