The GravityLight is an LED unit that uses weights and a pulley system to power a drive sprocket. A polymer gear train turns this input into a high speed, low-torque output that drives a DC generator at thousands of rotations per minute, generating about a tenth of a watt of power to an onboard LED and ancillary devices.
Rendering: GravityLight.org

Light in the Absence of an Electrical Grid Twenty percent (1,476,800,000) of the world's population (7.384 billion) are not on an electrical grid. The lighting option for most of these people is the use of biofuel, such as kerosene, to make a crude "lamp." This is often done simply by pouring kerosene into most any old vessel (can, bottle, etc.), placing some sort of wick in it and lighting it.

One source asserts that such a crude and simple lamp was first devised in the 9th century in Bagdad using mineral oil.

Such a kerosene light produces a weak light, but is also a smelly and unhealthy to breath lighting source. And, of course, knocking over a kerosene lamp can cause an instant fire.

Clearly people without access to electricity need a better solution than kerosene lamps.

A company called GravityLight has come up with an ingenious solution that requires no biofuel, but rather turns potential energy into kinetic energy to generate enough power to light an LED.

Here's how it works. An LED light unit (GravityLight GL01) is connected to a strap that is attached to a ceiling. A nylon bag with two metal rings is filled with rocks (17.6 lbs to 26 lbs). A hook connects the rings, which are connected to a nylon strap, which is threaded through the base of the GravityLight housing and back out to form a pulley. A smaller weighted bag is attached the other pulley strap, then the heavier bag is lifted close to the GravityLight. The heavier bag slowly (1mm/sec.) begins its 6 ft. drop. This movement powers a drive sprocket, which rotates very slowly with high torque (force). A polymer gear train running through the product turns this input into a high speed, low-torque output that drives a DC generator at thousands of rotations per minute. This generates just under a tenth of a watt (deciwatt) to power an onboard LED and ancillary devices. Once the larger weighted bag reaches the floor it is simply lifted to repeat the process.

The GravityLight reportedly produces a light over 5 times brighter than a typical open-wick kerosene lamp.

This single light at ceiling height is great, but two other LEDs ("Satlite") with cables and connections can be added to this system. One Satlite plugs directly into the GravityLight, and its cable length allows it to be positioned where additional light is needed; the second Satlite is connected to the top of the other Satlite, and its cable allows placement to yet another location, such as at reading level.

You can view two videos on the lighting system at https://gravitylight.org/how-it-works/.