A gravity light is a light (an LED) that is powered for a useful amount of time by a slowly falling object, or mass. In the photo below, my masses are two jugs of water, and I'm using the LED as a reading light. Once the object has fallen all the way, you simply lift it back up and the light is lit again.
This was first invented by two fellows now selling it through a company called Deciwatt for selling cheaply to remote villages to use instead of kerosene lamps.
I decided to try making my own, and as you can see below, I succeeded.
The basic principle of the gravity light
As shown in the diagram below, the whole idea is to have the masses fall as slowly as possible, taking as long as possible, while still causing the generator to turn fast enough to power the LED light. With the system below, even though the small sprocket is turning slowly, the outer edge of the large pulley/wheel that it's attached to is turning fast. That's where the conversion from slow to fast is done.
How long my gravity light runs for
With version 1 I get a run time of 8 minutes before the masses have fallen all the way but my drop height is only 2 1/2 feet/0.75 meters. If I lift the whole thing to the ceiling I would get a run time of 15 minutes. Plans are under way for one that runs even longer.
Gravity light calculations
The following are the calculations I did to figure out if it would work given the things I had to work with.
The diagram above has Ar, Br and Cr added to it. These are the:
I started with a generator, in this case actually a motor taken from a microwave oven. When you turn the shaft of a motor, the motor acts like a generator, producing electricity. I knew how fast I could turn it without damaging it and that it would be enough to light my LED. I had to turn it 1/2 turn each second.
I had a small pulley that I could attach to the generator shaft and its diameter is 1 inch, which is 25.4 mm (millimeters). To get the circumference of the pulley you simply multiply the diameter by the mathematical constant pi, which is 3.14159.
As I said above, I needed to turn it only 1/2 turn each second. That's a distance of 1/2 the circumference, so 1/2 of 80 mm is 40 mm. And I needed to turn it that distance every 1 second. That gives me the desired rotational speed of Ar as 40 mm/second.
Now that I knew I needed a rotational speed for the small pulley of 40 mm/second (Ar), I needed to find some combination of large pulley with attached small pulley (or sprocket in this case), and possibly more than one of them that would cause a mass to fall at a reasonably slow speed for long enough to light the LED for a reasonably long time.
I had a bicycle wheel and made measurements of it to see if it would work. The bicycle wheel, which is my large pulley (B), is connected by belt to the small pulley (A). If the circumference of the small pulley is moving at a speed of 40 mm/second then the circumference of the large pulley is also moving at 40 mm/second. So Br is 40 mm/second.
The small sprocket (C) is pysically connected to the large pulley (B). A single turn of the large pulley results in a single turn of the small sprocket.
The circumference of the large pulley is 2010 mm and the circumference of the small sprocket is 157 mm. Since one turn of the large pulley means one turn of the small sprocket, that means when the large pulley goes 2010 mm, the small sprocket goes only 157 mm. The ratio of those two lengths is 0.078.
Since the rotational speed of the large pulley is 40 mm/second (Br), the rotational speed speed of the small sprocket is 3.1 mm/second, i.e. Cr is 3.1 mm/second.
That means that the mass will be falling at 3.1 mm/second. If the distance it has to fall is 1 meter, 1000 mm, then that will take 5.4 minutes.
That means the calculated run time is 5.4 minutes, not taking into account losses, and assuming a reasonably sized mass can do it. As you'll see in the second video below on making the gravity light, I achieved an actual run time of 4 minutes using the bicycle wheel, 8 kilograms of mass and a 710 gram counterweight hanging from the other end of the chain.
Video - Gravity Light - a Homemade/DIY one (version 1)
This video gives an overview of the above homemade gravity light, the principle of how it works, and a demonstration of it in action.
Note that this video says the run time is 2 minutes but with the addition of a counterweight as shown in the next video below, along with a simple electrical circuit the run time is 8 minutes.
Here are step-by-step details for how I made the gravity light, as well as the simple addition of a counterweight that brought the run time from the 2 minutes reported in the above video to 4 minutes and the electrical circuit which brought it to 8 minutes. Since the microwave oven motor I'm using puts out AC, the circuit converts it to DC so that both halves of the cycle can be used. The capacitor smooths things out and the resistor gives the LEDs some protection. Note that 3 LEDs are now used. It's been suggested that using Schottky diodes in place of the 1N4002 diodes would be more efficient.