A gravity ligh t is a lig ht (an LED) that is pow ered for a us eful am ount of time by a slo wly fall falling ing object, ob ject, or mas s . In In the photo below, my mas ses are two jugs o f water, water, and I'm I'm us ing the LED as a reading li ght. Once Once the object has fallen all the way, way, you s imp ly lift it back up and the light is lit again. This was first invented invented by two fellows no w se lling it through a comp any called Deciwatt Deciwatt for for selling cheaply to to remo te villages to us e instead of kerosene lamps . I decided to try try making my own, and as you can see be low, I succeeded.
Deciwatt's gravity light.
My homemade gravity light Putting on the masses (two jugs of water.)
Ready Read y to turn out the house lights.
Gravity powered reading light.
The basic b asic principle of the gravity light As As s hown in i n the diag ram b elow, elo w, the whole id ea is to have the the m as s es fal l as s lowl y as p os s ible , taking taking as a s long l ong as pos sib le, while s till causi ng the generator to turn turn fast enough to power the LED light. With With the system bel ow, even even though the sm all s procket is turning s lowly, the outer edge of the large pull ey/wheel ey/wheel that it's it's attached attached to is turning fast. That's That's where the conversio conversio n from slo w to fas fas t is done.
Gravity light light calculations calcu lations The following are the calculations I did to figure out if it would work given the the things I had to work with.
The diagram above has Ar, Br and Cr added to it. These are the: rotational speed Ar of the sm all pulle y A with the gen erator attached, rotational speed Br of the large pulley B, and rotational speed Cr of the small sprocket C that's attached to the large pull ey. I started with a gen erator, in this cas e actually a motor taken from a m icrowave oven. When you turn the sha ft of a motor, the motor acts like a generator, producing electricity. I knew how fast I could turn it without damagin g it and that it would be enough to lig ht my LED. I had to turn it 1/2 turn each second. I had a sma ll pulley that I could attach to the generator shaft and its diam eter is 1 inch, which is 25.4 mm (mil lim eters). To get the circumference of the pulley you s imp ly mu ltiply the diam eter by the mathem atical constant pi, which is 3.14159.
As I said above, I needed to turn it only 1/2 turn each s econd . That's a distance of 1/2 the circumfere nce, so 1/2 of 80 mm i s 40 m m. And I needed to turn it that dis tance every 1 s econd. That gives me the desi red rotational sp eed of Ar as 40 mm/second.
Now that I knew I needed a rotational sp eed for the sm all pulle y of 40 mm /second (Ar), I needed to find s ome combin ation of large pulley with attached s mal l pulley (or sprocket in this case ), and pos si bly mo re than one of them that would cause a mas s to fall at a reaso nably slow s peed for long enough to light the LED for a reas onably long tim e. I had a bicycle wheel an d ma de m eas ureme nts of it to se e if it would work. The bicycle wheel, which is my large pulley (B), is conn ected by belt to the s mal l pulley (A). If the circumference of the sm all pull ey is moving at a speed of 40 mm /second then the circum ference of the large pulley is als o mo ving at 40 m m/s econd. SoBr is 40 mm/second. The sm all s procket (C) is p ysi cally connected to the large pull ey (B). A sin gle turn of the large pull ey resul ts in a s ingle turn of the small sprocket. The circumference of the large pulley is 20 10 mm and the circumference of the sm all s procket is 157 mm . Since one turn of the large pulley means one turn of the sm all s procket, that me ans wh en the large pulley goes 2010 m m, the sm all sprocket goes on ly 157 mm . The ratio of those two lengths i s 0.078.
Since the rotational spee d of the large pulley is 40 m m/s econd (Br), the rotational sp eed s peed of the smal l sprocket is 3.1 mm/second, i.e. Cr is 3.1 mm /second.
That mea ns that the mas s wil l be falling at 3.1 mm /second. If the distance it has to fall is 1 m eter, 1000 m m, then that will take 5.4 minutes.
Video - Gravity Light - Homemade/DIY, How to Make This video will be available Friday, April 18th. In it are s tep-by-step de tails for how to m ake the gravity light, as well as the sim ple addition of a counterweight that brought the run time from the 2 m inutes repo rted in the above video to 4 m inutes.
