This version ran for 12 minutes on the smallest sprocket of the bicycle wheel and produced enough light to illuminate a book and some of the surroundings. Improvements over version 1 were:
Below is the circuit. It's almost the same as the one for gravity light v1 except the diodes are Schottky diodes instead of silicon diodes to have less loss. Also, I got rid of the resistor since I didn't think I needed it.
The lights were three of the LEDs (shown below) which I bought on ebay here. Each LED is actually made of multiple tiny LEDs. If you watch the Gravity light - Making video below carefully at 2:44 you'll see that only some of those tiny LEDs actually light up, though as the power is increased they light up very brightly. But the price was low enough for experimenting like this. They are 10 watt, 9-12 volts, pure white 800-900 lumens. A single LED (really is bunch of tiny LEDs) starts to turn on above around 8 volts.
Each has a positive (+) and a negative (-) side. I actually bought five of them but from experimenting I found that three connected in series (+ of one connected to the - of the next) gave the best light. That meant the whole series string doesn't turn on until the capacitor reaches 24 volts.
Three in parallel instead gave around the same light but that meant the capacitor would charge up to only 8 volts before the lights turned on. I wanted the capacitor to charge up more so I'd have more energy stored for when the masses reached the floor.
To mount them in the lamp, I made the DIY LED lightbulb shown below by mounting them on a breadboard and connecting that to a base I'd previously cut from a compact fluorescent lightbulb (CFL) for a joule thief project.
I measured both the current and voltage going into the 3 LEDs in series while turning the wheel and got around 2.7 milliamps at 24 volts. This means that the gravity light is not supplying anywhere near the LEDs rated 10 watts.
Power and efficiency calculations
These are calculations done to get some idea of the efficiency.
Amount of falling water: 15.3 liters/4 US gallons
1 liter of water = 1 kg (kilogram)
I measured the voltage and current while the lights were lit.
power = current x voltage = 0.0027 amps x 24 volts =
Comparing the two, (0.065 watts / 0.25 watts) x 100 = 26% efficiency. 26% is actually surprisingly good given what the losses must be in the bicycle wheel, the generator mounting bearings, the generator and the circuit.
Video - Gravity Light - a Homemade/DIY one (version 2)
This video gives an overview of the above homemade gravity light, the principle of how it works, and a demonstration of it in action.
Here are most of the step-by-step details for how I made this gravity light, and includes the improvements made over version 1.