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Playing around with a simple circuit idea with friends but none of us have really worked with circuits since college so we need a little guidance. The thought is to have a small generator (someone said a stepping generator) being turned by a wheel. Then we connect some sort of rectifier then to SOMETHING (the part we can't remember) that will pass current to about 8 LEDs but divert any excess current to either a battery or capacitor to store until the wheel stops being turned then the battery/capacitor will discharge to the LEDs.

Guys I know this is a very simple circuit so sorry I can't remember my electronics from college. Any assistance is appreciated.

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That sounds like any number of things, many of which aren't so simple. What do you want to do? Turn a crank, light LEDs? Or is it more complicated? –  Phil Frost Jan 18 '13 at 2:00
    
Mainly turn a crank, light LEDs. Figured that the crank turning would produce excess so thought I would store the overflow to then keep the LEDs lit until the stored energy is discharged through the LEDs. It's more novelty/toy for friends and amusement so not wanting to go too complex and expensive to build –  Andy Jan 18 '13 at 6:17
    
As @Phil says, to do this efficiently is not so simple. The regulation of a widely varying power source and charging part particularly. How long do you want the LEDs to be lit after the generator stops turning? Do you mind if it's pretty inefficient? –  Oli Glaser Jan 18 '13 at 10:39
    
Honestly I could care less how inefficient it is. It's just a novelty so my thought on the LEDs staying lit was just added to that. I figured there was a potential to overload the LEDs and so I was as much as anything looking for a way to give an outlet and not pop he LEDs. The circuit diagram Phil did is great I just will have to play and figure out how much power I'm generating from the wheel spinning to appropriately gauge the rest of the circuit. –  Andy Jan 18 '13 at 16:21

1 Answer 1

The simplest way to do this would be to simply put LEDs in series with the generator. There are DC generators that wouldn't require a rectifier, and in fact, any DC motor is also a DC generator. As the generator turns faster, the current through the LEDs increases, and the generator becomes harder to turn. Depending on the rating of the LEDs, the efficiency of the generator, and how hard you turn it, you may overpower the LEDs. Putting a resistor in series with them will dissipate some of the power as heat.

If you wanted the LEDs to continue glowing after the generator is no longer turning, you could add to this circuit a capacitor in parallel. You will want a resistor in series with the LEDs so that the LEDs don't clamp the voltage across the capacitor. As you turn the generator the LEDs will glow. Also, current will flow through the capacitor to keep its voltage equal to the voltage generated by the generator, and in doing so, energy will be stored. When you stop turning the generator, this voltage will persist, which will light the LEDs and also keep the generator spinning, until the capacitor has no energy left. I haven't done the math, but intuition says you will need the biggest capacitor you can find.

To prevent the capacitor from turning the generator, thus reserving more of its energy for the LEDs, you can put a diode in series with the motor. You could even make it an LED, which is a kind of diode, to visualize the flow of electricity. Here's the circuit so far:

schematic

It's hardly efficient, but it is simple.

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