This is a little project I made to charge up my iPod using a toy steam engine, though you could use it to charge any USB device.
I coupled a Lego Technic Motor to a Jensen #75 steam engine to make a crude generator. From there I built a 5V regulator circuit and soldered in a female USB connection to power any USB device. Since I wanted to use it to charge my iPod, I put in a diode and a .5 amp fuse to provide some circuit protection.
This is a good approximation of power is normally generated. Also, this is a good DIY power generation project, though its not very practical.
Step 1: Obtain a Steam Engine
The first step is to get a steam engine. you can buy kits or complete engines from ministeam.com. Some of them can get pretty expensive though. For this project I used a Jensen #75 engine though others would probably work too. I tried to get an estimate of how many watts a little engine like this puts out, but even the manufactuers didn't have a good idea. Based on the amount of output power and some efficiency estimates, I guess about 10 watts.
Step 2: Couple the Lego Motor and Flywheel
This was one of the hardest steps. I experimented with wood and other ideas but eventually it turned out to be the simplest way. I just used a big lego 'plate' and jammed it beneath the little stand that the engine sits. The best part is that its adjustable, just move the red piece to change how far away the lego motor is from the flywheel.
I experimented with a couple different ways to couple the flywheel with the lego motor. However, just using a rubber band worked fine.
Step 3: Make USB Voltage Regulator
This was the most intensive step, you need to be able to solder to complete this. The circuit diagram is attached below. I had a spare UV LED so I used that, though any LED will work to give you indication that your circuit is working.
Since I was going to use this on sensitive electronics, I added several capacitors on the input. The data sheet recommends .33uF but I just used 3 .1uF caps in parallel to achieve this. The output also has a cap on it to smooth the output. I found the output is a rock steady 5.025V. I also added a diode at the very beginning to ensure the iPod battery doesn't try to turn the motor. Additionally, there is a 1/2 amp fuse since the max rating of USB devices are 500mA.