This is my very first instructable! I have made a machine using a motor, some electronic components, gears and pulleys to very efficiently convert the kinetic energy from a persons hands into electricity. This can produce, at fastest speed, up to 12 volts, but it is controlled at 5 volts because I made it to only charge mobile devices. It has unlimited potential as more gears are added, and a bigger motor is used. It can also be connected to a bicycle or car wheel to produce amazingly high amounts of electricity. About 10 - 15 minutes of spinning would get your smart phone or tablet charged up to 40% - 50% if the product is built correctly. Good luck, and enjoy.
Step 1: Planning and Building
Sorry for the inconvenient format. All instructions are written in images. All the instructions are very clear and easy to follow.
Here is a video showing how it worked out at the end.