Human energy

You're looking for 0.5W, and your title suggests that you want a human to make this.

Piezo's won't work because they will never produce 0.5W continuously. I guess you'd be looking into small DC-motors, preferrably with a gearbox so you can move the slow speed of humans to a high rpm for the motor (generating a higher voltage). It's how these work: Bresser Junior Dynamo Torch: Amazon.co.uk: Electronics .

By human energy, I meant that while walking, your legs move in pendulum motion, to and fro, so when it moves, attach something that could oscillatory motion and generate electricity. The medium which generates electricity is at the knee joint. Two things will be attached one to the thigh and one below knee. So while walking, the thigh portion remains stagnant and the bellow knee portion will move turning the medium and generating electricity.

Ben heck actually tried this in one of his episodes, simply by attaching the handle of one of these torches to a piece of wood and then someone's leg. I can't remember how affective it was, but I think he was able to charge a smartphone.

Nice idea. One of the issues you'll run into is that we, humans, are very easily getting irritated by objects that downgrade our own efficiency. We're very used at how our limbs work, and dampening our oscillating legs by draining energy is something that won't go unnoticed during walking. If you'd be able to brand it as 'exercising equipment' you might have a chance....

Nice idea. One of the issues you'll run into is that we, humans, are very easily getting irritated by objects that downgrade our own efficiency. We're very used at how our limbs work, and dampening our oscillating legs by draining energy is something that won't go unnoticed during walking. If you'd be able to brand it as 'exercising equipment' you might have a chance....

Any practical answers, how can I use it to charge phones, which motors to use or gearbox's gear ratio or any direct motor which can generate 5V without much effort?

You can do an inverse calculation: if you know the speed at which the knee will be travelling, you can make an estimate at the amount of power you'll want to drain, and then you can try to find a motor that does that power (0.5W?) at 5V+ DC, at that speed. Pololu sells a nice range of motors (25D and 37D series) that already have gearboxes. Because you'll probably need to do something for slightly lower speeds too, you'd better use a power converter to try to get maximum power. You can take a look at the Energy Harvesting Solutions group, where a few converters from Linear were used in combination with Wurth coils.