We all know that we exert energy while doing exercise, and riding a bicycle is no exception. We also know that electricity can be generated by spinning a turbine with the power of wind or the rush of falling water — so why can't electric power be produced by the spinning of a "turbine" when riding a bicycle?
This is an idea proposed by many a gadget hacker out there, including Doug Costlow, who has worked to perfect a bicycle-powered electric generator over three of his Instructables projects, that can charge 2 AA batteries, power bicycle lights — or even recharge your iPhone or other mobile device via USB connection.
His DIY project is simple enough, if you know your way around basic circuitry; he pulled it off with parts and tools from a hardware store, a Radio Shack, and an old printer he bought at a Goodwill thrift store. Using a circuit board from the printer, he was able to build a circuit with wires and a soldering gun, and then connect it to a stepper motor (also from the printer), before attaching it to a Tamiya wheel.
The small wheel is mounted to the frame and makes contact with the bike's back wheel, so that it can spin with the rotation of the bicycle — which captures the energy and eventually transforms it into electricity. The power is harnessed and can outputted a number of ways, including a USB connector mounted to the seat post or extended out to a bike rack.
Of course this is a very simplified version of what it takes to make your own DIY Bike Charger. All the steps can be found by following Costlow's first Bike Generator project, then his second, and his third — each version an enhancement of the previous. And if you follow all his steps, who knows — maybe you'll be able to power your dead iPhone if you're ever lost out in the countryside, or at least play some music to be lost to.