I’m a mechanical engineer who loves bicycling. When gas prices soared through the roof in 2009, I rode my bike to save money. I wanted to charge USB devices, such as my phone, during commutes but didn’t want to spend $150 (or more) on a commercial wheel-hub generator. In the end, I was able to build a cheaper system.

Electrical work wasn’t my expertise, so I had to teach myself how to build circuits from the ground up. I’m now on version three of my two-port USB generator, which is about 70 percent efficient above 12 mph and creates less drag on bike wheels than other designs. With the right equipment, you can build one yourself.