When your phone runs out of juice, the normal procedure is to plug it in to a USB connector of some kind. However, wireless charging is becoming more and more common. While this might seem like magic, it uses the same basic inductive principle as an electrical transformer, passing power magnetically from a charging coil to a corresponding device on your phone.
That’s the simple explanation, but if you truly want to understand the concept, you could “simply” build your own. Vinod S. did just this, creating a receiver that uses the Qi charging standard. Although a simple power transfer device could be made with a 555 timer and MOSFET-based circuitry, he instead implemented ATtiny13 microcontroller on the build. This allows the transmitter to recognize the newly-built receiver via signals sent through its coils, and adjust the power supplied in response.
As shown in the video below, Vinod’s DIY receiver is functional and capable of delivering a full amp of current at 5 volts. If you’re just interested in charging your phone or other smart device, off-the-shelf solutions will certainly be a better bet, but this type of build is a great way to explore. A whiteboard schematic, as well as an explanation of the protocol is available on the project write-up, and source code is found here on GitHub.