Lost in the woods near a stream, with some empty water bottles, wire, plastic plates, and a stepper motor, *and* you need to charge your smartphone? Try this.
OK, so perhaps you wouldn't ever be caught away from the grid with all the necessary parts, and have running water nearby, and have cell service but a dying phone battery, but in case you'd like to MacGyver together a working hydroelectric generator just for fun (and free electricity) or for a child's science project, YouTuber Thomas Kim has you covered.
Kim, a power plant operator and science enthusiast, shows us how it's done in this short video, which is short on technical details but long on showing the viability of a small-scale DIY hydro generator for charging a smartphone and/or LED lights:
According to the video description, Kim's hydroelectric generator employs plastic bottles and 'disposable platters' for the waterwheel, which then turns a shaft in a 3-phase stepping motor, generating electricity that then flows through a rectifier circuit (which converts the AC current into the DC current needed for charging a mobile device).
Although not specifically mentioned, it appears that he also integrated a voltage regulator (and USB connector) to keep the output from damaging the phone, which would seem to be an important factor. From the video, the output on Kim's device looks to be about 10V, and he's able to get a smartphone charging and light up a small LED device with this setup.
For those with easy access to running water, this type of micro-hydro project could be a fun and useful way to harness some carbon-free renewable energy and use it to power small lighting systems or charge battery banks, and probably at a minimal cost as well.