The brief I gave myself was simple: turn flowing water into electricity for as little money as possible. It landed at around $15 total, cheap enough to actually deploy somewhere that needs it off-grid homes, refugee camps, disaster zones where the power just isn't there.
The core of it is an Archimedes screw turbine, a design that's been lifting water since ancient Greece. Run it backwards, with water flowing through instead of being pushed up, and the screw spins a shaft connected to a DC generator. Faster water, more voltage. In this build, that's enough to power a couple of LEDs or a small motor through a 2-pin connector.
What hooked me was how much physics was hiding inside something that looks this simple. I derived the output voltage, current, RPM, and the screw blade's helical path length from scratch the voltage formula alone ties together water density, flow velocity, screw geometry, and generator efficiency. A lot happens inside a piece of PVC pipe.
The frame is wood, PVC, and salvaged plastic. I soldered every connection, epoxied the screw blades to the shaft by hand, and built the housing out of a transparent plastic container so you can actually watch it work. And it does. That part still gets me.