Pressure v5: A 20 cm Buoyancy-Driven "Droplet Sub" with Brushless Micro-Generator and Capacitor Power

We present a buildable 20 cm buoyancy-driven micro-submarine (“droplet sub”) that powers a small buoyancy actuator using only hydrokinetic energy harvested from its own rise–sink motion. A tail propeller drives a brushless micro-generator on both ascent and descent; a low-drop rectifier charges a storage capacitor that delivers short pulses to a syringe or wax actuator to toggle net buoyancy. The result is a sealed, self-cycling platform requiring no tether, pumps, or wireless power. We give sizing rules, an energy budget closed with reference estimates, and minimal electronics/mechanics for benchtop replication in a vertical, water-filled tube.