Most of today's candidates are hashtag-stuffed Shorts or vague "free energy" clickbait. This one from Hetronics stands out because it pairs a practical end goal — automatic shutoff for an overflowing water tank — with a teachable circuit element: using a MOSFET as a low-side switch driven by a simple water-level sense path.

The interesting part for hobbyists is why a MOSFET is the right choice here over a BJT or relay. Water itself is a high-impedance, slightly conductive medium, so the gate's near-infinite input impedance lets you trigger the device with the tiny current that flows between two probes dipped in the tank, without needing an op-amp front end. Once the gate goes high, the MOSFET sinks current through a load (pump, buzzer, or solenoid) cleanly with very little voltage drop.

It's a good template project: the same topology — conductive-medium probe into a MOSFET gate with a pulldown resistor — works for soil moisture sensors, rain detectors, and leak alarms. If you've only ever used MOSFETs as logic-driven switches from a microcontroller, watching one used as a standalone analog threshold detector is a useful mental model expansion.