YF-S201 is a water flow sensor that operates on the Hall effect principle, commonly used to measure water flow rates in various applications. It typically comes with a G3/4" (3/4 inch) thread size, which might be referred to as "3/4 YF-S201.
The sensor has three pins: Power (Red), Ground (Black), and Signal Output (Yellow). When water flows through the sensor, the internal rotor spins, and the Hall effect sensor outputs a pulse signal corresponding to the flow rate. By counting these pulses, you can determine the amount of water passing through the sensor.