YF-S201 Water Flow Measurement Sensor with 1-30Liter/min Flow Rate white sensor. This sensor sits in line with your water line and contains a pinwheel sensor to measure how much liquid has moved through it. There’s an integrated magnetic hall effect sensor that outputs an electrical pulse with every revolution. The hall effect sensor is sealed from the water pipe and allows the sensor to stay safe and dry.
 
By counting the pulses from the output of the sensor, you can easily calculate water flow. Each pulse is approximately 2.25 milliliters.
 
Connection
Red wire: +5V.
Black wire: GND.
Yellow wire: PWM output.