This water flow sensor is basically a small turbine whose output signal is a series of digital pulses. The frequency of the pulses is proportional to the flow rate of the liquid passing through the sensor. That digital signal, whose frequency is in the range between OHz and 100Hz, can be read directly through one of the digital input/output pins of a microcontroller.

This is YF-S401 Water Flow Measurement Sensor with 0.3-6Liter/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.