The heart of a water flow sensor consists of a Hall effect sensor (https://en.wikipedia.org/wiki/Hall_effect_sensor) that outputs pulses for magnetic field changes. Inside the housing, there is a small pinwheel with a permanent magnet attached to it. When the water flows through the housing, the pinwheel begins to spin, and the magnet attached to it passes very close to the Hall effect sensor in every cycle. The Hall effect sensor is covered with a separate plastic housing to protect it from the water. The result generates an electric pulse that transitions from low voltage to high voltage, or high voltage to low voltage, depending on the attached permanent magnet's polarity. The resulting pulse can be read and counted using the Arduino.

For this project, we will use a Liquid Flow sensor from Futurlec (http://www.futurlec.com/FLOW25L0.shtml). The following image shows the external view of a Liquid Flow Sensor:

Liquid flow sensor – the flow direction is marked with an arrow...