The green wire is connected to input pin 2 on the Arduino, and that 10kΩ resistor is functioning as a pull-down resistor. The pull-down resistor holds the logic signal near to zero volts (0V) when no ...

Since there is an internal pull-down resistor on the Arduino, it will be pulled down to 0V when the water stops flowing. But I think it's better to use an external pull-down resistor to avoid the ...