Help understanding simple Arduino circuit from a datasheet example

Question

I'm trying to use a water sensor in a new design, and I don't understand the example circuit that is given in the datasheet, even though it looks very very simple. I must be missing something. Any help explaining why this example circuit works would be much appreciated.

I don't get how the example circuit shown in the image “Arduino circuit I don't understand” works. The datasheet for the water sensor can be found here.

In the example in question, it says that the Arduino Mega should be in a pull up mode, which I believe should pull the digital input pin up to 5 V. But the sensor is hooked up to 5 V on the other side, so when water is present and it conducts, it seems like the input pin should also go to 5 V. We tested the real device though and found that when there is water the Arduino reads a digital low.

Please explain how this circuit works and why we see a digital low when water is present. To me it looks like the following two conditions would happen:

No water present: Water sensor doesn't conduct. The Arduino input is pulled high to 5 V by the internal pull-up.

Water present: 5V is detected by the Arduino input (a high impedance digital input state) since the water allows the signal to propagate.

What am I missing? Thanks!

Answer 1

Your analysis is correct: the datasheet circuit will not work and I doubt they've actually tested it in that configuration. Given the relatively high MΩ impedance of your sensor and the nature of its signal, I recommended using an analog front-end with a voltage divider and Schmitt trigger comparator to give a clean digital signal to your Arduino. Or use a normal (probably rail-to-rail) opamp to buffer the signal into one of your Arduino's analog inputs if you want to measure the level (not just above/below a setpoint) and don't mind cleaning up the signal in software.

Answer 2

I think you might misinterpreted the information provided by the datasheet. The datasheet suggested two type of configurations for interfacing with Arduino/Rapsberry Pi.

This, when translating into a schematic would looks something like the following:

^{simulate this circuit – Schematic created using CircuitLab}

So if you write your code to read the sensor using ADC with the following code:

int value = analogRead(LS_2600);
int sensed_voltage = value * 5 / 1023;

It would shows a voltage of 0v when it is not in-contact with water because the LS-2600 has an infinitely high impedance like an open contact, and the pulldown resistor would pull the analog input to ground; and it will shows an voltage of 3.9v when the sensor is in-contact with water based on the assumption that the resistance is 1.4M-ohm. This method will only applicable to Arduino but not Raspberry Pi because Raspberry Pi does not have analog input.

The method two uses Digital Input with internal pull-up and noticed the connection wiring as provided by the datasheet, the red wire is connected directly to the Digital Input pin of Arduino/RPI, and white wire is connected to the GND, together with internal pull-up form the voltage divider.

pinMode(LS_2600, INPUT_PULLUP);
if (digitalRead(LS_2600) // if the reading is HIGH
  // no leakage
else
  // leakage

Assuming the internal pull-up resistance is around 50k-ohm, the voltage presented at the Digital Input pin is around .95V, whether this will be considered as logical LOW would need to further consult the datasheet of the MCU.