I've connected the FC-37 rain sensor module to my Arduino Uno, and I'm reading analog values from it. What puzzles me is that the more water I add, the lower the values become. My expectation was that since water is a good conductor of electricity, increasing the water should result in higher values. Could you please explain this phenomenon?

 

Do you have a tutorial for the FC-37 and an Arduino? The link points to one. It describes what happens with more water.

When the board in the sensor is:

• Wet: the resistance increases, and the output voltage decreases
• Dry: the resistance is lower, and the output voltage is higher

Since the voltage output from the sensor decreases when the board gets wetter, the Arduino will read lower values. (Note the Arduino reads the voltage on an analog pin and returns 0 for no voltage and as much as 1023 for the highest voltage).

This is exactly what you are seeing!

Just one more point. Water is NOT a good conductor of electricity. Distilled water does not conduct electricity. It is ions dissolved in water that give the appearance that water is a good conductor. Such as ions from a contamination of the water, like the addition of salt or dissolved minerals in soil.

 

Do you have a tutorial for the FC-37 and an Arduino? The link points to one. It describes what happens with more water.

Since the voltage output from the sensor decreases when the board gets wetter, the Arduino will read lower values. (Note the Arduino reads the voltage on an analog pin and returns 0 for no voltage and as much as 1023 for the highest voltage).

This is exactly what you are seeing!

Just one more point. Water is NOT a good conductor of electricity. Distilled water does not conduct electricity. It is ions dissolved in water that give the appearance that water is a good conductor. Such as ions from a contamination of the water, like the addition of salt or dissolved minerals in soil.

Thank you very much for your reply. I still have some doubts. Please clarify.

When rain drop falls on the exposed copper strips, won't the conductivity will increase and resistivity will decrease?
Shouldn't the result should be,
The more water on the surface means better conductivity and will result in a lower resistance.
The less water on the surface means poor conductivity and will result in a higher resistance.

 

That is correct for just the sensing element comprising two inter-digitated electrodes. However, the FC-37 includes a circuit board which conditions the resistance presented the sensing element to provide a voltage which can be read by an MCU such as an Arduino module.

 

That is correct for just the sensing element comprising two inter-digitated electrodes. However, the FC-37 includes a circuit board which conditions the resistance presented the sensing element to provide a voltage which can be read by an MCU such as an Arduino module.

Thank you for replying.
Is,the board inverting the values? or The analogRead() is giving resistance values?
https://electronics.stackexchange.com/questions/173067/pull-down-resistor-why-is-it-crucial

 

No, it reads a voltage. The sensor you have provides a voltage that is apparently inversely proportional to the rain level. Do you have a datasheet? It should all be explained there.

 

Hello,

The following page has a nice desciption of the sensor and its control board:
https://diyi0t.com/rain-sensor-tutorial-for-arduino-and-esp8266/

Bertus

 

Hello,

The following page has a nice desciption of the sensor and its control board:
https://diyi0t.com/rain-sensor-tutorial-for-arduino-and-esp8266/

Bertus

Thank you very much.

 

Do you have a tutorial for the FC-37 and an Arduino? The link points to one. It describes what happens with more water.

When the board in the sensor is:

• Wet: the resistance increases, and the output voltage decreases
• Dry: the resistance is lower, and the output voltage is higher

But the claim that the resistance increases when the board is wet makes no sense. The board is clearly nothing more than two isolated strips in parallel to each other. When dry, the resistance of the sensor should be infinite. As water drops fall on the board, they bridge the conductors. Assuming each droplet has an effective resistance across the gap of Rd, the total resistance across the sensor is then just Rd/N where N is the number of droplets (and, if N=0, you get infinite resistance).

So the resistance should go DOWN as the board gets wetter.

But whether that results in the output voltage produced by the module to go up or down depends on the circuitry in the module. If it uses the sensor as one element in a voltage divider along with a fixed resistor, then as the sensor gets wet, more and more voltage will be shifted from the fixed resistor to the sensor. Whether the output goes up or down depends on whether the voltage that is output is taken across the fixed resistor or across the sensor.

 

But the claim that the resistance increases when the board is wet makes no sense. The board is clearly nothing more than two isolated strips in parallel to each other. When dry, the resistance of the sensor should be infinite. As water drops fall on the board, they bridge the conductors. Assuming each droplet has an effective resistance across the gap of Rd, the total resistance across the sensor is then just Rd/N where N is the number of droplets (and, if N=0, you get infinite resistance).

So the resistance should go DOWN as the board gets wetter.

But whether that results in the output voltage produced by the module to go up or down depends on the circuitry in the module. If it uses the sensor as one element in a voltage divider along with a fixed resistor, then as the sensor gets wet, more and more voltage will be shifted from the fixed resistor to the sensor. Whether the output goes up or down depends on whether the voltage that is output is taken across the fixed resistor or across the sensor.

Oh, I totally agree with you. My point was the exact same (I think) as you. I’m assuming poor documentation. What they say is “resistance” I take to be the output of the sensor circuitry which is inversely related to the resistance of the board.