Does moisture sensor need a resistor when connected to an Arduino Uno?

Projects General Guidance
1

Hello!

I'm pretty new here so please go easy on me if I posted in the wrong place.

My question is

  1. Let's say if I want to hook the moisture sensor up to an Arduino Uno board

    *Please be kind to replace the board in the pic with an Arduino Uno board.

  2. which is connect to an external power 9V DC 1A

  3. with this kind of moisture sensor instead of the one in the photo.
    http://www.banggood.com/Moisture-Sensor-For-Arduino-Automatic-Watering-System-p-995384.html

Here is the specs for the sensor.
Supply voltage: 3.3V or 5V
Working current: less than 20mA

I googled Arduino Uno + moisture sensor in the image tab and noticed that most of the set up do not have a resistor.

Wouldn't it make sense to have a resistor between the moisture sensor and the arduino uno? Due to the fact that the working current for the sensor is only 20mA.

2

The sensor seems to have some smd parts on it, so no external resistor needed.
Output is 0-2.3volt.
Try this first. Then think of converting the A/D value (0 to ~470) to a moisture percentage.
Leo..

void setup() {
Serial.begin(9600);
}

void loop() {
Serial.println(analogRead(A0));
delay(500);
}
3

If you keep a standard cheap Chinese unipolar conductivity sensor powered all the time,
it will be destroyed by electrolysis in a very short period.

Life: approximately 1 year (surface gold processing, enhanced electrical conductivity and corrosion resistance)

The pictures of the sensor do not show any goldplating.