Analog Data producing a sawtooth wave

Using Arduino Sensors
#1

I have a simple conductivity sensor setup for measuring soil moisture. The sensor uses this circuit: https://www.faludi.com/2006/11/02/moisture-sensor-circuit/

I’m using an Arduino Nano and one of the .96 OLed screens. I have the Nano configured to use A2 as the sensor input and A4 & A5 as SDA & SDL to drive the screen.

The strange part is that when I have the sensor pins apart, the Arduino reads a strange pattern like so (serial output):

Raw: 0
24%
Raw: 626
32%
Raw: 297
31%
Raw: 0
25%
Raw: 566
31%
Raw: 474
34%
Raw: 0

The Raw reading is the data from analogRead(), the percentage is from a mapping 0-1023 to 0-100

Any idea why the sensor reads like this?

#2

I have a feeling that there’s something wrong with the code that I can’t see.

Steve

#3

Adding full code.

#include <Adafruit_ssd1306syp.h>
#include <math.h>

#define SDA_PIN A4
#define SCL_PIN A5
const int analogInPin = A2;
float sensorValue = 0;
int moist = 0;
Adafruit_ssd1306syp display(SDA_PIN,SCL_PIN);

void setup() {
  Serial.begin(9600);
  delay(1000);
  display.initialize();
}

float readMoisture(){
  float tmpVal=0;
  for (int i=0; i <= 25; i++){
    tmpVal+=analogRead(analogInPin);
    delay(10);
  }
  return (tmpVal / 25);
}

void loop() {
  // put your main code here, to run repeatedly:
  sensorValue = readMoisture();
  display.setTextSize(2);
  display.setTextColor(WHITE);
  display.setCursor(20,0);
  display.println("Moisture");
  display.setTextColor(WHITE);
  display.setCursor(38,18);
  display.println("Level");
  Serial.print("Raw: ");
  Serial.println(sensorValue);
  moist=map(sensorValue, 0, 1023, 0, 100);
  Serial.print(moist);
  Serial.println("%");
  if (moist < 10) {
    display.setCursor(48,40);
  }else if(moist < 100){
    display.setCursor(40,40);
  }else{
    display.setCursor(32,40);
  }
  display.print(moist);
  display.println("%");
  display.update();
  delay(1000);
  display.clear();
}
#4