If not already installed, install QTRSensors library from Sketch->Include Library-> Search and install from Arduino library manager.

Code

#include <ESP32Servo.h>
#include <QTRSensors.h>
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>

#define NUM_SENSORS 5 // number of sensors used
#define NUM_SAMPLES_PER_SENSOR 4 // average 4 analog samples per sensor reading
#define EMITTER_PIN 15 // emitter is controlled by digital pin 15
#define MOTOR1_P 16
#define MOTOR1_N 17
#define MOTOR2_P 27
#define MOTOR2_N 18
#define RIGHT_BUTTON 34
#define SENSOR1_PIN 32
#define SENSOR2_PIN 14
#define SENSOR3_PIN 26
#define SENSOR4_PIN 4
#define SENSOR5_PIN 13

Adafruit_SSD1306 display(128, 64);
QTRSensors qtra;
uint16_t sensorValues[NUM_SENSORS];
int position, proportional, power_difference, last_proportional = 0, integral = 0;

void setup() {

qtra.setTypeAnalog();
qtra.setSensorPins((const uint8_t[]) {
SENSOR1_PIN, SENSOR2_PIN, SENSOR3_PIN, SENSOR4_PIN, SENSOR5_PIN}, 5);

display.begin(SSD1306_SWITCHCAPVCC, 0x3C);

display.setTextSize(1);
display.setTextColor(WHITE);
display.setCursor(0, 0);
pinMode(MOTOR1_P, OUTPUT);
pinMode(MOTOR1_N, OUTPUT);
pinMode(MOTOR2_P, OUTPUT);
pinMode(MOTOR2_N, OUTPUT);
pinMode(EMITTER_PIN, OUTPUT);
digitalWrite(EMITTER_PIN, HIGH);
digitalWrite(MOTOR1_P, LOW);
digitalWrite(MOTOR1_N, LOW);
digitalWrite(MOTOR2_P, LOW);
digitalWrite(MOTOR2_N, LOW);
pinMode(RIGHT_BUTTON, INPUT);
Serial.begin(9600);
delay(500);
display.clearDisplay();
display.setTextSize(1);
display.println("Line Follower");
display.println();
display.println("Place robot on a black line on a white surface and press the right button");
display.display();
delay(30);
while (digitalRead(RIGHT_BUTTON))
{
//wait until button is pressed
}
display.setCursor(0, 0);
display.clearDisplay();
display.setTextSize(1);
display.println("Calibrating");
for (int i = 0; i < 34; i++) // make the calibration take about 10 seconds

{
display.print(".");
display.display();

if (i < 7)
{
analogWrite(MOTOR1_P, 0);
analogWrite(MOTOR1_N, 100);
analogWrite(MOTOR2_P, 100);
analogWrite(MOTOR2_N, 0);

}
else if (i < 21)
{
analogWrite(MOTOR1_P, 100);
analogWrite(MOTOR1_N, 0);
analogWrite(MOTOR2_P, 0);
analogWrite(MOTOR2_N, 100);
}
else
{
analogWrite(MOTOR1_P, 0);
analogWrite(MOTOR1_N, 100);
analogWrite(MOTOR2_P, 100);
analogWrite(MOTOR2_N, 0);
}
qtra.calibrate(); // reads all sensors 10 times at 2.5 ms per six sensors (i.e. ~25 ms per call)
}
analogWrite(MOTOR1_P, 0);
analogWrite(MOTOR1_N, 0);
analogWrite(MOTOR2_P, 0);
analogWrite(MOTOR2_N, 0);

display.clearDisplay();
display.setTextSize(2);
display.setCursor(0, 0);
display.println("Start Now");
display.setTextSize(1);
display.println("Press right button to start");
display.println();
display.display();
while (digitalRead(RIGHT_BUTTON));
display.clearDisplay();
display.display();
}

void loop() {

// Get the position of the line.
position = qtra.readLineBlack(sensorValues);

// The "proportional" term should be 0 when we are on the line.
proportional = ((int)position) - 2000;

// Compute the derivative (change) and integral (sum) of the
// position.
int derivative = proportional - last_proportional;
integral += proportional;

// Remember the last position.
last_proportional = proportional;

// Compute the difference between the two motor power settings,
// m1 - m2. If this is a positive number the robot will turn
// to the left. If it is a negative number, the robot will
// turn to the right, and the magnitude of the number determines
// the sharpness of the turn.

power_difference = proportional / 5 + derivative / 2 ;

// Compute the actual motor settings. We never set either motor
// to a negative value.

const int maximum = 250; // the maximum speed
if (power_difference > maximum)
power_difference = maximum;
if (power_difference < -maximum)
power_difference = -maximum;

Serial.println(power_difference);
if (power_difference > 0)
{
analogWrite(MOTOR1_P, 0);
analogWrite(MOTOR2_P, 0);
analogWrite(MOTOR1_N, maximum - power_difference);
analogWrite(MOTOR2_N, maximum);
}
else
{
analogWrite(MOTOR1_P, 0);
analogWrite(MOTOR2_P, 0);
analogWrite(MOTOR2_N, maximum + power_difference);
analogWrite(MOTOR1_N, maximum);
}
delay(10);
}