line_follower.ino

int motor1_1 = 0;
int motor1_2 = 1;
int motor1_speed = 3;
int motor2_1 = 2;
int motor2_2 = 4;
int motor2_speed = 5;
int motor3_1 = 6;
int motor3_2 = 7;
int motor3_speed = 9;
int motor4_1 = 8;
int motor4_2 = 10;
int motor4_speed = 11;

void setup() {
  pinMode(motor1_1, OUTPUT);
  pinMode(motor1_2, OUTPUT);
  pinMode(motor2_1, OUTPUT);
  pinMode(motor2_2, OUTPUT);
  pinMode(motor3_1, OUTPUT);
  pinMode(motor3_2, OUTPUT);
  pinMode(motor4_1, OUTPUT);
  pinMode(motor4_2, OUTPUT);
  
  pinMode(motor1_speed, OUTPUT);
  pinMode(motor2_speed, OUTPUT);
  pinMode(motor3_speed, OUTPUT);
  pinMode(motor4_speed, OUTPUT);

  pinMode(A0, INPUT);
  
  digitalWrite(motor1_1, LOW);
  digitalWrite(motor1_2, HIGH);
  digitalWrite(motor2_1, LOW);
  digitalWrite(motor2_2, HIGH);
  digitalWrite(motor3_1, LOW);
  digitalWrite(motor3_2, HIGH);
  digitalWrite(motor4_1, LOW);
  digitalWrite(motor4_2, HIGH);
}  

void loop() {
  if(digitalRead(A0) == HIGH)
  {
    analogWrite(motor1_speed, 250);
    analogWrite(motor2_speed, 120);
    analogWrite(motor3_speed, 250);
    analogWrite(motor4_speed, 120);  
  }  
  else 
  {
    analogWrite(motor1_speed, 120);
    analogWrite(motor2_speed, 250);
    analogWrite(motor3_speed, 120);
    analogWrite(motor4_speed, 250);  
  }  

}