Hi...
I'm building an arduino line follower robbot using an arduino uno a motor shield. For the optical sensors I'm using three qre1113. It works perfect follows the line perfect. Here is the code.
#define lights 4
int LDR1, LDR2, LDR3; // sensor values
// calibration offsets
int leftOffset = 0, rightOffset = 0, centre = 0;
// pins for motor speed and direction
int speed1 = 3, speed2 = 11, direction1 = 12, direction2 = 13;
// starting speed and rotation offset
int startSpeed = 90, rotate = 80; /* CHANGE START SPEED IF YOUR ROBOT IS TOO SLOW, AND CHANGE THE ROTATE VALUE ACCORDING TO YOUR
TURNING ANGLE OF BLACK LINE.*/
// sensor threshold
int threshhold = 6;
/*Threshold is the difference in values required between the center sensor and the left or right sensors
before the robot decides to make a turn. In my case, a setting of 5 worked well. This means that the left and right
sensors would need to detect a value greater than the value read from the center sensor plus the
threshold value before action is taken. In other words, if the center sensor is reading a value of 600 and
the left sensor is reading 603, then the robot will keep going straight. However, a left sensor value of 612
(which is higher than the center value plus threshold) means that the left sensor is detecting the back
line, indicating that the robot is too far over to the left. So the motors would adjust to make the robot
turn to the right to compensate.
The threshold value will vary depending on the contrast between your floor (or whatever surface
you use) and the black line. This may need to be adjusted to ensure the robot only turns when it has
detected enough of a difference between floor and line to ascertain it had moved too far left or right.*/
// initial speeds of left and right motors
int left = startSpeed, right = startSpeed;
// Sensor calibration routine
void calibrate() {
for (int x=0; x<10; x++) { // run this 10 times to obtain average
digitalWrite(lights, HIGH); // lights on
delay(100);
LDR1 = analogRead(3); // read the 3 sensors
LDR2 = analogRead(4);
LDR3 = analogRead(5);
leftOffset = leftOffset + LDR1; // add value of left sensor to total
centre = centre + LDR2; // add value of centre sensor to total
rightOffset = rightOffset + LDR3; // add value of right sensor to total
delay(100);
digitalWrite(lights, LOW); // lights off
delay(100);
}
// obtain average for each sensor
leftOffset = leftOffset / 10;
rightOffset = rightOffset / 10;
centre = centre /10;
// calculate offsets for left and right sensors
leftOffset = centre - leftOffset;
rightOffset = centre - rightOffset;
}
void setup()
{
// set the motor pins to outputs
pinMode(lights, OUTPUT); // lights
pinMode(speed1, OUTPUT);
pinMode(speed2, OUTPUT);
pinMode(direction1, OUTPUT);
pinMode(direction2, OUTPUT);
// calibrate the sensors
calibrate();
delay(3000);
digitalWrite(lights, HIGH); // lights on
delay(100);
// set motor direction to forward
digitalWrite(direction1, HIGH);
digitalWrite(direction2, HIGH);
// set speed of both motors
analogWrite(speed1,left);
analogWrite(speed2,right);
}
void loop() {
// make both motors same speed
left = startSpeed;
right = startSpeed;
// read the sensors and add the offsets
LDR1 = analogRead(3) + leftOffset;
LDR2 = analogRead(4);
LDR3 = analogRead(5) + rightOffset;
// if LDR1 is greater than the centre sensor + threshold turn right
if (LDR1 > (LDR2+threshhold)) {
left = startSpeed + rotate;
right = startSpeed - rotate;
}
// if LDR3 is greater than the centre sensor + threshold turn left
if (LDR3 > (LDR2+threshhold)) {
left = startSpeed - rotate;
right = startSpeed + rotate;
}
// send the speed values to the motors
analogWrite(speed1,left);
analogWrite(speed2,right);
}
Now I want my robot to stop with an object on the black line. I use a HC-SR04 board with some changes in my code, but the robot sometimes looses the line and I can't understand why. I have little experience with programming so I put the new code bellow for you to see where I have made mistake. Can you help me? 
#define trigPin 5
#define echoPin 6
#define ledRed 7
#define ledGreen 10
#define beep 2
#define lights 4
int LDR1, LDR2, LDR3; // sensor values
// calibration offsets
int leftOffset = 0, rightOffset = 0, centre = 0;
// pins for motor speed and direction
int speed1 = 3, speed2 = 11, direction1 = 12, direction2 = 13;
// starting speed and rotation offset
int startSpeed = 90, rotate = 80; /* CHANGE START SPEED IF YOUR ROBOT IS TOO SLOW, AND CHANGE THE ROTATE VALUE ACCORDING TO YOUR
TURNING ANGLE OF BLACK LINE.*/
// sensor threshold
int threshhold = 5;
/*Threshold is the difference in values required between the center sensor and the left or right sensors
before the robot decides to make a turn. In my case, a setting of 5 worked well. This means that the left and right
sensors would need to detect a value greater than the value read from the center sensor plus the
threshold value before action is taken. In other words, if the center sensor is reading a value of 600 and
the left sensor is reading 603, then the robot will keep going straight. However, a left sensor value of 612
(which is higher than the center value plus threshold) means that the left sensor is detecting the back
line, indicating that the robot is too far over to the left. So the motors would adjust to make the robot
turn to the right to compensate.
The threshold value will vary depending on the contrast between your floor (or whatever surface
you use) and the black line. This may need to be adjusted to ensure the robot only turns when it has
detected enough of a difference between floor and line to ascertain it had moved too far left or right.*/
// initial speeds of left and right motors
int left = startSpeed, right = startSpeed;
// Sensor calibration routine
void calibrate() {
for (int x=0; x<10; x++) { // run this 10 times to obtain average
digitalWrite(lights, HIGH); // lights on
delay(100);
LDR1 = analogRead(3); // read the 3 sensors
LDR2 = analogRead(4);
LDR3 = analogRead(5);
leftOffset = leftOffset + LDR1; // add value of left sensor to total
centre = centre + LDR2; // add value of centre sensor to total
rightOffset = rightOffset + LDR3; // add value of right sensor to total
delay(100);
digitalWrite(lights, LOW); // lights off
delay(100);
}
// obtain average for each sensor
leftOffset = leftOffset / 10;
rightOffset = rightOffset / 10;
centre = centre /10;
// calculate offsets for left and right sensors
leftOffset = centre - leftOffset;
rightOffset = centre - rightOffset;
}
void setup()
{
// set the motor pins to outputs
pinMode(lights, OUTPUT); // lights
pinMode(speed1, OUTPUT);
pinMode(speed2, OUTPUT);
pinMode(direction1, OUTPUT);
pinMode(direction2, OUTPUT);
// calibrate the sensors
calibrate();
delay(3000);
digitalWrite(lights, HIGH); // lights on
delay(100);
pinMode(beep,OUTPUT);
pinMode(trigPin, OUTPUT);
pinMode(echoPin, INPUT);
pinMode(ledRed, OUTPUT);
pinMode(ledGreen, OUTPUT);
// set motor direction to forward
digitalWrite(direction1, HIGH);
digitalWrite(direction2, HIGH);
// set speed of both motors
analogWrite(speed1,left);
analogWrite(speed2,right);
}
void loop() {
digitalWrite(ledGreen,HIGH);
long duration, distance;
digitalWrite(trigPin, LOW); // Added this line
digitalWrite(trigPin, HIGH);
// delayMicroseconds(1); - Removed this line
delayMicroseconds(1);
digitalWrite(trigPin, LOW);
duration = pulseIn(echoPin, HIGH);
distance = (duration/2) / 29.1;
// make both motors same speed
left = startSpeed;
right = startSpeed;
// read the sensors and add the offsets
LDR1 = analogRead(3) + leftOffset;
LDR2 = analogRead(4);
LDR3 = analogRead(5) + rightOffset;
// if LDR1 is greater than the centre sensor + threshold turn right
if (LDR1 > (LDR2+threshhold)) {
left = startSpeed + rotate;
right = startSpeed - rotate;
digitalWrite(ledGreen,HIGH);
digitalWrite(ledRed,LOW);
digitalWrite (beep,LOW);
}
// if LDR3 is greater than the centre sensor + threshold turn left
if (LDR3 > (LDR2+threshhold)) {
left = startSpeed - rotate;
right = startSpeed + rotate;
digitalWrite(ledGreen,HIGH);
digitalWrite(ledRed,LOW);
digitalWrite (beep,LOW);
}
// send the speed values to the motors
analogWrite(speed1,left);
analogWrite(speed2,right);
if (distance<8){
digitalWrite(ledRed,HIGH);
digitalWrite(ledGreen,LOW);
digitalWrite (beep,HIGH);
analogWrite(speed1,LOW);
analogWrite(speed2,LOW);
}
}