Hi guys, I’ve faced an obstacle in my project. I’m using a remote control car, an Arduino Uno R3, motor shield and Ping ultrasonic sensor to make an auto obstacle-avoidance car. My plan is when my car senses an obstacle, it will reverse for 2secs and then move forward again.
However, the result is the car moves forward (for few seconds) and reverse (for another few seconds) without any obstacle in front of the sensor, but it reverses full-time when there is an obstacle.
Here’s my code, anyone mind telling me where is the error? Thanks 
/*
RC Car to Robot Conversion
by Randy Sarafan
Used to convert an RC car into a robot that uses a PING sensor to avoid obstacles,
and an Arduino motor shield for motor control.
For more information see:
http://www.instructables.com/id/RC-Car-to-Robot/
Built atop Ping example code by Tom Igoe
*/
// this constant won't change. It's the pin number
// of the sensor's output:
const int pingPin = 7;
void setup() {
//establish motor direction toggle pins
pinMode(12, OUTPUT); //drive motor -- HIGH = forwards and LOW = backwards
//establish motor brake pins
pinMode(9, OUTPUT); //brake (disable) the drive motor
//Turns brake off for drive motor
digitalWrite(9, LOW);
//Sets initial speed of drive motor
analogWrite(3, 200);
//Sets initial direction of drive motor
digitalWrite(12, HIGH);
}
void loop()
{
// establish variables for duration of the ping,
// and the distance result in inches and centimeters:
long duration, inches, cm;
// The PING))) is triggered by a HIGH pulse of 2 or more microseconds.
// Give a short LOW pulse beforehand to ensure a clean HIGH pulse:
pinMode(pingPin, OUTPUT);
digitalWrite(pingPin, LOW);
delayMicroseconds(2);
digitalWrite(pingPin, HIGH);
delayMicroseconds(5);
digitalWrite(pingPin, LOW);
// The same pin is used to read the signal from the PING))): a HIGH
// pulse whose duration is the time (in microseconds) from the sending
// of the ping to the reception of its echo off of an object.
pinMode(pingPin, INPUT);
duration = pulseIn(pingPin, HIGH);
// convert the time into a distance
inches = microsecondsToInches(duration);
//
//if objects are less than 15 inches away
//the robot reverses for 2 seconds
//
if (inches < 15){
//brake drive motor and pause 1/10 second
digitalWrite(9, HIGH);
delay(100);
//
//setting drive motor
//
//turn off brake of drive motor
digitalWrite(9, LOW);
//set drive motor backwards direction
digitalWrite(12, LOW);
//activate the drive motor
analogWrite(3, 200);
//backup for 2 seconds
delay(2000);
//
//stopping
//
//brake both motors
digitalWrite(9, HIGH);
}
//
//when nothing is within 12"
//the robot simply drives forwards
//
else{
//
//Setting drive motor
//
//set drive motor forward direction
digitalWrite(12, HIGH);
//turn off brake of drive motor
digitalWrite(9, LOW);
//activate drive motor
analogWrite(3, 200);
}
delay(100);
}
long microsecondsToInches(long microseconds)
{
// According to Parallax's datasheet for the PING))), there are
// 73.746 microseconds per inch (i.e. sound travels at 1130 feet per
// second). This gives the distance travelled by the ping, outbound
// and return, so we divide by 2 to get the distance of the obstacle.
// See: http://www.parallax.com/dl/docs/prod/acc/28015-PING-v1.3.pdf
return microseconds / 74 / 2;
}