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/*
 HC-SR04 Ping distance sensor]
 VCC to arduino 5v GND to arduino GND
 Echo to Arduino pin 13 Trig to Arduino pin 12
 Red POS to Arduino pin 11
 Green POS to Arduino pin 10
 560 ohm resistor to both LED NEG and GRD power rail
 More info at: http://goo.gl/kJ8Gl
 Original code improvements to the Ping sketch sourced from Trollmaker.com
 Some code and wiring inspired by http://en.wikiversity.org/wiki/User:Dstaub/robotcar
 */

#define trigPin 13
#define echoPin 12
#define led 11
#define led2 10

void setup() {
  Serial.begin (9600);
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
  pinMode(led, OUTPUT);
  pinMode(led2, OUTPUT);
}

void loop() {
  long duration, distance;
  digitalWrite(trigPin, LOW);  // Added this line
  delayMicroseconds(2); // Added this line
  digitalWrite(trigPin, HIGH);
//  delayMicroseconds(1000); - Removed this line
  delayMicroseconds(10); // Added this line
  digitalWrite(trigPin, LOW);
  duration = pulseIn(echoPin, HIGH);
  distance = (duration/2) / 29.1;
  if (distance < 4) {  // This is where the LED On/Off happens
    digitalWrite(led,HIGH); // When the Red condition is met, the Green LED should turn off
  digitalWrite(led2,LOW);
}
  else {
    digitalWrite(led,LOW);
    digitalWrite(led2,HIGH);
  }
  if (distance >= 200 || distance <= 0){
    Serial.println("Out of range");
  }
  else {
    Serial.print(distance);
    Serial.println(" cm");
  }
  delay(500);
}

Minimum sensor recognition distance:

> Sensor’s minimum-distance does not have to be much smaller than 20cm (8“).

Infrared:

• Binary Output:               detects whether an object is within a threshold distance
• Multiple Bit Output:        detects the more or less accurate distance to an object
                     

Pro:

-       Quality products comparatively cheap

-       In some cases very high resolution (up to 1 mm)

Contra:

-       Problems with sun light

-       Problems with recognition of dark or black objects

-       Small range sizes

Recommendation after some research: 

Sharp GP2Y 0A02 (ca. $11, 20cm – 150cm Range, Analogue Output)

http://acroname.com/products/SHARP-GP2Y0A02YK0F-IR-PACKAGE

(see also: http://acroname.com/articles/sharp-infrared-ranger-comparison for comparison)

Ultrasonic

Pro:

-       Not depending on optical properties of an object -> more reliable in common usage

-       Larger range sizes

Contra:

-       Usually higher prize

-       Lower resolution (2-3 cm)

-       Problems detecting acoustic quieting objects

Recommendation after some research:

Parallax PING Ultrasonic Sensor (ca. $29, 2cm – 300cm Range)

https://www.parallax.com/product/28015

(maybe cheap replacement: http://www.dx.com/p/hc-sr04-ultrasonic-sensor-distance-measuring-module-133696?Utm_rid=37202148&Utm_source=affiliate#.VTsyqa3tmko)

FPV 433Mhz Radio Telemetry Kit 100mW V1.1:

http://www.hobbyking.com/hobbyking/store/__42847__FPV_433Mhz_Radio_Telemetry_Kit_100mW_V1_1.html