Notes

int greenLedPin = 2; // Pin Green LED is connected to
int yellowLedPin = 3; // Pin Yellow LED is connected to
int redLedPin = 4; // Pin Red LED is connected to

int temp_sensor = 5; // Pin DS18B20 Sensor is connected to

float temperature = 0; //Variable to store the temperature in
int lowerLimit = 15; //define the lower threshold for the temperature
int upperLimit = 35; //define the upper threshold for the temperature

OneWire oneWirePin(temp_sensor);

DallasTemperature sensors(&oneWirePin);

void setup(void){
Serial.begin(9600);

//Setup the LEDS to act as outputs
pinMode(redLedPin,OUTPUT);
pinMode(greenLedPin,OUTPUT);
pinMode(yellowLedPin,OUTPUT);

sensors.begin();
}

void loop(){
Serial.print("Requesting Temperatures from sensors: ");
sensors.requestTemperatures();
Serial.println("DONE");

temperature = sensors.getTempCByIndex(0);

digitalWrite(redLedPin, LOW);
digitalWrite(greenLedPin, LOW);
digitalWrite(yellowLedPin, LOW);

Serial.print("Temperature is ");
Serial.print(temperature);

//Setup the LEDS to act as outputs
if(temperature <= lowerLimit){
Serial.println(", Yellow LED is Activated");
digitalWrite(yellowLedPin, HIGH);
}
else if(temperature > lowerLimit && temperature < upperLimit){
Serial.println(", Green LED is Activated");
digitalWrite(greenLedPin, HIGH);
}
else if(temperature >= upperLimit){
Serial.println(", Red LED is Activated");
digitalWrite(redLedPin, HIGH);
}
delay(3000);
}