We are designing a boat that can be controlled by a DC motor, and a servo. We are also including a red LED to indicate that temperature is below some certain temperature (a temperature that we can set). We are also including a yellow LED to show that the temperature is now above the set temperature. Both action ( red or yellow) are going to be shown on a LCD with the current temperature at that moment.

hardware: 

1. temperature sensor.

2. LCD (16x2)

3. a red LED.

4. a yellow LED

5. potentiometer

6.  2 resistor.

 7. wires.

code:    

/*
Basic Analogue Input Control - Temperature Sensor
This example reads an analogue value from analogue in pin 1 and then compares this to a set point.
If the set point is threshold is crossed, the LED on digital pins 8,9 is turned on/off. A deadband is
provided to prevent high speed toggling at the set point transition.
The logic used below with act on Rising temperature.
The analogue input will come from a TMP36 temperature sensor.
-40'C to 125'C = 100mV to 1750mv (linear 10mV per 'C)

*/
int tempIn = 0;       // temperature sensor on Analogue Pin 3
int ledOut = 8;       // LED on Digital Pin 8
int LedOut1 = 9;      // LED on Digital Pin 9
int aiValue = 0;       // input value (0-1023 = 0 to 5v)
float setPoint =30;    // Trigger value in 'C
float deadband = 0.5;   // Differential for reset value in 'C
#include <LiquidCrystal.h>
LiquidCrystal lcd(12,11,5,4,3,2);
/*
  NOTE: the deadband action can be disabled by setting the deadband to 0
*/
float degC;            // The temperature in Degrees Centigrade
void setup()
{
  pinMode(ledOut, OUTPUT);  // Configure the Digital Pin Direction for the LED
  pinMode(LedOut1, OUTPUT);
  lcd.begin(16, 2);
  lcd.clear();
  Serial.begin(9600);
}
void loop()
{

  degC = getTemperature(getVolts());      //Get the Volts and the Temperature using Helper functions.
   
  if (degC > setPoint)
  {
    digitalWrite(ledOut, HIGH);      // Temperature Limit Reached, turn the LED on.
    Serial.println(" | Output: ON.");
     lcd.setCursor(0,1);
     lcd.print("  Yellow  ");
     lcd.print(degC);

   
   }
    else
  {
      if (degC > (setPoint - deadband))
      {
      digitalWrite(ledOut, LOW);       // Temperature dropped below SP and deadband, turn the LED off.
      Serial.println(" | Output: OFF.");
      lcd.setCursor(0,1);
      lcd.print("      ");
      }
  }
   if (degC < setPoint)
  {
    digitalWrite(LedOut1, HIGH);      // Temperature Limit Reached, turn the LED on.
    Serial.println(" | Output: ON.");
    lcd.setCursor(0,1);
    lcd.print("  RED   ");
    lcd.print(degC);
   }
    else
  {
      if (degC < (setPoint - deadband))
      {
      digitalWrite(LedOut1, LOW);       // Temperature dropped below SP and deadband, turn the LED off.
      Serial.println(" | Output: OFF.");
      lcd.setCursor(0,1);
      lcd.print("      ");
      }
  }
  
  delay(1000);
}
/*
Return the voltage for the input value; 0-1023 = 0 to 5V
*/
float getVolts()
{
  int inputValue;
  inputValue = analogRead(tempIn);
  float volts;
  volts = (((float)inputValue / 1024) * 5);
 
  Serial.print("Input Value: ") ; Serial.print(inputValue);
  Serial.print(" | Voltage: ") ; Serial.print(volts);
  return volts;
}
/*
Return the temperature in DegreesC for given voltage
*/
float getTemperature(float volts)
{
  float temp = (volts - 0.5) / 0.01 ;
 
  Serial.print(" | Temperature: "); Serial.print(temp); Serial.print(" 'C");
 
  return temp;
}
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