Arduino and LM35 Temperature Measurement

Abstract

The LM35 is a precision integrated-circuit temperature sensor, which is connected to Arduino and measures the temperature in Celsius format. The program converts the reading into either Fahrenheit, or Kelvin or Rankine, accordance with the user selection and display it on LCD screen.

Parts and components

Arduino Uno board
16x2 LCD
1 K ohm potentiometer
LM35
Push Button
10K resistor = 2 Nos

Step 1: Schematic

The 16x2 is very common type LCD, with two rows, and each row displays 16 characters of either 5x7 or 5x8 dot matrix characters.
The LCD is available in a 16 pin package. It consists of back light and contrast adjustment function and each dot matrix has 5×8 dot resolution.
The 16x2 LCD display is connected to the Arduino (2,3,4,5,6,7) digital IO pins, where LCD operates at 4 bit data mode.
If the display is not visible, adjust the Contrast pot (1K), to make it visible.
The LM35 is connected to the A0, analog input pin.
The LM35 is a precision integrated-circuit temperature sensor, whose output voltage is linearly proportional to the Celsius (Centigrade) temperature.
The LM35 does not require any external calibration or trimming to provide typical accuracies of ±1⁄4˚C at room temperature and ±3⁄4˚C over a full −55 to +150˚C temperature range.
The LM35 has linear scale factor that is + 10.0 mV/˚C.
The LM35 is rated to operate over a −55˚ to +150˚C temperature range.
It can be used with single power supply, operates from 4 to 30 volts. As it draws only 60 µA from its supply and it has very low self-heating, less than 0.1˚C in still air.
A push button is connected to digital IO pin 8. Which is used to select and display the measured temperature at Celsius, Fahrenheit, Kelvin and Rankine scales.
The conversion formula for Celsius to other scale are given below.
Fahrenheit:- T(°F) = T(°C) × 9/5 +32

Kelvin:- T(K) = T(°C) + 273.15
Rankine:- T(°R) = (T(°C) + 273.15) × 9/5

Step 2: Software

The Arduino LiquidCrystal library is used for displaying the measured temperature.
The push button switch is used for selecting the temperature scale from Celsius to Fahrenheit or Kelvin or Rankine.
The 1.1V internal reference is chosen for the ADC.
Hence the software will measure and display the values between 0 °C to 110°C, only.
10 bit ADC reading is mapped to 1100 mV, which is the vref mille volt.

/*
 Demonstrates the use ADC and 16x2 LCD display
 The Arduino circuit connection for LCD:
 * LCD RS pin to digital pin 7
 * LCD Enable pin to digital pin 6
 * LCD D4 pin digital pin 5
 * LCD D5 pin to digital pin 4
 * LCD D6 pin to digital pin 3
 * LCD D7 pin to digital pin 2
 *
 *  LM35 is connected with A0 ADC pin
 * Push button is connected with D8 analog pin
 Name:- M.Pugazhendi
 Date:-  08thJul2016
 Version:- V0.1
 e-mail:- muthuswamy.pugazhendi@gmail.com
 */
// include the library code:
#include  <LiquidCrystal.h>
int analogPin1 = 0;     // LM35 temperature sensor connected to analog pin 2
int val = 0;           // variable to store the value read
//initialize the library with the numbers of the interface pins
LiquidCrystal lcd(7,6,5,4,3,2);
// this constant won't change:
const int  buttonPin = 8;    // the pin that the pushbutton is attached to
// Variables will change:
int buttonPushCounter = 0;   // counter for the number of button presses
int buttonState = 0;         // current state of the button
int lastButtonState = 0;     // previous state of the button
void setup() 
{
  // initialize the button pin as a input:
  pinMode(buttonPin, INPUT);
  // set up the LCD's number of columns and rows:
  lcd.begin(16, 2);
  analogReference(INTERNAL);
  
  // Print a message to the LCD
  lcd.print("Start");
 
}
void loop() 
{
  // read the pushbutton input pin:
  buttonState = digitalRead(buttonPin);
  // compare the buttonState to its previous state
  if (buttonState != lastButtonState) 
  {
    // if the state has changed, increment the counter
    if (buttonState == HIGH) 
    {
      // if the current state is HIGH then the button
      // wend from off to on:
      buttonPushCounter++;
      
      // (note: line 0 is the first row, since counting begins with 0):
      lcd.setCursor(0, 0);
      lcd.print("                "); //Print blanks to clear the row
      lcd.setCursor(0, 0);
      switch(buttonPushCounter)
      {
        case 1:
          lcd.print("Celsius");
          break;
        case 2:
          lcd.print("Farenheit");
          break;
        case 3:
          lcd.print("Kelvin");
          break;
        case 4:
          lcd.print("Rankine");
          break;                              
        default:
         buttonPushCounter = 1;
         lcd.print("Celsius");
        break;      
      }            
    } 
    
    // Delay a little bit to avoid bouncing
    delay(50);
  }
  
  // save the current state as the last state,
  //for next time through the loop
  lastButtonState = buttonState;
  // Resets the buttonPushCounter to zero once every four button pushes.    
  //if (buttonPushCounter  == 5) 
  //{
  //  buttonPushCounter = 0;
  //}
  // set the cursor to column 0, line 1
  // (note: line 1 is the second row, since counting begins with 0):
  lcd.setCursor(12, 1);
  lcd.print("     "); //Print blanks to clear the row
  lcd.setCursor(0, 1);
  
  val = analogRead(analogPin1);    // read the LM35 sensor input pin
  float converted = fmap(val, 0, 1023, 0.0, 110.0);
 
  
      switch(buttonPushCounter)
      {
        case 1:
           //Celsius
            lcd.print("TEMP = ");
            lcd.print(converted);             // ADC value
            lcd.print("\337C");
          break;
        case 2:
            //Fahrenheit
            //T(°F) = T(°C) × 9/5 + 32
            converted = ( converted * 1.8 ) + 32;          
            lcd.print("TEMP = ");
            lcd.print(converted);             // ADC value
            lcd.print("\337F");
          break;
        case 3:
          //Kelvin
          //T(K) = T(°C) + 273.15          
          converted = converted + 273.15;          
          lcd.print("TEMP = ");
          lcd.print(converted);             // ADC value
          lcd.print("K");
          break;
        case 4:
          //Rankine
          //T(°R) = (T(°C) + 273.15) × 9/5          
          converted = converted + 273.15;
          converted = (converted * 1.8);          
          lcd.print("TEMP = ");
          lcd.print(converted);             // ADC value
          lcd.print("\337R");
          break;                              
        default:
         break;      
      }
  //One second delay between readings                  
  delay(1000);       
}
//Float interpolation function, for mapping the ADC reading
float fmap(float x, float in_min, float in_max, float out_min, float out_max)
{
 return (x - in_min) * (out_max - out_min) / (in_max - in_min) + out_min;
}