Introduction: Digital Thermometer Using Arduino UNO & LM35

Picture of Digital Thermometer Using Arduino UNO & LM35

In this instructables i will show, how to read the temperature using Arduino UNO & LM35

Step 1: Video

You can also watch the video or continue reading for detailed explanation

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Step 2: Components Required

Picture of Components Required

1. Arduino UNO

2. LM35

3. Bread Board

4. Jumper Wires

Step 3: Circuit Diagram

Picture of Circuit Diagram

Step 4: Sketch Explanation

Picture of Sketch Explanation

Declarations

This is where the variables are declared which will store the values during execution

float temperature, referenceTemp, referenceVolt;

int lm35Reading;

int lm35Pin = 5;

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Setup - Setting the arduino pins

void setup()
{

analogReference(INTERNAL);

Serial.begin(9600);

}

the statement analogReference(INTERNAL); is used toConfigures the reference voltage used for analog input (i.e. the value used as the top of the input range).

& the statement Serial.begin(9600); is for setting the baud rate

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Loop

This is the place where all the things happen. This is main core of the sketch or program.

void loop()
{

lm35Reading = analogRead(lm35Pin);

referenceVolt = 1 / 1024 ;

referenceTemp = 0.01 / referenceVolt ;

temperature = lm35Reading / referenceTemp ;

Serial.print("Temperature = ");

Serial.println( temperature );

delay(500);

}

  • lm35Reading = analogRead(lm35Pin); - This statement will the read the value from the LM35 which is an analog value.
  • referenceVolt = 1 / 1024 ; - This will be our reference voltage
  • referenceTemp = 0.01 / referenceVolt ; - Now if your wondering from where did i get the value 0.01. The answer is simple if you refer the datasheet it says "Linear a 10.0 mV/'C scale factor" which means for every 10m volt or 0.01 volt change there is a 1 degree change
  • temperature = lm35Reading / referenceTemp ; - This statement calculates the temperature
  • Serial.print("Temperature = "); & Serial.println( temperature ); - This statement is to print the values in the serial monitor
  • delay(500); - For every 500 milli second the loop will execute

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Important Note : I have used 1 volt has my reference in my sketch, so please kindly change it to 1.1 volt in the statement & comment the line analogReference(INTERNAL); ( i.e referenceVolt = 1 / 1024 ; into referenceVolt = 1.1 / 1024 ; ).

Using 1 volt will limit temperature range to 0 to 100'C approx and if you are using 1.1 volt you will get a range of approx 0 to 115'C.

If you refer the LM35 datasheet the temperature range is -55 to +150'C so we are not utilizing the full potential of LM35, i.e is the one drawback.

If your wondering why i used 1 volt instead of 1.1 volt, using 1 volt as my top range i got the near exact temperature and with 1.1volt i got nearly 1.5'c difference. This may be because a faulty LM35

If you are trying, I strongly Recommend you to use 1.1 volt as the reference

Step 5: Construction & Working

Picture of Construction & Working

1st & 2nd Image - Now connect the circuit as show in the Step 3 and upload the sketch. Once uploaded open the serial monitor which is under the TOOLS section or press Ctrl + Shift + M.

3rd Image - It is reading the temperature of the room

4th Image - When i touched the LM35 with my hand the temperature went up due to my body tempearature

5th Image - When i removed my hand the temperature kept and started showing the room temperature which is shown in the video

To watch the test video please CLICK HERE

Testing begins @ 00:51 sec

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