Digital Thermometer using Arduino and LM35 Temperature Sensor

Thermometers are useful apparatus being used since long time for temperature measurement. In this project we have made an Arduino based digital thermometer to display the current ambient temperature on a 16x2 LCD unit in real time . It can be deployed in houses, offices, industries etc. to measure the temperature. We can divide this Arduino based thermometer into three sections - The first section senses the temperature by using temperature sensor LM35, second section converts the temperature value into a suitable numbers in Celsius scale which is done by Arduino, and last part of system displays temperature on 16x2 LCD. The same is demonstrated in below block diagram.

In this digital temperature sensor with Arduino, Arduino Uno is used to control the whole process. An LM35 temperature sensor is used for sensing environment temperature which gives 1 degree temperature on every 10mV change at its output pin. You can easily check it with voltmeter by connecting Vcc at pin 1 and Ground at pin 3 and output voltage at pin 2 of LM35 sensor. For an example if the output voltage of LM35 sensor is 250m volt, that means the temperature is around 25 degree Celsius.

Arduino reads output voltage of temperature sensor by using Analog pin A0 and performs the calculation to convert this Analog value to a digital value of current temperature. After calculations arduino sends these calculations or temperature to 16x2 LCD unit by using appropriate commands of LCD. We have also built other digital thermometer projects using DHT11, DS18B20 and other temperature sensors.

 

Circuit Components

Arduino

In this project we have used a Arduino to control whole the process of system. Arduino is a controller which runs on ATmega AVR controller. Arduino is an open source hardware platform and very useful for project development purpose. There are many types of Arduino boards like Arduino UNO, arduino mega, arduino pro mini, Lilypad etc. available in the market or you can also build Arduino by yourself.

 

LM35 Temperature Sensor

LM35 is a 3 pin temperature sensor which gives 1 degree Celsius on every 10mVolt change. This sensor can sense up to 150 degree Celsius temperature. 1 number pin of lm35 sensor is Vcc, second is output and third one is Ground. LM35 is the most simplest temperature sensor and can be interfaced easily with any microcontroller. You can check various Temperature Measurement using LM35 based projects here.

 

Pin No	Function	Name
1	Supply voltage; 5V (+35V to -2V)	Vcc
2	Output voltage (+6V to -1V)	Output
3	Ground (0V)	Ground

LM35 can be easily interfaced with Raspberry Pi, NodeMCU, PIC microcontroller, etc to measure the temperature and can also be used standalone with Op-amp to indicate temperature levels.

 

LCD

16x2 LCD unit is widely using in embedded system projects because it is cheap, easily availablet, small in size and easy to interface. 16x2 have two rows and 16 columns, which means it consist 16 blocks of 5x8 dots. 16 pin for connections in which 8 data bits D0-D7 and 3 control bits namely RS, RW and EN. Rest of pins are used for supply, brightness control and for backlight.

 

Power Supply

Arduino Board already have an inbuilt power supply section. Here we only need to connect a 9 volt or 12 volt adaptors with the board.

 

Circuit Diagram and Explanation

Circuit digram for Digital Thermometer using Arduino and LM35 Temperature Sensor​, is shown in the above figure. Make the connections carefully as shown in the schematic. Here 16x2 LCD unit is directly connected to arduino in 4-bit mode. Data pins of LCD namely RS, EN, D4, D5, D6, D7 are connected to arduino digital pin number 7, 6, 5, 4, 3, 2. A temperature sensor LM35 is connected to Analog pin A0 of arduino, which generates 1 degree Celsius temperature on every 10mV output change at its output pin.

If you are new to Arduino then learn to interface 16x2 LCD with Arduino in our previous tutorial.

 

Arduino LM35 Code & Explanation

The code for Temperature Measurement using LM35 is simple and given at the end of this tutorial. First we include library for LCD unit and then we defines data and control pins for LCD and temperature sensor.

After getting analog value at analog pin we reads that value using Analog read function and stores that value in a variable. And then convert the value into temperature by applying the below given formula.

float analog_value=analogRead(analog_pin);           
float Temperature=analog_value*factor*100
where
factor=5/1023
analog_value= output of temperature sensor

Here degree symbol is created using custom character method

So this is how we can build a simple Digital Thermometer to measure temperature with Arduino. Complete code with a demo video is given below.