Measuring room temperature with 8051(89c51,89c52) microcontroller using lm35 temperature sensor

In this tutorial i am going to measure room temperature using LM35 temperature sensor with 8051 (89c51 , 89c52) microcontroller. Lm35 outputs an analog signal which is proportional to the temperature in Celsius. 89c51 microcontroller did not have analog to digital converter built in it unlike their relatives arduino, pic, nxp and stm32 micrococntrollers. So we have to use an external analog to digital converter to measure the lm35 output voltage, convert it to digital form and pass it to 89c51 microcontroller. I decided to use adc0804 analog to digital converter to convert analog voltage from the temperature sensor(LM35) in to 8-bit digital value and pass it to microcontroller when ever requested.

Room temperature monitoring project – working principle

Lm35 temperature sensor will measure room analog temperature and passes it adc0408 for analog to digital conversion. 89c51 microcontroller fetches temperature value from adc0804 analog to digital converter. Temperature value is then displayed on a 16×2 lcd interfaced with 89c51 microcontroller. I am not only displaying temperature value in Celsius on 16×2 lcd, but also converting it to Fahrenheit and displaying the Fahrenheit temperature too on the 16×2 lcd.

Lm35 temperature sensor analog room temperature measuring – Analog to digital converting principle

LM35 outputs voltage in Celsius. 10 milli volts output by LM35 represents 1 degree Celsius. So if LM35 outputs 90 mV it means the temperature is 9 degree Celsius.

If you are new and don’t know much about working and pin out of LM35 temperature sensor. Than please take the below tutorial. Its an introduction to LM35 temperature sensor, its pin out and working principal. The tutorial will help you in understanding the code below.

LM35 Pin Out, Working and Temperature Calculation Formula

Digital Thermometer with 8051 microcontroller – Project requirements

89c51 or 89c52 microcontroller (You can also use any other)
ADC0804 (Analog to digital converter)
LM35 temperature sensor
16×2 lcd
power supply(5 volts)
Bread board (to make circuit)
Crystal (11.0592 MHz)

If you are new and didn’t know about ADC0804 and 16×2 lcd. Below are some good tutorials.

8051 digital temperature monitor – Circuit connections

Port 1 of 89c52 microcontroller is connected to data pins of 16×2 lcd. Port 2 of 89c51 is connected to data pins of ADC0804. Enable pin of 16×2 lcd is connected to port 3 pin 6. RS (register select) pin is connected to port 3 pin 5. RW(read write) pin is connected to port 3 pin 7. CS(chip select) pin for ADC0804 is connected to port 3 pin 0. RD(read) pin of ADC0804 is connected to port 3 pin 2. WR (write) pin of ADC0804 is connected to port 3 pin 1. The INTR (interrupt) pin of ADC0804 is connected to port 3 pin 3 of 89c51 microcontroller.

LM35 temperature sensor has three legs one for input voltage (5v), one is GND (ground) and the third is output voltage or temperature in Celsius. LM35 gives 1 degree Celsius change in temperature on every 10 mv rise in temperature. Vref/2 pin is very important pin. It is used to set reference voltage for ADC0804. Apply 1.28 volts to this pin. This will make the ADC0804 to increment the value on every 10 mv rise in temperature. Now both ADC0804 and LM35 have same voltage configurations.

Ground lcd ground pin. Apply 5 volts to vcc pin of lcd. Set contrast on contrast pin of lcd use potentiometer/variable resistor for setting contrast. The CLK R and CLK IN pins of ADC0804 are clock pins you connect an external clock with these pins.But the better way is to use the configuration given below 10 k resistor in parallel with 150 pf capacitor it will utilize your internal clock. If you want to use external connect and external clock to CLK IN pin and leave CLK R void.

measuring room temperature with 89c51 microcontroller

89c51 digital thermometer – Project code

The code is complex lcdint() function is initializing our lcd. Character will be displayed on lcd in 5×7 matrix and our cursor is blinking. lcdcmd() function is selecting command register of lcd and sends commands to lcd command register. display() function is selecting lcd data register and sends data to lcd. farenheit() function is converting Celsius temperature in Fahrenheit and then displays the temperature on the lcd.

The above code is little hard and complex to understand. But if you took the recommended tutorial links given above in the post yon can easily understand the circuit working and code flow and each statement of code easily. Other tutorials on working with Lm35 temperature sensor and measuring temperature are below. Each tutorial is about interfacing Lm35 with different microcontroller and measuring temperature.

Measuring temperature with Lm35 and Nodemcu microcontroller over WiFi

Download the project files and code(C & HEX) written in C language and compiled using keil u vision 4. Project video is also given with output. Please give us your feed back on the post. If you have any queries please write them below in the comments section.

Temperature with 89c51 Files/Code