what is conversion system of LM35 (temperature sensor) in Celsius?

Question

I see formula like

temp = (5*val*100/1024) 

Can anyone tell me the details of this formula?

Solution 1

the truth always lies within the datasheets:

• http://www.ti.com/lit/ds/symlink/lm35.pdf on how to get the data from the LM35 ;
• http://atmel.com/Images/doc8161.pdf on how to use the Atmega ADC.

The Atmega ADC: Analog to Digital Converter

Your Atmega is powered by 5V, and the datasheet of the Atmega states that its ADC has a definition of 1024 values (i.e. 10bits). So in your formula, 5/1024 is representing each voltage step represented by a bit:

0.0000V -> 0b0000000000
0.0048V -> 0b0000000001
...
5.0000V -> 0b1000000000

Getting value out of the LM35

If you read the application notes in the LM35 datasheet, you'll find the following formula:

Vout=10mV/°C

if you're binding the LM35 with a 200ohms resistor. So if you use the rule of three, you'll get:

Vout=0.01/°C
°C=Vout/0.01
°C=Vout/0.01
°C=Vout*100

HTH

Solution 2

for Arduinos;

val = analogRead(tempPin);

float mv = ( val/1024.0)*5000; 

float cel = mv/10;

Comments

[shajib0o]

why there is 11 bit for voltage representation? :@zmo

[zmo]

because it should be 8 bits, it's been a typo when I wrote my answer :-)

[zmo]

thanks to a rejected edit, just noticed that I was wrong: this is indeed 10bits, as the datasheet specs it, and the voltage representation is on 10 bits. Aside from that detail, what I say stays right.