Temperature Sensor LM35 interfacing with GR-Sakura

LM 35 is one of the low cost and easily available temperature sensor used by most of the hobbyists.

Click here for Datasheet.

It has 3 interface pins,

1.      Vs

2.      Vout

3.      Gnd

Vs and Gnd are power supply connections (4 – 30 Volts) and Vout is the output voltage that is linearly proportional to Celsius temperature scaling.

Keep in mind the feature as stated in its datasheet, “Linear + 10.0 mV/°C scale factor”. This is useful in ADC calibrations.

I have used a 12 bit ADC in Sakura board. Hence the possible digital values range from 0 – 4095 (2¹² = 4096).  The default reference voltage for ADC in GR-Sakura is 3.3 V.

Hence, from the digital value, the voltage can be calculated as,

Voltage = ( (value from ADC) / 4096 ) x 3.3 v

The result is in volts. However, the LM35 corresponds to output in milliVolts(mV). Converting the result in milliVolts,

Voltage = ( (value from ADC) / 4096 ) x 3.3 x 1000 mV

Now consider the ‘+ 10.0 mV/°C scale factor’. This means that, every additional 10mV from LM35 corresponds to an increase in 1 degree. Since the scale factor is 10mV, dividing the result by 10 mV/C will give us the result in degrees

Voltage =[ ( (value from ADC) / 4096 ) x (1000 mV/10mV) x 3.3 ] degree Centigrade

Voltage = [ ( (value from ADC) / 4096 ) x (100) x 3.3 ] degree Centigrade

You can change the above formula for various resolution ADC’s (for 10 bit ADC, 2¹⁰ = 1024) and various reference voltage as

Voltage = [ ( (Value from ADC)/resolution) x 100 x reference voltage ]

The above formula is valid only for LM35 sensor.

 

A simple demo,

Code:

#include<rxduino.h>

#include<liquidcrystal.h>

LiquidCrystal lcd(12,11,10,5,4,3,2);

double T;

int main()

{
                                                  lcd.begin(16,2);

                                                  analogReference(RAW12BIT);

                                                  lcd.print("Temperature is ");

                                                  while(1)

                                                  {

                                                  int val=0;

                                                  int T;

                                                  val = analogRead(2);       // read from analog pin 2 (AN002)

                                                  T = (val * 0.08056640625); // calculation given above

                                                  lcd.setCursor(2,1);

                                                  lcd.print(T);  // print temperature value

                                                  lcd.setCursor(5,1);

                                                  lcd.print("degree C");

                                                  delay(500); // refresh rate

                                                  }

}