MERIN JOHN

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7 years, 1 month ago.

what coefficient value should be multiplied to get accurate LM35 reading

  1. include "mbed.h" DigitalOut green(LED_GREEN); DigitalOut blue(LED_BLUE);

AnalogIn T1(PTB2); temperature sensor T1 on A0 float T=0; average temperature float K=3.3*100; scaling coefficient for calculating analog value to temperature int main() { blue=0; blue=0; green=0; green=0; T= T1*K; average temperature value in Celsius (C)

Serial pc(USBTX,USBRX); pc.printf("LM35 temperature sensor\n"); wait(0.2f); wait a little pc.printf("Temperature (in Celsius) is %4.2f \r\n",T);

if ( T<=32) green=1; else blue=1; }

1 Answer

Graham S.
7 years, 1 month ago.

This seems to be a TI part, here is a datasheet:

http://www.ti.com/lit/ds/symlink/lm35.pdf

So the temperature in C is pegged directly to the voltage output of the sensor. Vout is independent of the supply voltage.

  • 0V = 0C
  • 1.0V = 100C
  • 1.5V = 150C
  • 3.3V = 330C

Let's assume the ADC on your board uses 3.3V for Vref+ and ground for Vref- (you should check this). The ADC read gives you a float value between 0 and 1 with 1 corresponding to 3.3V. So the scale factor of 330 looks right. Your program doesn't seem to have a loop. Maybe something more like this:

include "mbed.h"

DigitalOut green(LED_GREEN);
DigitalOut blue(LED_BLUE);
AnalogIn T1(PTB2); 		//Temperature sensor T1 on A0 

float T = 0; 			//average temperature 
const float K = 330; 	//scaling coefficient for calculating analog value to temperature 

Serial pc(USBTX, USBRX);

int main() {
	blue = 0;
	green = 0;
	
	pc.printf("\r\nLM35 temperature sensor\r\n");

	/* Loop */
	while (true) {

		//average temperature value in Celsius (C)
		T = T1 * K;  

		pc.printf("Temperature (in Celsius) is %4.2f \r\n", T);

		if (T <= 32) {
			green = 1;
			blue = 0;
		} else {
			green = 0;
			blue = 1;
		}
		
		//wait a little
		wait(0.2f);  
	}
}