Fawaz Sammani

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7 years, 3 months ago.

Temperature Sensor to only sense when there is a value

Hello. I am working with LM35 Temperature Sensor. I only want the temperature to print the temperature when it senses a new value (a value different from the one sensed and stored previously). This is my code, but i can't get it. 

#include "mbed.h"                   
AnalogIn analog_value(PA_0); 
Serial pc(SERIAL_TX, SERIAL_RX);   
int main() {
    float temp; 
    float prevalue=0; 
    float currvalue; 
    while(1) {
              currvalue = analog_value.read();
              if(prevalue==currvalue){
              wait(0.1);  }  
              else{
              temp = ((currvalue*5000)/10);
              pc.printf("temprature = %.0f ^C\r\n", temp);
              prevalue=currvalue; 
              } 
                 
}
}

1 Answer

Zoltan Hudak
7 years, 3 months ago.

Hello Fawaz,

Since the measured temperature is always a bit floating I would suggest to neglect small changes for example as follows:

#include "mbed.h"

AnalogIn    analog_value(PA_0);
Serial      pc(SERIAL_TX, SERIAL_RX);

int main(void) {
    const float DELTA = 1.0;    // in degree C
    const float CONV = 3300 / 10;
    float       prevvalue = 0;
    float       currvalue;

    while (1) {
        currvalue = analog_value.read() * CONV;
        if (((prevvalue - DELTA) < currvalue) && (currvalue < (prevvalue + DELTA))) {
            wait(0.1);
        }
        else {
            prevvalue = currvalue;
            pc.printf("temprature = %5.1f%cC\r\n", currvalue, 176);
        }
    }
}


Regarding the correctness and conversion of values measured with LM35 temperature sensor also have a look at:
https://developer.mbed.org/questions/6317/AnalogIn-problem-pics-included-strange-v/


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Thank you. But this did not work, instead it became worse than before, it will rapidly print he values.

posted by Fawaz Sammani 22 Aug 2017

Try to increase DELTA step by step until it doesn't print values rapidly.

posted by Zoltan Hudak 22 Aug 2017