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Topic: Temperature Sensor LM60 (Read 3 times) previous topic - next topic

KenshinPT

Nov 14, 2012, 12:20 am Last Edit: Nov 14, 2012, 12:25 am by KenshinPT Reason: 1
Hey mates!

I am experiencing a very strange problem with the LM60 sensor!

1) The formula is simple: Vo = (+6.25 mV/ºC x TºC) + 424 mV.

So, for:
+125°C     +1205 mV
+100°C     +1049 mV
+25°C       +580 mV
0°C          +424 mV
−25°C       +268 mV
−40°C       +174 mV

So far so good! :)

2) The real Vref of my ATmega328 is 1088mV (ok, it was measured with a multimeter and it has error... but for this question it's irrevelante, I think!)

3) Lets assume that I use as Vref 1100, because I don't trust my multimeter.

4) Code:

Code: [Select]

// Print temperature and milivolt on LCD
void WriteTemperatureOnLCD(void)
{
   
  k++;
  if (k == 50)
  {
   Vin=0;
   temp=0;
   
   for(j=0 ; j<=49 ; j++)
    {
     samples[j] = (analogRead(analogtemp));
     Vin = Vin + samples[j];
    }
   
   Vin=Vin/50;
   lcd.setCursor(9, 0);
   lcd.print(Vin, 1);
   lcd.print("mV");
  }
 
  if (k == 100)
  {
    temp = ( ((Vin/1024.00) * 1100.00) / 6.250) + minimum;   //minimum = (-424.0/6.25)

    if ( (temp >=0) & (temp <10) )
    {
      ................. etc.....



4) The sensor as a very big error.... I know that!!! :)
But when the ADC reads (right or wrong) 424mV ... Which temperature reading shoud I get?

In my oppinion: 0ºC

In my code oppinion: -4ºC

And in your oppinion?


To conclude, in my oppinion, if the ADC reads 424mV... whatever the error the sensor and/or Vref has, the temperature should follow the formula and point to 0ºC !! Or not?

Regards and thanks :)

MarkT

We need to see all the code - for instance is Vin declared as "int" or "unsigned int" or "long" or what?

And how is this wired up?  long cables?  Power source?

Have you checked the accuracy specification for the sensor?

Have you calibrated analogRead() with a known voltage?
[ I won't respond to messages, use the forum please ]

KenshinPT

#2
Nov 14, 2012, 12:59 am Last Edit: Nov 14, 2012, 01:11 am by KenshinPT Reason: 1

We need to see all the code - for instance is Vin declared as "int" or "unsigned int" or "long" or what?


Code: [Select]

const int analogtemp=6; // This is the analog pin which is measuring the input voltage from the LM60 temperature sensor
double temp=0, Vin=0, samples[50];
double minimum = (-424.0/6.25);




And how is this wired up?  long cables?  Power source?

It's wired correctly, regular cables and USB powered.



Have you checked the accuracy specification for the sensor?

The accuracy here is irrelevant... If I measure a wrong value but if the ADC says it's 424mV the temperature in the LCD (after formula calculation) should be 0ºC, right? And it's not! It's -4ºC.



Have you calibrated analogRead() with a known voltage?

I used the internal reference without any calibration. My multimeter says that Vref is 1088mV, instead of 1100mV.



Yah, I know the sensor has error but for this issue is irrelevant, I think :)

The error in the sensor is for this example: The real temperature is 0ºC, but the ADC is measuring 580mV, then the LCD shows 25ºC.

Regards and thanks

tw310367

@Kennshin

The measured -4°C should be correct.
Reason: The reference voltage for the AD converter has been defined by yourself with 1100mV.
According to the datasheet of the sensor, the voltage which will be provided by the sensor at +125°C will be 1205mV (1205mV > VRef).
The maximum voltage which will be converted by the ADC is 1100mV. In case you will set 1205mV on the ADC pin, I would expect that there will be an overflow in the µC. Due to that the reference voltage has not been adapted to the max sensor voltage, you will get a lower temparetaure as expected.
Also the behaviour U=f(T) of the sensor is not linear according the reference points you mentioned from the datasheet.

tw310367

@Kennshin
Forget my last sentence. I saw that the formular is also mentioned in the datasheet.

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