Temperature sensor TMP36 convert mV to Celcius degrees

Other Hardware Sensors
1

Hello,

I just bought a starter kit and I just started to use the temperature sensor TMP36. It is working properly as expected but I don't understand why we need to substract 0.5V to the reading to convert it to Celcius degrees. I read the documentation for the sensor but I don't get it. I suppose it is written somewhere in the documentation but I can't figure it out.

Thanks,

2

The TMP36 sensor already outputs 0.5volt at zero degrees C.
Or, said in another way, the sensor must be -50C for 0volt output.

Your code can use the voltage offset or the temperature offset.

"better" TMP36 code then in most starter kits attached.
Leo..

const byte tempPin = A0; // connect TPM36 to 3.3volt A0 and (not-shared) ground
float calibration = 0.1039; // calibrate temp by changing the last digit(s) of "0.1039"
float tempC;

void setup() {
  Serial.begin(9600);
  analogReference(INTERNAL); // use the stable internal 1.1volt Aref
}

void loop() {
  tempC = (analogRead(tempPin) * calibration) - 50.0;
  
  Serial.print("Temperature:  ");
  Serial.print(tempC, 1); // one meaningful decimal place is all you get
  Serial.println(" C");
  
  delay(1000); // use a non-blocking delay when combined with other code
}
3

Ok I understand what you mean. Do you see that only from graphic TPC1 on page 4 or it is written somewhere else in the document?

4

Not sure which document you mean. The TMP36 is made by many manufacturers.

10mV/degreeC and 750mV@25C should give you a clue.

Some graphs give 1V@50C and 10mV/C as reference.
Leo..

5

The document I'm talking about is here:

I am using a 5V supply for the sensor and I understand I could use 3.3V as supply voltage.

I see 750mV@25degC and 10mV/degC in the documentation and also that there is an offset voltage of 0.5V.

If I need to substrct 0.5V (and multiply by 100) to get the temperature in Celcius, does it mean I won't be able to read a temperature below 0degC even if I see on page 3 that the "Linear Operating Temperature Range" is -40degC to +125degC?

6

prodonshape:
I am using a 5V supply for the sensor and I understand I could use 3.3V as supply voltage.

Yes, The TMP36 can work on a 3.3volt supply, which is usually a bit 'cleaner' on 5volt Arduinos.
Make sure you give the TMP36 it's own ground return (not shared on a breadboard), because any ground voltage/noise will appear in the temp readout.

prodonshape:
If I need to substrct 0.5V (and multiply by 100) to get the temperature in Celcius

Better to subtract 50 degrees C instead of 0.5volt, as I do in the example code.
You're not making a voltmeter, so you should code in A/D values and temps.

prodonshape:
I won't be able to read a temperature below 0degC even if I see on page 3 that the "Linear Operating Temperature Range" is -40degC to +125degC?

Yes, you can read below 0C, because of that 50C offset.
The code I gave you has a range between about -40C and +55C.
The +55C restriction is because I have used the (more stable) 1.1volt Aref.
A higher Aref means you can measure higher temps, but it also means a lower resolution.
Standard 5volt Aref code has ~0.5C temp steps. The code I gave you has ~0.1C steps.
(5/1024 = ~5mV, 1.1/1024 = ~1mV).
Leo..

7

Thank you Leo!

I'm a beginner but your comments are definitely helping me a lot! I will have a closer look at your code and try it later. I know there is a project in the starter kit which allow to calibrate a sensor. It will probably teach me to change the range a sensor operates. I will also read on Aref which I don't know yet what it is.