Project 3 - Problem with temperature measurement

Hardware The Arduino Starter Kit
1

Hi everyone, I am a beginner with Arduino and I'm new in the forum.
I am trying to get the correct Celsius Temperature, however I'm getting values over 100.

Sensor Value: 291 , Volts: 1.42 , Temp: 102.09
Sensor Value: 287 , Volts: 1.40 , Temp: 100.14
Sensor Value: 286 , Volts: 1.40 , Temp: 99.65
Sensor Value: 291 , Volts: 1.42 , Temp: 102.09

Below the code. Can someone explain me why?

Thanks,

const int sensorPin = A0;
const float baselineTemp = 20.0;

// variables:
int sensorValue = 0;         // the sensor value
int sensorMin = 1023;        // minimum sensor value
int sensorMax = 0;           // maximum sensor value

void setup() {
  // put your setup code here, to run once:
  pinMode(4, OUTPUT);
  digitalWrite(4, HIGH);

  // calibrate during the first five seconds
  while (millis() < 7000) {
    sensorValue = analogRead(sensorPin);

    // record the maximum sensor value
    if (sensorValue > sensorMax) {
      sensorMax = sensorValue;
    }

    // record the minimum sensor value
    if (sensorValue < sensorMin) {
      sensorMin = sensorValue;
    }
  }

  // signal the end of the calibration period
  digitalWrite(4, LOW);

  Serial.begin(9600);
  for(int pinNumber = 2; pinNumber<5; pinNumber++) {
    pinMode(pinNumber, OUTPUT);
    digitalWrite(pinNumber, LOW);
  }

}

void loop() {
  // put your main code here, to run repeatedly:
  int sensorValue = analogRead(sensorPin);
  Serial.print("Sensor Value:  ");
  Serial.print(sensorValue);
  float voltage = (sensorValue/1024.0) * 5.0;
  Serial.print(" , Volts:  ");
  Serial.print(voltage);
  float temperature = (voltage - .4) * 100;
  Serial.print(" , Temp:  ");
  Serial.println(temperature);

}
2

Hi @andreacalcagni25 ,
These values you are getting are because of your formulas.
voltage = (sensorValue/1024.0) * 5.0;
and
temperature = (voltage - .4) * 100;
Where did you find these formulas?

With the readings 291, 287 and 286, which was the value you expected.

RV mineirin

3

What sensor?

4

The sensor is TMP36

5

HI RV,
thanks for the answer. I found these formulas both on the book as well as online.

By dividing sensor value by 1024 and multiplying by 5 I should get the voltage.
I'm subtracting - .5 and multiplying by 100 to convert from 10 mv per degree with 500 mV offset to degrees.

With those Sensor Values those are the temperature values I obtain.
I was expecting to get Celsius degrees close to 20° with that formula.

6

Here is the Adafruit tutorial on using the TMP36 sensor.

7

Hi GroundFungus,
thanks for the link. I tried using that code, but the values obtained are the same.
Maybe there is something wrong with the calibration?

8

Hi @andreacalcagni25 ,
The TMP36 measures temperatures from -40°C to +125°C.

At -40° the output is 100 mV, at 0° the output is 500 mV, at 25° the output is 750mV and at 125° the output is 1750 mV.

Assuming you are using an arduino UNO, the voltages will generate the following values.
at -40° -> 100 mV =~ 20, at 0° -> 500mV =~102, at 25° -> 750mV =~153 and 125° -> 1750 =~ 358.
The formulas entered in your sketch are correct, (except the value -.4 which for the TMP36 should be -.5, but this difference has very little effect on the temperature result).

These values

actually generate these temperatures.

What seems to me to be wrong are the values read: 291, 287, 286, 291....
Check that the sensor is connected correctly.

RV mineirin

9

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