LMT86 and Arduino

Temperature with an Arduino Uno and an
LMT86 Analog Temperature Sensor – by Greg Bushta

The Datasheet from TI for the LMT86 has a chart with the corresponding temperatures in °C and a mV. I noticed that the number read from the center pin is 1/5 that of what is in the chart. I am powering the LMT86 with 5v from the Arduino Uno. I noticed that there is about 11mV different from one °C to the next degree. However, when the number read on the pin goes down the corresponding temperature rises. I looked at the number corresponding to 0°C, 2100, and the number for 100°C, 997. These represent freezing and boiling of water. I subtracted 997 from 2100 and divided the result by 100 to get the approximate difference per degree C that the mV number would be. The result was 11.03. So, I figured that for every rise in degree C starting at 0° I could subtract 11.03. I thought why am I multiplying the number read on the pin by 5 and then doing my calculations with 11.03. Why not divide 11.03 by 5 and work with the number read from the pin. 2100 divided by 5 would give me the number read from the pin at zero degrees, or 420 as my starting point. So, I take the reading from the pin and subtract it from 420 then divide the answer by 2.206 to get my degrees in C. From there I convert °C to °F with °F = (°C * 9/5) + 32.

I think It is fairly accurate since it is reading 380 and feels just a little on the chilly side in the room. After all it is mid-January and 2:45 in the morning. It is raining outside. There is a fire going in the other room. I am off to sleep.
– Greg

(the next morning)

I tested the calculation with a piece of ice wrapped in plastic to keep the sensor from getting wet. The reading with the ice held upon the sensor was 426. That calcs to 27°F. So, that is not quite correct. Close but not quite right. I wonder if the correct starting number for 0°C should be 426.

Wouldn’t that be the correct way to do the measuring? Or, perhaps the scientific way? Get a starting number for zero degrees C and another number for a known temperature, perhaps boiling water. Then I could do the correct calculations. I would need to get the range and figure the steps per degree again. But my calculations would be much more accurate.

At boiling I got 212 on the read pin.
On ice I got 426 on the read pin.
Lets do the calculations with those numbers.

426 – 212 = 214
214 / 100 = 2.14
Boiling is 212. Freezing is 426. The difference is 214. From ice to boiling is 100 degrees, or steps. So, each step or degree is 2.14 on the pin read.

The LMT86 Sensor is hooked up
on the Arduino Uno with
+ pin on LMT86 is hooked to 5v
GND pin goes to Arduino GND
Data (center) pin goes to A0


int LMT86Pin = A0;
int TempRead = 0;
float voltage = 0;
float Temperature = 0;

void setup() {
// put your setup code here, to run once:
Serial.begin(9600);
Serial.println(“LMT86 test!”);
Serial.println(“Testing the Temperature Sensor”);
}

void loop() {
// put your main code here, to run repeatedly:
TempRead = analogRead(LMT86Pin);
Serial.print(“TempRead = “); Serial.println(TempRead);
Temperature = (426 – TempRead) / 2.14; // was 420 and 2.206
Serial.print(“The temperature is “);
Serial.print(int(Temperature));
Serial.println(“°C”);
Temperature = (Temperature * 9.0 / 5.0) + 32;
Serial.print(“The temperature is “);
Serial.print(int(Temperature));
Serial.println(“°F”);
delay(5000);
}

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