Temperature, humidity and air

MLX90614 IR thermometer with ESP32

The MLX90614 reads a surface's temperature from its infrared radiation, so it suits things you cannot touch: a rotating motor, a hot bearing or a heater plate. Melexis gives -70 to 380 °C for the object and -40 to 125 °C for the sensor itself, with ±0.5 °C around room temperature. That accuracy applies near room temperature only. It does not hold across the full range.

The reading is an average of everything the sensor can see. The common BAA version has a 90° field of view, so it needs to be very close to a small target: from a distance, it reports the target mixed with the background. Narrow versions exist, such as a 5° DCI, and they cost more. Check the part code before you buy for a distance measurement.

Polished metal is the other trap. The sensor is set for an emissivity of 1, which is close to skin or matt paint. Shiny metal reflects its surroundings and reads wrong, often far too low. A piece of matt black tape on the target fixes that without code. Emissivity can be changed in the sensor's EEPROM, but leave that until you need it. The bus has to run at 100 kHz or less, which may affect other chips sharing it, so the example sets that. The address is 0x5A by default, and modules with a regulator and pull-ups work on 3.3 V or 5 V.

#include <Wire.h>

const int SDA_PIN = 8, SCL_PIN = 9;
const uint8_t ADDR = 0x5A;

float readTemp(uint8_t reg) {           // 0x06 sensor, 0x07 object
  Wire.beginTransmission(ADDR);
  Wire.write(reg);
  Wire.endTransmission(false);
  Wire.requestFrom(ADDR, (uint8_t)3);   // low, high, PEC
  uint16_t raw = Wire.read();
  raw |= Wire.read() << 8;
  Wire.read();
  return raw * 0.02 - 273.15;
}

void setup() {
  Serial.begin(115200);
  Wire.begin(SDA_PIN, SCL_PIN);
  Wire.setClock(100000);
}

void loop() {
  Serial.printf("object %.2f C  sensor %.2f C\n", readTemp(0x07), readTemp(0x06));
  delay(1000);
}

Use this information at your own risk. It is for general guidance only: the example code compiles, but it has not been tested on every part, and modules differ between sellers. Check the manufacturer's data before you build. Product names belong to their owners, and LCD Ultra Pro is not affiliated with them. See our Terms.