IR thermometer, MLX90614

The MLX90614 on its GY-906 breakout reads the temperature of whatever it points at, without touching it. Aliases: GY-906, non-contact thermometer.

Behind the window a thermopile sees the infrared an object gives off, and a thermistor beside it measures the can (the ambient). The chip keeps both temperatures in RAM for an SMBus read.

Pins

Pin	What it does
VIN	Supply, 3 V to 5 V on the GY-906.
GND	Ground.
SCL	SMBus (I2C) clock.
SDA	SMBus data.

Properties

object is the temperature of what it points at, 36.6 C by default, and ambient its own, 22 C.

While it runs

The first slider is the object's temperature and the second the ambient.

What the model gets right

From the MLX90614 datasheet:

The read. An SMBus "read word": the command, a repeated start, then the low byte, the high byte and a packet error code (a CRC-8 over the whole exchange). The model computes the code the datasheet's way, so a library that checks it is satisfied.

The registers. RAM 0x06 is the ambient and 0x07 the object, in fiftieths of a kelvin: T = word x 0.02 - 273.15 C. 0x08, the second zone, answers with its error flag set on this single-zone part.

The EEPROM. The emissivity at 0x24 (0xFFFF for 1.0) and the address at 0x2E read back, and a write with a correct code is stored; a wrong code is refused.

The ranges. The ambient -40 to 125 C and the object -70 to 380 C.

What it does not model

The emissivity's effect on the reading, the field of view (it sees only the object on the slider), the chip's filter and its settling time, sleep mode and the PWM output.

Libraries

Adafruit MLX90614 Library (Adafruit_MLX90614.h): mlx.begin(), then mlx.readObjectTempC() and mlx.readAmbientTempC().

Common mistakes

Measuring something far away: a real sensor's 90 degree field of view takes in the whole room. Hold it a few centimeters off.

Another chip at 0x5A on the same bus, such as an MPR121.

See it in action

Fever thermometer reads a forehead and calls a fever over 38 C on an I2C LCD, with a beep. Open it at /templates.