diff --git a/pslab/external/MLX90614.py b/pslab/external/MLX90614.py
index 6b96367..ec16e81 100644
--- a/pslab/external/MLX90614.py
+++ b/pslab/external/MLX90614.py
@@ -1,64 +1,136 @@
-from pslab.bus import I2CSlave
+"""MLX90614 infrared thermometer.
+
+This module provides an interface for the MLX90614 non-contact infrared
+temperature sensor, connected to the PSLab via I2C.
+
+Examples
+--------
+Read object (target) temperature:
+
+>>> from pslab.external.MLX90614 import MLX90614
+>>> sensor = MLX90614()
+>>> sensor.get_object_temperature()
+25.73
+
+Read ambient (sensor body) temperature:
+
+>>> sensor.get_ambient_temperature()
+24.18
+"""
+
+import logging
+from typing import List, Optional
+
+from pslab.bus.i2c import I2CSlave
+from pslab.connection import ConnectionHandler
+
+logger = logging.getLogger(__name__)
+
 
 class MLX90614(I2CSlave):
+    """MLX90614 non-contact infrared temperature sensor.
+
+    The MLX90614 is a passive infrared (PIR) sensor that measures
+    temperature without physical contact. It can measure both the
+    temperature of a target object and its own ambient temperature.
+
+    The sensor communicates over SMBus (a subset of I2C) at address 0x5A
+    and supports bus speeds up to 100 kHz.
+
+    Parameters
+    ----------
+    device : :class:`ConnectionHandler`, optional
+        Serial connection to PSLab device. If not provided, a new one
+        will be created.
+
+    Attributes
+    ----------
+    NUMPLOTS : int
+        Number of data plots for GUI integration.
+    PLOTNAMES : list of str
+        Labels for data plots.
+    name : str
+        Human-readable sensor name.
+    """
+
     _ADDRESS = 0x5A
-    _OBJADDR = 0x07
-    _AMBADDR = 0x06
+    _OBJ_REGISTER = 0x07
+    _AMB_REGISTER = 0x06
     NUMPLOTS = 1
-    PLOTNAMES = ['Temp']
-    name = 'PIR temperature'
-
-    def __init__(self):
-        super().__init__(self._ADDRESS)
-
-        self.source = self._OBJADDR
-
-        self.name = 'Passive IR temperature sensor'
-        self.params = {'readReg': {'dataType': 'integer', 'min': 0, 'max': 0x20, 'prefix': 'Addr: '},
-                       'select_source': ['object temperature', 'ambient temperature']}
-
-        # try:
-        #     print('switching baud to 100k')
-        #     self.I2C.configI2C(100e3)
-        # except Exception as e:
-        #     print('FAILED TO CHANGE BAUD RATE', e.message)
-
-    def select_source(self, source):
-        if source == 'object temperature':
-            self.source = self._OBJADDR
-        elif source == 'ambient temperature':
-            self.source = self._AMBADDR
-
-    def readReg(self, addr):
-        x = self.getVals(addr, 2)
-        print(hex(addr), hex(x[0] | (x[1] << 8)))
-
-    def getVals(self, addr, numbytes):
-        vals = self.read(numbytes, addr)
-        return vals
-
-    def getRaw(self):
-        vals = self.getVals(self.source, 3)
-        if vals:
-            if len(vals) == 3:
-                return [((((vals[1] & 0x007f) << 8) + vals[0]) * 0.02) - 0.01 - 273.15]
-            else:
-                return False
-        else:
-            return False
-
-    def getObjectTemperature(self):
-        self.source = self._OBJADDR
-        val = self.getRaw()
-        if val:
-            return val[0]
-        else:
-            return False
-
-    def getAmbientTemperature(self):
-        self.source = self._AMBADDR
-        val = self.getRaw()
-        if val:
-            return val[0]
-        else:
-            return False
+    PLOTNAMES = ["Temp"]
+    name = "PIR temperature"
+
+    def __init__(self, device: Optional[ConnectionHandler] = None):
+        super().__init__(self._ADDRESS, device=device)
+        self._source = self._OBJ_REGISTER
+        self.name = "Passive IR temperature sensor"
+
+    def select_source(self, source: str):
+        """Select which temperature source to read.
+
+        Parameters
+        ----------
+        source : str
+            Either ``'object temperature'`` or ``'ambient temperature'``.
+        """
+        if source == "object temperature":
+            self._source = self._OBJ_REGISTER
+        elif source == "ambient temperature":
+            self._source = self._AMB_REGISTER
+
+    def read_reg(self, register: int):
+        """Read and log a 16-bit register value.
+
+        Parameters
+        ----------
+        register : int
+            Register address to read (0x00–0x20).
+        """
+        data = self.read(2, register)
+        value = data[0] | (data[1] << 8)
+        logger.info("Register %s: %s", hex(register), hex(value))
+
+    def get_raw(self) -> Optional[List[float]]:
+        """Read raw temperature from the currently selected source.
+
+        The raw value is read as a 3-byte SMBus word (LSB, MSB, PEC)
+        and converted from the sensor's internal unit (0.02 K per LSB)
+        to degrees Celsius.
+
+        Returns
+        -------
+        list of float or None
+            Single-element list with temperature in °C, or None if
+            the read failed.
+        """
+        data = self.read(3, self._source)
+
+        if data and len(data) == 3:
+            raw = (((data[1] & 0x007F) << 8) + data[0]) * 0.02 - 0.01
+            return [raw - 273.15]
+
+        return None
+
+    def get_object_temperature(self) -> Optional[float]:
+        """Read the temperature of the target object.
+
+        Returns
+        -------
+        float or None
+            Object temperature in °C, or None if the read failed.
+        """
+        self._source = self._OBJ_REGISTER
+        result = self.get_raw()
+        return result[0] if result else None
+
+    def get_ambient_temperature(self) -> Optional[float]:
+        """Read the ambient (sensor body) temperature.
+
+        Returns
+        -------
+        float or None
+            Ambient temperature in °C, or None if the read failed.
+        """
+        self._source = self._AMB_REGISTER
+        result = self.get_raw()
+        return result[0] if result else None
diff --git a/tests/test_mlx90614.py b/tests/test_mlx90614.py
new file mode 100644
index 0000000..16ff4e5
--- /dev/null
+++ b/tests/test_mlx90614.py
@@ -0,0 +1,188 @@
+"""Tests for MLX90614 infrared thermometer (Issue #182).
+
+These are mock-based unit tests that run without a physical PSLab device.
+"""
+
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+from pslab.external.MLX90614 import MLX90614
+
+
+@pytest.fixture
+def mock_device():
+    """Return a mock ConnectionHandler."""
+    device = MagicMock()
+    device.send_byte = MagicMock()
+    device.send_int = MagicMock()
+    device.get_ack = MagicMock(return_value=1)
+    return device
+
+
+@pytest.fixture
+def sensor(mock_device):
+    """Return an MLX90614 with a mocked device."""
+    with patch("pslab.bus.i2c.autoconnect", return_value=mock_device):
+        return MLX90614(device=mock_device)
+
+
+# ============================================================
+# Test 1: Initialization
+# ============================================================
+class TestInitialization:
+
+    def test_default_address(self, sensor):
+        """Sensor should use address 0x5A."""
+        assert sensor.address == 0x5A
+
+    def test_default_source_is_object(self, sensor):
+        """Default measurement source should be the object register."""
+        assert sensor._source == 0x07
+
+    def test_name_is_set(self, sensor):
+        """Sensor should have a descriptive name."""
+        assert sensor.name == "Passive IR temperature sensor"
+
+    def test_accepts_device_parameter(self, mock_device):
+        """Sensor should accept an optional device parameter."""
+        with patch("pslab.bus.i2c.autoconnect", return_value=mock_device):
+            s = MLX90614(device=mock_device)
+            assert s.address == 0x5A
+
+
+# ============================================================
+# Test 2: Source selection
+# ============================================================
+class TestSourceSelection:
+
+    def test_select_object_temperature(self, sensor):
+        """Selecting 'object temperature' sets source to OBJ register."""
+        sensor.select_source("object temperature")
+        assert sensor._source == 0x07
+
+    def test_select_ambient_temperature(self, sensor):
+        """Selecting 'ambient temperature' sets source to AMB register."""
+        sensor.select_source("ambient temperature")
+        assert sensor._source == 0x06
+
+    def test_invalid_source_no_change(self, sensor):
+        """Selecting an invalid source should not change the current one."""
+        original = sensor._source
+        sensor.select_source("invalid source")
+        assert sensor._source == original
+
+
+# ============================================================
+# Test 3: Temperature calculation
+# ============================================================
+class TestTemperatureCalculation:
+
+    def test_known_temperature_conversion(self, sensor, mock_device):
+        """Verify temperature conversion math with known raw bytes.
+
+        Raw value 0x3A98 (15000) at 0.02 K per LSB = 300.00 K - 0.01
+        = 299.99 K = 26.84 °C.
+        """
+        # 15000 = 0x3A98: LSB = 0x98, MSB = 0x3A, PEC = 0x00
+        mock_device.read = MagicMock(return_value=b"\x98\x3A\x00")
+
+        with patch.object(sensor, "read", return_value=bytearray(b"\x98\x3A\x00")):
+            result = sensor.get_raw()
+
+        assert result is not None
+        assert len(result) == 1
+        expected = ((((0x3A & 0x7F) << 8) + 0x98) * 0.02 - 0.01) - 273.15
+        assert result[0] == pytest.approx(expected)
+
+    def test_get_raw_returns_none_on_empty_read(self, sensor):
+        """get_raw should return None when read returns empty data."""
+        with patch.object(sensor, "read", return_value=bytearray()):
+            result = sensor.get_raw()
+            assert result is None
+
+    def test_get_raw_returns_none_on_short_read(self, sensor):
+        """get_raw should return None when fewer than 3 bytes are read."""
+        with patch.object(sensor, "read", return_value=bytearray(b"\x00\x01")):
+            result = sensor.get_raw()
+            assert result is None
+
+
+# ============================================================
+# Test 4: High-level temperature methods
+# ============================================================
+class TestTemperatureMethods:
+
+    def test_get_object_temperature_sets_source(self, sensor):
+        """get_object_temperature should set source to OBJ register."""
+        with patch.object(sensor, "read", return_value=bytearray(b"\x98\x3A\x00")):
+            sensor.get_object_temperature()
+            assert sensor._source == 0x07
+
+    def test_get_ambient_temperature_sets_source(self, sensor):
+        """get_ambient_temperature should set source to AMB register."""
+        with patch.object(sensor, "read", return_value=bytearray(b"\x98\x3A\x00")):
+            sensor.get_ambient_temperature()
+            assert sensor._source == 0x06
+
+    def test_get_object_temperature_returns_float(self, sensor):
+        """get_object_temperature should return a single float."""
+        with patch.object(sensor, "read", return_value=bytearray(b"\x98\x3A\x00")):
+            result = sensor.get_object_temperature()
+            assert isinstance(result, float)
+
+    def test_get_ambient_temperature_returns_none_on_failure(self, sensor):
+        """Should return None when the read fails."""
+        with patch.object(sensor, "read", return_value=bytearray()):
+            result = sensor.get_ambient_temperature()
+            assert result is None
+
+
+# ============================================================
+# Test 5: read_reg uses logging
+# ============================================================
+class TestReadReg:
+
+    def test_read_reg_logs_value(self, sensor, caplog):
+        """read_reg should log the register value, not print it."""
+        with patch.object(sensor, "read", return_value=bytearray(b"\x34\x12")):
+            import logging
+            with caplog.at_level(logging.INFO):
+                sensor.read_reg(0x07)
+
+            assert "0x1234" in caplog.text
+
+
+# ============================================================
+# Test 6: Return types
+# ============================================================
+class TestReturnTypes:
+
+    def test_get_raw_returns_list_of_float(self, sensor):
+        """get_raw should return a list containing one float."""
+        with patch.object(sensor, "read", return_value=bytearray(b"\x98\x3A\x00")):
+            result = sensor.get_raw()
+            assert isinstance(result, list)
+            assert isinstance(result[0], float)
+
+    def test_temperature_is_reasonable(self, sensor):
+        """A known raw value should produce a reasonable temperature."""
+        # Room temp ~25°C: raw ~14915 (0x3A43)
+        with patch.object(sensor, "read", return_value=bytearray(b"\x43\x3A\x00")):
+            temp = sensor.get_object_temperature()
+            assert -40 < temp < 85  # Sensor operating range
+
+
+# ============================================================
+# Test 7: Class attributes
+# ============================================================
+class TestClassAttributes:
+
+    def test_numplots(self):
+        assert MLX90614.NUMPLOTS == 1
+
+    def test_plotnames(self):
+        assert MLX90614.PLOTNAMES == ["Temp"]
+
+    def test_class_name(self):
+        assert MLX90614.name == "PIR temperature"