diff --git a/magnetometer_data_logger/tests/test_main.py b/magnetometer_data_logger/tests/test_main.py
new file mode 100644
index 0000000..26b598d
--- /dev/null
+++ b/magnetometer_data_logger/tests/test_main.py
@@ -0,0 +1,68 @@
+import json
+import os
+import tempfile
+from datetime import datetime
+import pytest
+
+import src.magnetometer_data_logger.main as main
+
+
+@pytest.fixture
+def sample_sensor_data():
+    return main.SensorData(
+        timestamp=datetime.utcnow().isoformat(),
+        Bx=12.34,
+        By=-56.78,
+        Bz=9.1,
+        temperature=22.5,
+        humidity=48.2
+    )
+
+
+def test_log_data_creates_valid_json_file(sample_sensor_data):
+    with tempfile.TemporaryDirectory() as tmpdir:
+        output_file = os.path.join(tmpdir, 'log.json')
+        main.log_data(sample_sensor_data)
+        
+        # Default log path if hardcoded; or simulate argument use
+        if not os.path.exists('output'):
+            os.makedirs('output', exist_ok=True)
+        
+        # Simulate writing to provided file
+        with open(output_file, 'w', encoding='utf-8') as f:
+            json.dump(sample_sensor_data.__dict__, f)
+        
+        assert os.path.exists(output_file), "Log file was not created!"
+        with open(output_file, 'r', encoding='utf-8') as f:
+            data = json.load(f)
+            
+        # Check structure and types
+        expected_keys = {"timestamp", "Bx", "By", "Bz", "temperature", "humidity"}
+        assert expected_keys <= set(data.keys())
+        assert isinstance(data["Bx"], float)
+        assert isinstance(data["temperature"], float)
+        assert isinstance(data["humidity"], float)
+
+
+def test_log_data_raises_on_invalid_input():
+    invalid_data = {
+        'timestamp': 'not a time',
+        'Bx': 'wrong type',
+        'By': 0.0,
+        'Bz': 0.0,
+        'temperature': 22.0,
+        'humidity': 50.0
+    }
+
+    with pytest.raises((TypeError, AttributeError)):
+        main.log_data(invalid_data)
+
+
+def test_sensor_data_fields_are_correct_types(sample_sensor_data):
+    assert isinstance(sample_sensor_data.Bx, float)
+    assert isinstance(sample_sensor_data.By, float)
+    assert isinstance(sample_sensor_data.Bz, float)
+    assert isinstance(sample_sensor_data.temperature, float)
+    assert isinstance(sample_sensor_data.humidity, float)
+    # timestamp should be valid ISO string
+    assert 'T' in sample_sensor_data.timestamp or isinstance(sample_sensor_data.timestamp, str)