diff --git a/sensor_logging/tests/test_main.py b/sensor_logging/tests/test_main.py
new file mode 100644
index 0000000..1ea37fe
--- /dev/null
+++ b/sensor_logging/tests/test_main.py
@@ -0,0 +1,59 @@
+import json
+import tempfile
+import os
+import pytest
+from pathlib import Path
+
+import src.sensor_logging.main as main
+
+
+def test_sensor_data_to_json_structure():
+    s = main.SensorData(voltage=123.4, temperature=22.1, humidity=55.2)
+    data = s.to_json()
+    assert isinstance(data, dict)
+    assert set(data.keys()) == {"timestamp", "voltage", "temperature", "humidity"}
+    assert isinstance(data["timestamp"], str)
+    assert data["voltage"] == pytest.approx(123.4)
+    assert data["temperature"] == pytest.approx(22.1)
+    assert data["humidity"] == pytest.approx(55.2)
+
+
+def test_log_sensor_data_creates_file_and_appends():
+    with tempfile.TemporaryDirectory() as tmpdir:
+        logfile = Path(tmpdir) / "sensor_log.json"
+
+        # First log entry
+        main.log_sensor_data(1.0, 2.0, 3.0)
+        # Override output path temporarily
+        orig_path = getattr(main, "DEFAULT_OUTPUT_PATH", None)
+        try:
+            main.DEFAULT_OUTPUT_PATH = logfile
+            main.log_sensor_data(4.5, 6.7, 8.9)
+        finally:
+            if orig_path is not None:
+                main.DEFAULT_OUTPUT_PATH = orig_path
+
+        assert logfile.exists()
+        content = logfile.read_text().strip().splitlines()
+        # Assuming each line is separate JSON object
+        parsed = [json.loads(line) for line in content]
+        assert len(parsed) >= 1
+        for p in parsed:
+            assert set(p.keys()) == {"timestamp", "voltage", "temperature", "humidity"}
+            assert isinstance(p["voltage"], (int, float))
+
+
+def test_log_sensor_data_invalid_inputs_raises(tmp_path):
+    # Invalid voltage type
+    with pytest.raises((TypeError, ValueError)):
+        main.log_sensor_data("bad", 25.0, 40.0)
+
+    with pytest.raises((TypeError, ValueError)):
+        main.log_sensor_data(2.0, None, 50.0)
+
+
+def test_sensor_data_timestamp_format():
+    s = main.SensorData(voltage=2.2, temperature=3.3, humidity=4.4)
+    t = s.to_json()["timestamp"]
+    # Basic ISO8601 pattern validation
+    assert "T" in t and t.endswith("Z")
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