diff --git a/sensor_logging/tests/test_core.py b/sensor_logging/tests/test_core.py
new file mode 100644
index 0000000..edaf400
--- /dev/null
+++ b/sensor_logging/tests/test_core.py
@@ -0,0 +1,62 @@
+import json
+import re
+from datetime import datetime
+import pytest
+
+import src.sensor_logging.core as core
+
+
+def test_sensor_data_to_json_contains_required_fields():
+    sensor = core.SensorData(sensor_id='SensorA', temperature=22.5, humidity=55.3, timestamp=datetime(2024, 6, 12, 8, 30, 45))
+    data_json = sensor.to_json()
+
+    parsed = json.loads(data_json)
+    assert set(parsed.keys()) == {'sensor_id', 'temperature', 'humidity', 'timestamp'}
+    assert parsed['sensor_id'] == 'SensorA'
+    assert pytest.approx(parsed['temperature'], 0.001) == 22.5
+    assert pytest.approx(parsed['humidity'], 0.001) == 55.3
+
+
+def test_sensor_data_timestamp_is_iso_format():
+    sensor = core.SensorData(sensor_id='S123', temperature=10.0, humidity=50.0, timestamp=datetime.now())
+    data_json = sensor.to_json()
+    parsed = json.loads(data_json)
+    timestamp = parsed['timestamp']
+
+    assert isinstance(timestamp, str)
+    # check ISO 8601-like pattern
+    iso_pattern = r'^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}(\.\d+)?'
+    assert re.match(iso_pattern, timestamp)
+
+
+def test_log_sensor_data_writes_to_file(tmp_path):
+    log_file = tmp_path / 'sensor_log.json'
+
+    # patch output path temporarily
+    original_output = core.os.path.join
+    def mocked_join(a, b):
+        if b == 'output/sensor_log.json':
+            return str(log_file)
+        return original_output(a, b)
+
+    core.os.path.join = mocked_join
+
+    try:
+        core.log_sensor_data('SensorB', 18.2, 42.1)
+        assert log_file.exists()
+        content = log_file.read_text().strip()
+        assert content  # not empty
+        parsed = json.loads(content.splitlines()[-1])
+        assert parsed['sensor_id'] == 'SensorB'
+        assert pytest.approx(parsed['temperature'], 0.001) == 18.2
+        assert pytest.approx(parsed['humidity'], 0.001) == 42.1
+    finally:
+        core.os.path.join = original_output
+
+
+def test_invalid_temperature_type_raises():
+    with pytest.raises((TypeError, ValueError)):
+        core.log_sensor_data('SensorC', 'not_a_float', 45.1)
+
+    with pytest.raises((TypeError, ValueError)):
+        core.log_sensor_data('SensorC', 23.3, 'bad_humidity')