diff --git a/temperature_logger/tests/test_core.py b/temperature_logger/tests/test_core.py
new file mode 100644
index 0000000..56564e7
--- /dev/null
+++ b/temperature_logger/tests/test_core.py
@@ -0,0 +1,107 @@
+import json
+import tempfile
+import os
+import time
+from pathlib import Path
+from unittest import mock
+import pytest
+
+import temperature_logger.core as core
+
+
+def fake_serial_readline_factory(seq):
+    """Erzeugt eine Fake-Readline-Funktion, die Werte aus seq zurückgibt."""
+    it = iter(seq)
+
+    def fake_readline():
+        try:
+            val = next(it)
+        except StopIteration:
+            time.sleep(0.1)
+            val = b''
+        return f"{val}\n".encode()
+
+    return fake_readline
+
+
+@pytest.fixture
+def mock_serial(monkeypatch):
+    class FakeSerial:
+        def __init__(self, *args, **kwargs):
+            self.port = args[0]
+            self.is_open = True
+            self.readline = fake_serial_readline_factory([22.3, 23.0, 21.7])
+
+        def close(self):
+            self.is_open = False
+
+    monkeypatch.setattr(core, 'serial', mock.Mock(Serial=FakeSerial))
+    return FakeSerial
+
+
+def test_log_temperature_data_creates_json(tmp_path, mock_serial, monkeypatch):
+    output_file = tmp_path / 'temperature_log.json'
+    monkeypatch.setattr(core, 'OUTPUT_PATH', output_file)
+
+    # Reduce logging duration by limiting loop count via mock
+    monkeypatch.setattr(core.time, 'sleep', lambda x: None)
+
+    # Run function under test
+    result = core.log_temperature_data(sensor='/dev/ttyFAKE', interval=0.01)
+
+    assert result is None
+    assert output_file.exists(), 'Output JSON file sollte existieren.'
+
+    with output_file.open('r', encoding='utf-8') as f:
+        data = json.load(f)
+
+    assert isinstance(data, list)
+    assert len(data) >= 1, 'Es sollten mindestens ein Messwert geloggt werden.'
+
+    sample = data[0]
+    assert all(k in sample for k in ('timestamp', 'temperature', 'location'))
+    assert isinstance(sample['temperature'], (float, int))
+    assert isinstance(sample['timestamp'], str)
+    assert isinstance(sample['location'], str)
+
+
+def test_invalid_sensor_port(monkeypatch):
+    class FakeSerialError(Exception):
+        pass
+
+    def raise_serial_error(*args, **kwargs):
+        raise FakeSerialError('Invalid port')
+
+    monkeypatch.setattr(core, 'serial', mock.Mock(Serial=raise_serial_error))
+
+    with pytest.raises(Exception):
+        core.log_temperature_data(sensor='INVALID', interval=0.1)
+
+
+def test_interval_validation(monkeypatch, tmp_path):
+    monkeypatch.setattr(core, 'OUTPUT_PATH', tmp_path / 'temp.json')
+    monkeypatch.setattr(core, 'serial', mock.Mock())
+
+    with pytest.raises((AssertionError, ValueError, TypeError)):
+        core.log_temperature_data(sensor='/dev/ttyFake', interval=-1)
+
+
+def test_json_structure_fields(monkeypatch, tmp_path, mock_serial):
+    output_file = tmp_path / 'temperature_log.json'
+    monkeypatch.setattr(core, 'OUTPUT_PATH', output_file)
+    monkeypatch.setattr(core.time, 'sleep', lambda x: None)
+
+    core.log_temperature_data(sensor='/dev/ttyFAKE', interval=0.01)
+
+    with output_file.open('r', encoding='utf-8') as f:
+        logs = json.load(f)
+
+    for entry in logs:
+        assert 'timestamp' in entry
+        assert 'temperature' in entry
+        assert 'location' in entry
+        # Dataintegrity check
+        assert isinstance(entry['temperature'], float)
+        assert isinstance(entry['timestamp'], str)
+        assert isinstance(entry['location'], str)
+