diff --git a/python_sensor_logger/tests/test_core.py b/python_sensor_logger/tests/test_core.py
new file mode 100644
index 0000000..da20475
--- /dev/null
+++ b/python_sensor_logger/tests/test_core.py
@@ -0,0 +1,94 @@
+import json
+import os
+import time
+import tempfile
+from pathlib import Path
+from unittest import mock
+
+import pytest
+
+import src.python_sensor_logger.core as core
+
+
+def test_read_temperature_returns_float(monkeypatch):
+    mock_value = 23.456
+    with mock.patch('src.python_sensor_logger.core.read_temperature', return_value=mock_value):
+        result = core.read_temperature()
+        assert isinstance(result, float), 'Rückgabewert sollte float sein'
+        assert result == pytest.approx(mock_value)
+
+
+def test_log_temperature_creates_valid_json(tmp_path):
+    file_path = tmp_path / 'temperature_log.json'
+
+    timestamp = time.time()
+    temperature = 22.5
+
+    # Sicherstellen, dass Datei noch nicht existiert
+    if file_path.exists():
+        file_path.unlink()
+
+    # Funktion aufrufen
+    with mock.patch('src.python_sensor_logger.core.os.path.exists', return_value=False):
+        core.log_temperature(timestamp, temperature)
+
+    # Wenn Datei nicht vom Mock erstellt, selbst prüfen
+    if not file_path.exists():
+        # Versuch, Standardpfad zu prüfen
+        default_path = Path('output/temperature_log.json')
+        if default_path.exists():
+            file_path = default_path
+
+    assert file_path.exists(), 'JSON-Datei sollte nach log_temperature existieren'
+
+    with open(file_path, 'r', encoding='utf-8') as f:
+        data = json.load(f)
+
+    # Je nach Implementation: Liste oder einzelner Eintrag
+    if isinstance(data, list):
+        entry = data[-1]
+    else:
+        entry = data
+
+    assert 'timestamp' in entry, 'timestamp-Feld fehlt'
+    assert 'temperature' in entry, 'temperature-Feld fehlt'
+
+    # Validate value types
+    assert isinstance(entry['timestamp'], float)
+    assert isinstance(entry['temperature'], float)
+    assert entry['temperature'] == pytest.approx(temperature)
+
+
+def test_log_temperature_appends_data(tmp_path):
+    file_path = tmp_path / 'temp_log.json'
+
+    # Erstes Logging
+    first_ts = time.time()
+    first_temp = 20.0
+    core.log_temperature(first_ts, first_temp)
+
+    assert os.path.exists('output/temperature_log.json') or file_path.exists()
+
+    # Zweites Logging
+    second_ts = time.time() + 5
+    second_temp = 21.0
+    core.log_temperature(second_ts, second_temp)
+
+    # Datei überprüfen
+    path_to_check = Path('output/temperature_log.json') if Path('output/temperature_log.json').exists() else file_path
+    with open(path_to_check, 'r', encoding='utf-8') as f:
+        records = json.load(f)
+
+    assert isinstance(records, list), 'JSON sollte eine Liste sein'
+    timestamps = [r['timestamp'] for r in records]
+    temps = [r['temperature'] for r in records]
+
+    assert first_ts in timestamps or pytest.approx(first_ts) in timestamps
+    assert any(pytest.approx(second_temp, rel=1e-3) == t for t in temps)
+
+
+def test_temperature_validation():
+    invalid_inputs = [None, 'NaN', [], {}]
+    for invalid_temp in invalid_inputs:
+        with pytest.raises((TypeError, AssertionError)):
+            core.log_temperature(time.time(), invalid_temp)