diff --git a/donau_listener/tests/test_core.py b/donau_listener/tests/test_core.py
new file mode 100644
index 0000000..bfb3e69
--- /dev/null
+++ b/donau_listener/tests/test_core.py
@@ -0,0 +1,85 @@
+import json
+import os
+import pytest
+from pathlib import Path
+from unittest import mock
+
+import donau_listener.core as core
+
+@pytest.fixture
+def mock_output_path(tmp_path):
+    output_file = tmp_path / "recording_log.json"
+    yield output_file
+
+@pytest.fixture
+def sample_recording_data():
+    return {
+        "timestamp": "2024-05-15T12:00:00Z",
+        "water_level": 0.85,
+        "ground_vibration": 0.12,
+        "ai_label": "wave"
+    }
+
+def test_start_recording_valid(monkeypatch):
+    # Mock audio library to avoid real recording
+    with mock.patch("donau_listener.core.sd") as mock_sd:
+        mock_stream = mock.Mock()
+        mock_sd.InputStream.return_value.__enter__.return_value = mock_stream
+        result = core.start_recording(0.8, 0.03)
+        assert isinstance(result, str)
+        assert "started" in result.lower() or "recording" in result.lower()
+        mock_sd.InputStream.assert_called()
+
+def test_start_recording_invalid_gain():
+    # Gain outside valid range should raise ValueError
+    with pytest.raises(ValueError):
+        core.start_recording(-0.5, 0.02)
+
+    with pytest.raises(ValueError):
+        core.start_recording(1.5, 0.02)
+
+def test_start_recording_invalid_threshold():
+    with pytest.raises(ValueError):
+        core.start_recording(0.5, -0.1)
+
+
+def test_log_data_creates_file(monkeypatch, mock_output_path, sample_recording_data):
+    # Patch output path in module
+    monkeypatch.setattr(core, "OUTPUT_PATH", mock_output_path)
+
+    success = core.log_data(**sample_recording_data)
+    assert success is True
+    assert mock_output_path.exists()
+
+    # Validate JSON structure
+    with open(mock_output_path, 'r', encoding='utf-8') as f:
+        data = json.load(f)
+
+    assert isinstance(data, list)
+    assert data[0]["timestamp"] == sample_recording_data["timestamp"]
+    assert isinstance(data[0]["water_level"], float)
+    assert data[0]["ai_label"] == "wave"
+
+def test_log_data_appends_entries(monkeypatch, mock_output_path, sample_recording_data):
+    monkeypatch.setattr(core, "OUTPUT_PATH", mock_output_path)
+
+    core.log_data(**sample_recording_data)
+    second_entry = {
+        "timestamp": "2024-05-15T12:01:00Z",
+        "water_level": 0.9,
+        "ground_vibration": 0.11,
+        "ai_label": "boat"
+    }
+    core.log_data(**second_entry)
+
+    with open(mock_output_path, 'r', encoding='utf-8') as f:
+        data = json.load(f)
+
+    assert len(data) == 2
+    assert data[1]["ai_label"] == "boat"
+
+
+def test_log_data_invalid_input(monkeypatch, mock_output_path):
+    monkeypatch.setattr(core, "OUTPUT_PATH", mock_output_path)
+    with pytest.raises(ValueError):
+        core.log_data(timestamp="not-a-time", water_level="abc", ground_vibration=0.1, ai_label="test")
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