diff --git a/data_logging/tests/test_core.py b/data_logging/tests/test_core.py
new file mode 100644
index 0000000..c4db934
--- /dev/null
+++ b/data_logging/tests/test_core.py
@@ -0,0 +1,68 @@
+import json
+import pytest
+from pathlib import Path
+from io import StringIO
+
+import data_logging.core as core
+
+@pytest.fixture
+def temp_output(tmp_path):
+    file_path = tmp_path / "wifi_log.json"
+    return file_path
+
+@pytest.fixture
+def sample_entry():
+    return {
+        "timestamp": "2024-07-10T23:59:59Z",
+        "frequency": 2.437,
+        "signal_strength": -67.0
+    }
+
+def read_json_file(path: Path):
+    if not path.exists():
+        pytest.fail(f"Log file {path} does not exist")
+    with path.open('r', encoding='utf-8') as f:
+        data = json.load(f)
+    return data
+
+def test_log_wifi_data_creates_file(temp_output, sample_entry, monkeypatch):
+    monkeypatch.setattr(core, 'LOG_PATH', temp_output)
+    core.log_wifi_data(**sample_entry)
+    assert temp_output.exists(), "JSON Log file should be created."
+    data = read_json_file(temp_output)
+    assert isinstance(data, list), "Log file should contain a list of entries."
+    assert data[-1]['timestamp'] == sample_entry['timestamp']
+    assert data[-1]['frequency'] == pytest.approx(sample_entry['frequency'])
+    assert data[-1]['signal_strength'] == pytest.approx(sample_entry['signal_strength'])
+
+def test_log_wifi_data_appends_entries(temp_output, sample_entry, monkeypatch):
+    monkeypatch.setattr(core, 'LOG_PATH', temp_output)
+    core.log_wifi_data(**sample_entry)
+    second_entry = sample_entry.copy()
+    second_entry['signal_strength'] = -72.5
+    core.log_wifi_data(**second_entry)
+    data = read_json_file(temp_output)
+    assert len(data) == 2, "File should contain two log entries."
+    assert data[1]['signal_strength'] == pytest.approx(-72.5)
+
+def test_invalid_inputs_raise_error(monkeypatch, temp_output):
+    monkeypatch.setattr(core, 'LOG_PATH', temp_output)
+    # missing timestamp
+    with pytest.raises((TypeError, ValueError)):
+        core.log_wifi_data(timestamp=None, frequency=2.4, signal_strength=-60.0)
+    # invalid types
+    with pytest.raises((TypeError, ValueError)):
+        core.log_wifi_data(timestamp="2024-01-01T00:00:00Z", frequency="2.4GHz", signal_strength=-60.0)
+    with pytest.raises((TypeError, ValueError)):
+        core.log_wifi_data(timestamp="2024-01-01T00:00:00Z", frequency=2.4, signal_strength="strong")
+
+def test_multiple_calls_persist_data(monkeypatch, temp_output, sample_entry):
+    monkeypatch.setattr(core, 'LOG_PATH', temp_output)
+    for idx in range(3):
+        se = sample_entry.copy()
+        se['signal_strength'] -= idx * 1.5
+        core.log_wifi_data(**se)
+    data = read_json_file(temp_output)
+    signals = [entry['signal_strength'] for entry in data]
+    assert len(signals) == 3
+    assert signals == sorted(signals, reverse=True), "Signal strengths should decrease stepwise."
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