fix(adsb): disable bias-T on stop and warn when toggled while running (#207)

* fix(adsb): disable bias-T on stop and warn when toggled while running

The RTL-SDR bias-T hardware register persists after the device is closed,
so toggling bias-T off in the UI and stopping the SDR had no effect on the
actual hardware — verified with a multimeter in issue #205.

- Add disable_bias_t_via_rtl_biast() to rtlsdr.py (mirrors enable, uses -b 0)
- Track adsb_bias_t_active in adsb.py; call disable on stop_adsb() so the
  hardware register is cleared when ADS-B is stopped
- Show an inline warning in the UI when the bias-T checkbox is toggled while
  any SDR mode is active, since the setting only takes effect at start time

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(lint): remove unused imports in tscm sweep.py

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

routes/adsb.py

@@ -79,6 +79,7 @@ adsb_bytes_received = 0
 adsb_lines_received = 0
 adsb_active_device = None  # Track which device index is being used
 adsb_active_sdr_type: str | None = None
+adsb_bias_t_active = False  # Track if bias-t was enabled at start (for cleanup on stop)
 _sbs_error_logged = False  # Suppress repeated connection error logs
 
 # Track ICAOs already looked up in aircraft database (avoid repeated lookups)
@@ -859,7 +860,7 @@ def adsb_session():
 @adsb_bp.route('/start', methods=['POST'])
 def start_adsb():
     """Start ADS-B tracking."""
-    global adsb_using_service, adsb_active_device, adsb_active_sdr_type
+    global adsb_using_service, adsb_active_device, adsb_active_sdr_type, adsb_bias_t_active
 
     with app_module.adsb_lock:
         if adsb_using_service:
@@ -991,6 +992,7 @@ def start_adsb():
 
     # Build ADS-B decoder command
     bias_t = data.get('bias_t', False)
+    adsb_bias_t_active = bias_t
     cmd = builder.build_adsb_command(
         device=sdr_device,
         gain=float(gain),
@@ -1139,7 +1141,7 @@ def start_adsb():
 @adsb_bp.route('/stop', methods=['POST'])
 def stop_adsb():
     """Stop ADS-B tracking."""
-    global adsb_using_service, adsb_active_device, adsb_active_sdr_type
+    global adsb_using_service, adsb_active_device, adsb_active_sdr_type, adsb_bias_t_active
     data = request.get_json(silent=True) or {}
     stop_source = data.get('source')
     stopped_by = request.remote_addr
@@ -1162,6 +1164,13 @@ def stop_adsb():
             clear_dump1090_pid()
             logger.info("ADS-B process stopped")
 
+        # Turn off bias-T if it was enabled at start — the hardware register
+        # persists after the device is closed, so we must explicitly disable it.
+        if adsb_bias_t_active and (adsb_active_sdr_type or 'rtlsdr') == 'rtlsdr':
+            from utils.sdr.rtlsdr import disable_bias_t_via_rtl_biast
+            disable_bias_t_via_rtl_biast(adsb_active_device or 0)
+        adsb_bias_t_active = False
+
         # Release device from registry
         if adsb_active_device is not None:
             app_module.release_sdr_device(adsb_active_device, adsb_active_sdr_type or 'rtlsdr')

routes/tscm/sweep.py

@@ -19,12 +19,9 @@ from flask import Response, jsonify, request
 from data.tscm_frequencies import get_all_sweep_presets, get_sweep_preset
 from routes.tscm import (
     _baseline_recorder,
-    _current_sweep_id,
     _emit_event,
     _start_sweep_internal,
-    _sweep_running,
     tscm_bp,
-    tscm_queue,
 )
 from utils.database import get_tscm_sweep, update_tscm_sweep
 from utils.event_pipeline import process_event

templates/index.html

@@ -6349,6 +6349,14 @@
         function saveBiasTSetting() {
             const enabled = document.getElementById('biasT')?.checked || false;
             localStorage.setItem('biasTEnabled', enabled);
+            // Warn if any SDR mode is currently running — bias-T is applied at
+            // start time and cannot be toggled on a running device.
+            const anyRunning = isRunning || isSensorRunning
+                || (typeof isAdsbRunning !== 'undefined' && isAdsbRunning)
+                || (typeof isAisRunning !== 'undefined' && isAisRunning);
+            if (anyRunning) {
+                showInfo('Bias-T change will take effect after restarting the active SDR mode');
+            }
         }
 
         function getBiasTEnabled() {

utils/sdr/rtlsdr.py

@@ -75,6 +75,35 @@ def enable_bias_t_via_rtl_biast(device_index: int = 0) -> bool:
         return False
 
 
+def disable_bias_t_via_rtl_biast(device_index: int = 0) -> bool:
+    """Disable bias-t power using rtl_biast (RTL-SDR Blog drivers).
+
+    Should be called when stopping an SDR mode that had bias-t enabled,
+    since the hardware register persists after the device is closed.
+
+    Returns True if bias-t was disabled successfully.
+    """
+    rtl_biast_path = get_tool_path('rtl_biast') or 'rtl_biast'
+    try:
+        result = subprocess.run(
+            [rtl_biast_path, '-b', '0', '-d', str(device_index)],
+            capture_output=True,
+            text=True,
+            timeout=5
+        )
+        if result.returncode == 0:
+            logger.info(f"Bias-t disabled via rtl_biast on device {device_index}")
+            return True
+        logger.warning(f"rtl_biast failed (exit {result.returncode}): {result.stderr.strip()}")
+        return False
+    except FileNotFoundError:
+        logger.warning("rtl_biast not found — bias-t may remain on after stop")
+        return False
+    except Exception as e:
+        logger.warning(f"Failed to disable bias-t via rtl_biast: {e}")
+        return False
+
+
 def _get_dump1090_bias_t_flag(dump1090_path: str) -> str | None:
     """Detect the correct bias-t flag for the installed dump1090 variant.