bazauto · Sep 23, 2026 · Sep 23, 2026
diff --git a/CLAUDE.md b/CLAUDE.md
@@ -154,11 +154,11 @@ directory on the path for the same reason.
 - **Any `mpremote` command stops the running node**, dropping it to the raw REPL until the
   next reset.
 - `AT+MQTTPUB` honours backslash escaping, so JSON needs no `AT+MQTTPUBRAW`.
-- **Both boards read active low** — occupied/triggered is 0 — but they are different devices
-  and only agree by coincidence. `ACTIVE_LOW = True` is one global flag covering both
-  (`main.py` passes `config.ACTIVE_LOW` for every sensor); the day a batch disagrees, it has
-  to become per-sensor. An earlier "active high" note here was #8's, superseded by #11; see
-  `docs/block-detector-wiring.md`.
+- **Polarity is per sensor**, because it belongs to the device, not the board. Every
+  `SENSORS` entry names its device's constant (`LM_ID_ACTIVE_LOW`, `WAVESHARE_IR_ACTIVE_LOW`,
+  `BAZAUTO_BD_ACTIVE_LOW`), and startup refuses an entry without a real bool. Both fitted
+  devices read active low; the `bazauto/block-detection` board is active high. An earlier
+  "active high" note here was #8's, superseded by #11; see `docs/block-detector-wiring.md`.
 
 ### Traps
 
@@ -187,9 +187,10 @@ directory on the path for the same reason.
   `normal` → `unknown` → `reverse`. The backend expects it; its confirmation timeout is 8 s.
 - **A broken sensor wire reads as `clear`, not `occupied`** (#9). Both sensor types switch to
   ground, so an open circuit floats up to the pull-up — the permissive state. The re-assert
-  cannot catch it: the node is alive and republishing. Accepted knowingly — but #9 rejected
-  the closed-circuit fix on a premise since shown false, so it is reopened
-  (`docs/block-detector-wiring.md`).
+  cannot catch it: the node is alive and republishing. The fix for board 1 is the
+  `bazauto/block-detection` board, active high, so a broken signal wire reads occupied;
+  swap a `cs---` entry to `BAZAUTO_BD_ACTIVE_LOW` only after its wire-pull bench check.
+  Board 2 is still unfixed. Status in #9.
 - **A dead IR beam fails toward overrun, not toward stopping.** Same broken wire, different
   consequence: a `block_detection` sensor wrongly says empty track, an `ir_position` beam
   wrongly says *not yet reached*, so a berthing run never gets its stop trigger. Cheaper to

diff --git a/docs/block-detector-wiring.md b/docs/block-detector-wiring.md
@@ -47,7 +47,7 @@ transistor — a switch to ground and nothing else. With `A` powered, the same p
 5 V from an internal pull-up. The vendor's *"the LM-iD will act without outside power, but
 powering external devices needs a power source added"* is describing exactly this.
 
-**Occupied reads 0 in both modes**, which is why `config.py`'s `ACTIVE_LOW = True` is correct
+**Occupied reads 0 in both modes**, which is why `config.py`'s `LM_ID_ACTIVE_LOW = True` is correct
 regardless of how `A` is wired, and why the node has never mis-reported occupancy over this.
 
 ## Why this repo held two contradictory conclusions
@@ -142,7 +142,7 @@ LM-iD B ──┬── 74LVC14A input ──► MCP23017 input
 
 That is the closed-circuit principle #9 was reaching for, achieved with one 14-pin package
 per six channels and no change to the detectors. It requires `A` to be powered, and it
-requires `ACTIVE_LOW` to be flipped to `False`, because the inverter has moved the polarity.
+requires those sensors' `active_low` to be `False`, because the inverter has moved the polarity.
 
 **The important half is not the circuit, it is what it means:** powering `A` is what turns
 `clear` from an *absence of signal* into an *assertion*. With `A` unpowered, clear and
@@ -155,6 +155,12 @@ Current position is unchanged — `A` stays unpowered while the layout is being
 the broken-wire limitation stands as #9 describes it. This document records that the door #9
 believed was shut is open, and what it costs to walk through it.
 
+**Superseded as the plan for board 1 (2026-09).** Rather than powering `A` and adding an
+inverter, the LM-iD.1 is being replaced by the `bazauto/block-detection` board: a CT detector
+with a push-pull, **active-high** 3.3 V output, so a broken signal wire reads occupied against
+the expander's pull-up, with no level shifting. It is under bench test and not yet fitted to
+any sensor. The firmware side is ready: polarity is per sensor (`docs/pin-allocation.md`).
+
 ## The IR sensors are not these
 
 Board 2 carries [Waveshare Infrared Reflective Sensor](https://www.waveshare.com/wiki/Infrared_Reflective_Sensor)
@@ -172,10 +178,9 @@ which means, unlike an unpowered LM-iD, **its high level is asserted at the sens
 than being an absence. There is no 5 V hazard and no mode to get wrong. The module also brings
 out `AOUT` and a sensitivity potentiometer; neither is used.
 
-Both boards happen to read **active low**, so the single global `ACTIVE_LOW = True` is correct
-for both. That is a coincidence of two unrelated devices, not a property of the design —
-`main.py` passes `config.ACTIVE_LOW` for every sensor, so the first batch that disagrees makes
-it a per-sensor field.
+Both boards happen to read **active low**. That is a coincidence of two unrelated devices, not
+a property of the design, which is why polarity is a per-sensor field in `config.py` with one
+constant per device (`docs/pin-allocation.md`).
 
 ### The broken-wire exposure is still there, and it fails the other way
 

diff --git a/docs/pin-allocation.md b/docs/pin-allocation.md
@@ -107,15 +107,29 @@ If in doubt, do not count — measure. `mpremote run` a scan of all 32 pins with
 triggered and see which one moves. That took under a minute and settled it definitively both
 times counting got it wrong.
 
-## Sensor polarity: active LOW
+## Sensor polarity: per sensor
 
-Both boards pull to ground when asserting — occupied on board 1, triggered on board 2 — with
-the internal pull-up holding the line high otherwise. **Occupied reads 0**, so `config.py` has
-`ACTIVE_LOW = True`.
+Polarity is a property of the device on the far end of the wire, not of the board or the
+node, so every `SENSORS` entry in `config.py` carries its own `active_low`, set from a
+per-device constant. There is no default: startup refuses an entry without one, or with
+anything other than `True` or `False`.
 
-**The two boards are different devices that agree by coincidence, not by design.** `main.py`
-passes one global `config.ACTIVE_LOW` for every sensor, so the first batch that disagrees
-makes it a per-sensor field rather than a config edit.
+Both fitted devices pull to ground when asserting — occupied on board 1, triggered on board
+2 — with the internal pull-up holding the line high otherwise. **Occupied reads 0**, so both
+constants are `True`. They are different devices that agree by coincidence, not by design.
+
+The `bazauto/block-detection` board, built to replace the LM-iD.1 on board 1, is the first
+that disagrees. It drives its output push-pull, 3.3 V when occupied, so it is
+`BAZAUTO_BD_ACTIVE_LOW = False`. Against the pull-up, a broken signal wire from it reads
+**occupied**, which is the point of it (#9). Moving a `cs---` entry onto it is a one-line
+change, to make only after that channel's wire-pull check on the bench. The pull-up has to
+stay on for those inputs: without it, the broken wire would float instead.
+
+| Constant | Device | Occupied reads |
+|---|---|---|
+| `LM_ID_ACTIVE_LOW = True` | Legacy Models LM-iD.1 | 0 |
+| `WAVESHARE_IR_ACTIVE_LOW = True` | Waveshare IR reflective | 0 |
+| `BAZAUTO_BD_ACTIVE_LOW = False` | `bazauto/block-detection` rev 1.0 | 1 |
 
 | Board | Device | Output |
 |---|---|---|

diff --git a/src/apps/io-node/config.py b/src/apps/io-node/config.py
@@ -40,43 +40,69 @@
 EXPANDER_IR = 0x21
 EXPANDER_POINTS = 0x22
 
-# The sensors are switches to ground: closed (conducting) when the block is occupied,
+# Output polarity, per device. Polarity belongs to the device on the far end of the wire,
+# not to the board or the node, so every SENSORS entry names the device it is.
+#
+# The two fitted devices are switches to ground: closed (conducting) when asserting,
 # open otherwise, with the MCP23017's internal pull-up holding the line high when open.
 # So occupied reads 0. Confirmed on the bench 2026-08-23 by scanning all 32 pins with a
 # loco in Goods Shed and again with it removed — pin 8 on both boards was the only one
 # that moved.
 #
-# **A broken wire reads as `clear`, not `occupied`** (#9). The pull-up gives a defined
-# level on an open circuit, but that level is the permissive one, so a severed wire
-# asserts empty track and the re-assert keeps confirming it. Accepted knowingly for now;
-# see #9 for the closed-circuit alternatives.
-ACTIVE_LOW = True
+# **For both, a broken wire reads as `clear`, not `occupied`** (#9). The pull-up gives a
+# defined level on an open circuit, but that level is the permissive one, so a severed
+# wire asserts empty track and the re-assert keeps confirming it.
+LM_ID_ACTIVE_LOW = True         # Legacy Models LM-iD.1, board 1: pulls to 0 V occupied
+WAVESHARE_IR_ACTIVE_LOW = True  # Waveshare IR (LM393), board 2: pulls to 0 V triggered
+
+# The bazauto/block-detection board drives its output push-pull: 3.3 V occupied, 0 V
+# clear. Against the pull-up a broken signal wire therefore reads **occupied** — the fix
+# for #9 on board 1. Not yet fitted to any sensor. Swap a board 1 entry to it only once
+# that channel has passed its wire-pull check on the bench (see #9).
+BAZAUTO_BD_ACTIVE_LOW = False
 
 DEBOUNCE_MS = 200
 
 # The full allocation. Pin N is the same block on both boards; board 2 pin 5 is reserved
 # and empty because Engine / Goods Transfer has current sensing and no IR beam.
 SENSORS = (
     # --- board 1, current sensing (block_detection) ---
-    {"sensor_id": "cs---fiddle-yard-1", "expander": EXPANDER_CS, "pin": 0},
-    {"sensor_id": "cs---fiddle-yard-2", "expander": EXPANDER_CS, "pin": 1},
-    {"sensor_id": "cs---siding-1", "expander": EXPANDER_CS, "pin": 2},
-    {"sensor_id": "cs---siding-2", "expander": EXPANDER_CS, "pin": 3},
-    {"sensor_id": "cs---siding-3", "expander": EXPANDER_CS, "pin": 4},
-    {"sensor_id": "cs---engine-goods-transfer", "expander": EXPANDER_CS, "pin": 5},
-    {"sensor_id": "cs---engine-shed-1", "expander": EXPANDER_CS, "pin": 6},
-    {"sensor_id": "cs---engine-shed-2", "expander": EXPANDER_CS, "pin": 7},
-    {"sensor_id": "cs---goods-shed", "expander": EXPANDER_CS, "pin": 8},
+    {"sensor_id": "cs---fiddle-yard-1", "expander": EXPANDER_CS, "pin": 0,
+     "active_low": LM_ID_ACTIVE_LOW},
+    {"sensor_id": "cs---fiddle-yard-2", "expander": EXPANDER_CS, "pin": 1,
+     "active_low": LM_ID_ACTIVE_LOW},
+    {"sensor_id": "cs---siding-1", "expander": EXPANDER_CS, "pin": 2,
+     "active_low": LM_ID_ACTIVE_LOW},
+    {"sensor_id": "cs---siding-2", "expander": EXPANDER_CS, "pin": 3,
+     "active_low": LM_ID_ACTIVE_LOW},
+    {"sensor_id": "cs---siding-3", "expander": EXPANDER_CS, "pin": 4,
+     "active_low": LM_ID_ACTIVE_LOW},
+    {"sensor_id": "cs---engine-goods-transfer", "expander": EXPANDER_CS, "pin": 5,
+     "active_low": LM_ID_ACTIVE_LOW},
+    {"sensor_id": "cs---engine-shed-1", "expander": EXPANDER_CS, "pin": 6,
+     "active_low": LM_ID_ACTIVE_LOW},
+    {"sensor_id": "cs---engine-shed-2", "expander": EXPANDER_CS, "pin": 7,
+     "active_low": LM_ID_ACTIVE_LOW},
+    {"sensor_id": "cs---goods-shed", "expander": EXPANDER_CS, "pin": 8,
+     "active_low": LM_ID_ACTIVE_LOW},
     # --- board 2, IR (ir_position) ---
-    {"sensor_id": "ir---fiddle-yard-1", "expander": EXPANDER_IR, "pin": 0},
-    {"sensor_id": "ir---fiddle-yard-2", "expander": EXPANDER_IR, "pin": 1},
-    {"sensor_id": "ir---siding-1", "expander": EXPANDER_IR, "pin": 2},
-    {"sensor_id": "ir---siding-2", "expander": EXPANDER_IR, "pin": 3},
-    {"sensor_id": "ir---siding-3", "expander": EXPANDER_IR, "pin": 4},
+    {"sensor_id": "ir---fiddle-yard-1", "expander": EXPANDER_IR, "pin": 0,
+     "active_low": WAVESHARE_IR_ACTIVE_LOW},
+    {"sensor_id": "ir---fiddle-yard-2", "expander": EXPANDER_IR, "pin": 1,
+     "active_low": WAVESHARE_IR_ACTIVE_LOW},
+    {"sensor_id": "ir---siding-1", "expander": EXPANDER_IR, "pin": 2,
+     "active_low": WAVESHARE_IR_ACTIVE_LOW},
+    {"sensor_id": "ir---siding-2", "expander": EXPANDER_IR, "pin": 3,
+     "active_low": WAVESHARE_IR_ACTIVE_LOW},
+    {"sensor_id": "ir---siding-3", "expander": EXPANDER_IR, "pin": 4,
+     "active_low": WAVESHARE_IR_ACTIVE_LOW},
     # pin 5 reserved — no IR beam on Engine / Goods Transfer
-    {"sensor_id": "ir---engine-shed-1", "expander": EXPANDER_IR, "pin": 6},
-    {"sensor_id": "ir---engine-shed-2", "expander": EXPANDER_IR, "pin": 7},
-    {"sensor_id": "ir---goods-shed", "expander": EXPANDER_IR, "pin": 8},
+    {"sensor_id": "ir---engine-shed-1", "expander": EXPANDER_IR, "pin": 6,
+     "active_low": WAVESHARE_IR_ACTIVE_LOW},
+    {"sensor_id": "ir---engine-shed-2", "expander": EXPANDER_IR, "pin": 7,
+     "active_low": WAVESHARE_IR_ACTIVE_LOW},
+    {"sensor_id": "ir---goods-shed", "expander": EXPANDER_IR, "pin": 8,
+     "active_low": WAVESHARE_IR_ACTIVE_LOW},
 )
 
 # Physically wired, and therefore the only sensors published. Both on Goods Shed, which
@@ -104,7 +130,7 @@
 #
 # Each input is pulled to 0 V against the expander's pull-up when its side is made, so
 # a made input reads 0. Its own flag
-# rather than `ACTIVE_LOW` above: this is a third device that happens to agree with the
+# rather than the sensor polarities above: this is a third device that happens to agree with the
 # other two, and treating that coincidence as a rule is a mistake this repo has already
 # made once.
 POINTS_ACTIVE_LOW = True

diff --git a/src/apps/io-node/main.py b/src/apps/io-node/main.py
@@ -51,7 +51,11 @@ def get_expander(expanders, i2c, available, address, what):
 
 
 def build_sensors(i2c, available, expanders, wiring):
-    """Configure one input pin per installed sensor."""
+    """Configure one input pin per installed sensor, returned with its polarity.
+
+    The pull-up stays on for every sensor, whatever its polarity. For an active-high
+    device it is what makes a broken wire read `occupied` rather than float (#9).
+    """
     sensors = []
 
     for entry in wiring:
@@ -60,7 +64,7 @@ def build_sensors(i2c, available, expanders, wiring):
         pin = expander.create_pin(
             entry["pin"], mode="input", debounce_ms=config.DEBOUNCE_MS)
         pin.setup()
-        sensors.append((entry["sensor_id"], pin))
+        sensors.append((entry["sensor_id"], pin, entry["active_low"]))
 
     return sensors
 
@@ -171,7 +175,7 @@ def bring_up():
         on_publish_failure=lambda topic_id, exc: print(
             "io-node: publish failed for %s (%s)" % (topic_id, exc)),
     )
-    for sensor_id, _pin in sensors:
+    for sensor_id, _pin, _active_low in sensors:
         layout.register_sensor(sensor_id)
     for point_id, _normal, _reverse in points:
         layout.register_point(point_id)
@@ -185,8 +189,8 @@ def bring_up():
     # Publish what is on the track right now. Without this the first reading waits for
     # an edge, and a block occupied since before boot would never be reported at all.
     now = ticks_ms()
-    for sensor_id, pin in sensors:
-        layout.publish_sensor(sensor_id, is_occupied(pin.state, config.ACTIVE_LOW), now)
+    for sensor_id, pin, active_low in sensors:
+        layout.publish_sensor(sensor_id, is_occupied(pin.state, active_low), now)
     for point_id, normal_pin, reverse_pin in points:
         position, cross_wired = read_point(normal_pin, reverse_pin)
         if cross_wired:
@@ -201,10 +205,10 @@ def run(sensors, points, layout, client):
     while True:
         now = ticks_ms()
 
-        for sensor_id, pin in sensors:
+        for sensor_id, pin, active_low in sensors:
             if pin.update_state():
                 layout.publish_sensor(
-                    sensor_id, is_occupied(pin.state, config.ACTIVE_LOW), now)
+                    sensor_id, is_occupied(pin.state, active_low), now)
 
         for point_id, normal_pin, reverse_pin in points:
             # Both, every pass, and never short-circuited: a throw moves one contact

diff --git a/src/lib/point_wiring.py b/src/lib/point_wiring.py
@@ -117,7 +117,7 @@ def is_closed(level, active_low):
 
     Each input is pulled to 0 V when its side is made, whether by a contact, an opto or a
     Hall switch, so a made input reads 0 against the internal pull-up. Its own flag rather
-    than the sensors' `ACTIVE_LOW`: this is a third device that happens to agree with the
+    than the sensors' per-device polarity: this is a third device that happens to agree with the
     other two, and the repo has already been bitten once by treating a coincidence as a
     rule.
     """

diff --git a/src/lib/sensor_wiring.py b/src/lib/sensor_wiring.py
@@ -26,12 +26,21 @@ def select_installed(sensors, installed):
             sensor_id = entry["sensor_id"]
             expander = entry["expander"]
             pin = entry["pin"]
+            active_low = entry["active_low"]
         except (KeyError, TypeError):
             raise ValueError(
-                "every SENSORS entry needs sensor_id, expander and pin — got %r" % (entry,))
+                "every SENSORS entry needs sensor_id, expander, pin and active_low — got %r"
+                % (entry,))
 
         validate_sensor_id(sensor_id)
 
+        # No default, and nothing truthy standing in for a bool. Polarity is a property of
+        # the device on the far end of the wire, and a guessed one publishes `clear` for
+        # every occupied block that sensor watches.
+        if active_low is not True and active_low is not False:
+            raise ValueError(
+                "%s: active_low must be True or False — got %r" % (sensor_id, active_low))
+
         if sensor_id in by_id:
             raise ValueError("sensor id %r appears twice in SENSORS" % sensor_id)
         by_id[sensor_id] = entry
@@ -69,9 +78,9 @@ def is_occupied(level, active_low):
 
     One line, and it has its own name because getting it backwards publishes `clear`
     for an occupied block — which for a block_detection sensor is enough on its own to
-    let the orchestrator clear the block. Westgate Hollow's block detectors are active
-    **low** — occupied reads 0 (measured, see docs/pin-allocation.md). The polarity stays
-    configurable because the next batch of hardware may not be, and because fitting the
-    inverting stage in docs/block-detector-wiring.md would flip this one.
+    let the orchestrator clear the block. `active_low` comes from the sensor's own
+    SENSORS entry, because it is a property of the device on the far end of the wire:
+    the LM-iD.1 detectors and the IR modules are active low, the bazauto/block-detection
+    board is active high (see docs/pin-allocation.md).
     """
     return (level == 0) if active_low else (level == 1)
diff --git a/tests/test_io_node_config.py b/tests/test_io_node_config.py
@@ -54,14 +54,31 @@ def test_pin_n_is_the_same_block_on_both_boards():
         assert cs[pin] == block, "pin %d names different blocks on the two boards" % pin
 
 
-def test_sensors_are_active_low():
-    """Switches to ground: closed when occupied, so occupied reads 0.
+def test_the_installed_sensors_are_active_low():
+    """Both Goods Shed devices are switches to ground: occupied reads 0.
 
     Confirmed by scanning all 32 pins with a loco in the block and again with it
     removed. Note the consequence recorded in #9: a broken wire floats to the pull-up
     and therefore reads `clear`, which is the permissive state, not the safe one.
+    Swapping `cs---goods-shed` to the bazauto/block-detection board changes this.
     """
-    assert config.ACTIVE_LOW is True
+    by_id = {e["sensor_id"]: e for e in select_installed(config.SENSORS, config.INSTALLED)}
+
+    assert by_id["cs---goods-shed"]["active_low"] is config.LM_ID_ACTIVE_LOW is True
+    assert by_id["ir---goods-shed"]["active_low"] is config.WAVESHARE_IR_ACTIVE_LOW is True
+
+
+def test_the_bazauto_detector_is_active_high():
+    """Push-pull, 3.3 V when occupied, so a broken wire into the pull-up reads occupied."""
+    assert config.BAZAUTO_BD_ACTIVE_LOW is False
+
+
+def test_every_ir_sensor_uses_the_ir_polarity():
+    """The bazauto/block-detection board replaces current sensing only. An IR entry
+    given a detector's polarity would read every beam upside down."""
+    for entry in config.SENSORS:
+        if entry["expander"] == config.EXPANDER_IR:
+            assert entry["active_low"] is config.WAVESHARE_IR_ACTIVE_LOW, entry["sensor_id"]
 
 
 def test_no_outputs_are_configured():