Wattplot v2 — Sensor Placement

STALE — written for the v2.4 architecture (BMI160 IMU, GPIO33 battery, GPIO4 DS18B20, GPIO32 soil, WS2812B status LED, limit switches). The current hardware is ESP32-S3, schematic rev B (2026-08-03), where the IMU / WS2812B / limit switches are removed, battery is on GPIO7, soil is on GPIO6, DS18B20 is on GPIO16. The sensor mounting philosophy (where on the apparatus each sensor should live, and why) is still mostly valid; the wire colors / pin numbers are not. Cross-check against:

• Pin map: docs/pinmap.html
• Schematic: docs/schematic.html (rev B, 2026-08-03)
• Firmware config: firmware/wattplot.yaml

A future pass will regenerate this doc. Tracked in ROADMAP.md.

Where each sensor mounts on the apparatus, why that location, and how to route its cable. Reference the side view at the end of this doc.

1. BMI160 IMU (panel tilt feedback)

Mount location: Centered on the underside of the panel frame’s NORTH rail (the 2x6 PT clevis that the actuator pushes). Specifically: screwed to the inside face of the north rail, with the IMU’s X axis aligned along the bed’s long axis (east-west) and the Z axis pointing up (panel normal).

Why the north rail, not on the panel itself:

• The frame is the structural member; the IMU is the structural reference.
• The panel sits inside the frame with some compliance; measuring the frame is more accurate.
• The IMU is closer to the actuator, where the actual tilt movement is being applied. Less slop.
• The cable to the IMU runs through the cable carrier with the actuator cable, no extra routing.

Why the underside (not the topside):

• Weather protection: the panel frame’s top side is exposed to rain and UV. Underside is sheltered.
• The IMU’s Z axis (up) needs to point in the panel normal direction. Mounting on the underside means the IMU’s “up” matches the panel’s “up” when the IMU is right-side-up (Z+ out of the chip).
• The IMU faces DOWN, so the +Z direction (panel up) is opposite of the IMU’s +Z. The firmware needs to invert the Z reading (or mount the IMU Z+ up — depends on chip orientation, check the BMI160 datasheet).

Orientation: IMU X axis = bed long axis (east-west). IMU Y axis = bed short axis (north-south). IMU Z axis = panel normal (up when panel is flat).

Mounting: 2× #4 wood screws through the IMU breakout board’s mounting holes, into the underside of the north rail. Use a small adhesive foam pad between the IMU and the wood to dampen vibration.

Cable: 4-wire I2C (3V3, GND, SDA, SCL) through the cable carrier to the PCB. ~36” total length.

2. INA219 current/power sensor (motor current feedback)

Mount location: Inside the PCB enclosure, on the high side of the actuator’s motor lead. The INA219 measures the current going TO the actuator, so it goes in series with the motor wire.

Why inside the enclosure (not at the actuator):

• The enclosure is already weatherproof.
• The INA219 is connected to the I2C bus; keeping it close to the ESP32 keeps the I2C wires short.
• The actuator wire enters the enclosure through a cable gland, connects to the INA219, then exits to the actuator. Net: 6” of wire between enclosure and actuator.

Wiring: The INA219 has 4 screw terminals: VCC, GND, SDA, SCL (for I2C), and 2 high-side terminals (VIN+ and VIN−). The actuator motor wire goes: PCB J6 → INA219 VIN+ → INA219 VIN− → actuator lead A. The other motor lead (B) goes from PCB J6 directly to actuator lead B.

Cable: 2-wire I2C (SDA, SCL) from INA219 to ESP32 (4” inside the enclosure). 2-wire motor (red/black) from INA219 to actuator (~46” total length, through cable carrier).

3. DS18B20 soil temperature sensor

Mount location: 6” deep in the bed soil, 12” from the south wall, centered between the east and west walls.

Why 6” deep:

• This is the root zone for tomato plants. Soil temperature at root depth affects germination, growth, and yield.
• 6” is below the surface heating zone (the top 1-2” swings 20°F between day and night, the 6” zone is more stable).
• Above the deepest root zone (tomato roots go to 18-24” but the active root mass is 6-12”).

Why 12” from the south wall (not the center):

• The south wall is the warmest wall (faces the sun when the panel is tilted). Placing the sensor near the south wall gives a “warm zone” reading. (For an average reading, center the sensor.)
• The frame’s south rail is at the south wall, so the wall is partially shaded. This is a representative spot.

Mounting: Stainless steel probe (DS18B20 in a waterproof stainless housing, ~6mm diameter). Insert into the soil so the probe tip is at 6” depth. The cable exits the soil near the south wall and routes through a grommet in the south wall, into the cable carrier, to the PCB.

Cable: 3-wire (data, 3V3, GND), 26 AWG, ~30” total length.

4. Capacitive soil moisture sensor

Mount location: 4” deep in the bed soil, 18” from the south wall, centered between the east and west walls.

Why 4” deep:

• Different from the temperature sensor (which is 6” deep) on purpose. This gives a vertical profile of the soil (one temp at 6”, one moisture at 4”).
• 4” is the active root zone for seedlings and shallow-rooted plants.

Why 18” from the south wall:

• Different from the temperature sensor (12” from south). Gives a horizontal profile: temp is 12” south, moisture is 18” south.

Why capacitive (not resistive):

• Capacitive sensors don’t corrode in moist soil (resistive sensors do).
• Capacitive sensors measure dielectric permittivity, which correlates well with volumetric water content.
• Capacitive sensors need AC excitation (the sensor IC handles this).

Mounting: Capacitive sensor v1.2 (or similar) inserted into the soil with the tip at 4” depth. The PCB (the sensor’s own small PCB) is above the soil line, with the cable exiting upward. Hot-glue or epoxy the cable entry to seal against moisture intrusion into the sensor’s own PCB.

Cable: 2-wire (signal, GND), 26 AWG, ~30” total length.

5. Battery voltage sense (10k/10k divider)

Mount location: ON the PCB, not external. The divider resistors (R1 = 10 kΩ from 12V rail, R2 = 10 kΩ from divider tap to GND) are soldered on the PCB near the 12V input. The tap goes to GPIO33.

Why on the PCB:

• The 12V is already on the PCB (at J1). Routing the divider output through a separate wire to a remote divider is asking for noise pickup.
• The PCB has stable ground; the divider is referenced to PCB ground.
• Saves a cable run.

Range: With 10k/10k on a 12V battery, the divider output is 6V at 12V input. GPIO33 max is 3.3V. So we use a 3-resistor divider for 12V → 3.0V (10k from 12V, 10k from tap to GND, and a 33k from tap to GPIO33 as a current limiter — no wait, that’s wrong, you can’t have a 3-resistor divider that way).

Let me re-think. With 12V across 10k+10k, the tap is at 6V. To bring this down to 3V, use 10k+3.4k (10k from 12V to tap, 3.4k from tap to GND, gives V_tap = 12 × 3.4 / 13.4 = 3.04V). Or use the original 10k+10k and just clamp the GPIO33 input with a 3.3V Zener diode to protect against over-voltage (the GPIO reads 3.3V when input > 3.3V, but the Zener clamps at 3.3V — this works as long as the input current is limited, which the 10k series resistor does at 12V: 6V across 10k = 0.6 mA, well within the Zener’s rating).

Cleanest: use a 30k + 10k divider on the PCB. Tap = 12 × 10 / 40 = 3V. That’s directly within the ESP32 ADC range. No Zener needed.

Update the PCB spec: R1 = 30kΩ (from 12V to tap), R2 = 10kΩ (from tap to GND). The tap goes to GPIO33. This is documented in docs/pcb_design.md (J10 section needs updating).

6. Limit switches (0° and 90° end-stops)

6a. Limit switch 0° (south wall, near hinge)

Mount location: On the bed’s south wall top edge, just below the hinge axis. Specifically: a roller switch mounted on a small wood block attached to the south wall, with the roller positioned so the frame’s south rail presses it when the frame is at 0° (fully flat).

Why a roller switch:

• A roller switch activates on contact, not on a specific angle. As the frame tilts down to 0°, the rail presses the roller.
• Limit switches (vs. just relying on IMU for position) provide a hard end-stop, important for safety (the firmware can detect “limit switch activated but IMU says 5°” = IMU drift, recalibrate).

Wiring: NO (normally open) contact. 26 AWG 2-wire (signal, GND) from the switch to the PCB’s J7. The 10 kΩ pullup is on the PCB (not at the switch — long wire runs benefit from PCB-side pullup).

6b. Limit switch 90° (frame, near vertical position)

Mount location: On the frame’s east or west cross rail (the 2x6 at the east or west end of the frame), positioned so the rail touches a fixed stop on the bed when the frame reaches 90° (vertical).

Why on the cross rail (not on the bed):

• The cross rail moves with the frame, so the switch moves with the frame. The switch is wired to the PCB via the cable carrier.
• The fixed stop is a small wood block mounted on the bed’s north wall (or attached to the actuator mount). The switch contacts the stop at 90°.

Alternative: mount the switch on the bed’s north wall and have it contact the frame’s north rail at 90°. Same effect, but the wire is fixed to the bed (no cable carrier needed). This is the cleaner design — let me update.

Updated 90° switch design:

• Mount the 90° switch on a small wood block attached to the bed’s north wall, near the actuator mount.
• The frame’s north rail contacts the switch when the frame is at 90°.
• The wire is fixed to the bed (no cable carrier).

Wiring: 2-wire (signal, GND) from the switch to J7 on the PCB. The J7 connector is 3-pin (signal, signal, GND), so both switches share the GND wire.

7. WS2812B status LED

Mount location: On the PCB (or in the enclosure wall, with the LED protruding). The LED indicates controller state:

• Solid red = NORMAL
• Solid blue = FOLDING
• Solid green = LOCKED
• Solid yellow = MONITORING
• Flashing red = error
• Off = off

Why on the PCB: the LED is a status indicator. Visible from outside the enclosure (if the enclosure has a transparent window) or via a panel mount LED extension. No need to be on the apparatus itself.

Wiring: 1-wire (data) with 100 Ω series resistor on the PCB. The LED’s VDD and GND come from the PCB’s 3V3 and GND rails.

8. Grow light fixture

Mount location: Above the panel (the panel frame is the support structure), pointing down at the bed. Or on a separate mast above the bed. Or on a wire strung between two posts.

Why above the panel (frame-supported):

• The frame can hold the weight of a small grow light (~5-10 lb).
• The light is at a fixed distance from the bed, regardless of panel tilt. Simplifies light integration calculations.
• No need for a separate structure.

Mounting: Bolt the grow light fixture to the top of the panel frame’s east and west cross rails, with a small bracket.

Wiring: 2-wire (12V switched, GND) from PCB J5, through the cable carrier, to the light. ~60” total.

9. Sunapex 10A MPPT charge controller (mini) / larger MPPT (full-size)

v2.4 (mini build): the standalone Sunapex 10A MPPT is the charge controller. It is IP67, has its own internal MPPT, bulk/absorption/float for LiFePO4, and all charging protections. It has no host connection — the ESP32 only reads the resulting battery voltage via the on-PCB 10 kΩ / 10 kΩ divider. ~80 lines of UART/MPPT code were removed from the firmware when this swap landed.

Full-size v2 (620 W panel): the Sunapex is undersized (10 A max, 30 V max PV — the 620 W panel is Voc 40 V, Imp 19 A). The full-size build needs a 30 A+ MPPT sized for the 620 W panel — recommended: Victron SmartSolar 100/30 (30 A, 100 V max Voc, has VE.Direct UART for telemetry and Bluetooth for phone monitoring) or EPEver Tracer 4210AN (40 A, 100 V max Voc, RS-485 Modbus). Both are 12/24 V auto-detect and have explicit LiFePO4 charge profiles. The full-size build re-instantiates the UART/RS-485 connection on the PCB’s J4 footprint (GPIO 26 / 27) for telemetry and host-side MPPT verification.

Mount location (mini): On the bed’s east wall, under the panel edge — the Sunapex is IP67, so no separate enclosure is needed. Mount with two #6 × 1” screws through the Sunapex’s mounting tabs into the bed’s east wall. Orient the SAE connectors facing down so water can’t pool on them.

Wiring (mini):

• Panel PV+ (MC4 red) → Sunapex PV+ (via the included SAE polarity-reversal adapter). The Sunapex ships with SAE connectors on both sides, so no MC4 crimping is needed.
• Panel PV− (MC4 black) → Sunapex PV− (via the SAE adapter).
• Sunapex BAT+ (SAE) → 3 A in-line fuse → battery + (within 6” of the battery). The Sunapex’s battery lead is bare wire on the far end; cut the SAE end off if you prefer, or buy a separate SAE-to-bare-wire pigtail. 14 AWG silicone, ~18” run.
• Sunapex BAT− (SAE) → battery − (common GND). 14 AWG, ~18”.

The Sunapex has no host connection. Its LCD shows charging state, battery voltage, PV voltage, and charge current. There is no telemetry path back to the ESPHome controller.

Mode button: on first power-up, press the Sunapex’s MODE button to cycle to the LiFePO4 charge profile. The LCD should show “Li” or “LiFePO4” — out-of-box default is sometimes sealed lead-acid. The charge profile is the most important setting; verify it before leaving the build unattended.

Wiring (full-size v2, with the larger MPPT):

• Panel PV+ (MC4 red) → MPPT PV+ (10 AWG, ~30” — much higher current than the mini)
• Panel PV− (MC4 black) → MPPT PV− (10 AWG)
• MPPT BAT+ → fuse → battery +
• MPPT BAT− → battery −
• MPPT comms (VE.Direct or RS-485) → PCB J4 → ESP32 GPIO 26/27 (re-uses the J4 footprint reserved on the mini)
• For grid-tie: panel MC4 also branches to the microinverter via a Y-splitter (one panel leg → microinverter, other leg → MPPT).

10. Side view summary

   ↑ Y (up)
   │
40 ┤       ╱│ ←─── panel frame (tilted at 35°)
   │      ╱ │
   │     ╱  │
30 ┤    ╱   │
   │   ╱    │
   │  ╱     │
20 ┤ ╱      │
   │╱       │
   ┌───────┐│
10 ┤soil   ││ ←─── IMU on underside of north rail
   │       ││
   ├───────┤│
 0 ┤bed    ││
   │       ││
   ├───────┤│
-3 ┤skid   ││
   └───────┘└──
   Z (south ↑ north)