steveandClaude Opus 5
ba0c3b58b6
Feeds third-party BBQ probes into a pellet grill's wired probe jacks by presenting the resistance a real PT1000 RTD would show at that temperature. The grill sees an ordinary wired probe, so its display, app, target-temp alarms and Keep Warm all work with no protocol reversing involved. Pluggable probe sources: ThermoWorks RFX (via ThermoWorks Cloud), MEATER (community-derived BLE decode), a synthetic simulator for hardware-free development, and a Combustion stub. Two safety invariants are load-bearing: - Stale, missing or implausible readings drive the channel cold, never hot. A stuck-hot channel could convince the grill a cook finished early. - Unimplemented sources raise rather than returning plausible numbers, since the grill acts on these values. Cloud sources are dated by the cloud's own timestamp rather than by fetch time, because ThermoWorks serves a dead probe's last value with a fresh 200. Co-Authored-By: Claude Opus 5 <[email protected]>
175 lines
6.7 KiB
Markdown
175 lines
6.7 KiB
Markdown
# Hardware
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Target grill: **Traeger Ironwood (2022, touchscreen WiFire)** — the non-XL,
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non-850 model. It has two wired probe jacks on the controller, labelled 1 and 2,
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which take **PT1000 RTD** probes (1000 Ω at 0 °C, ~3.85 Ω/°C).
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We present a resistance to those jacks. The grill has no idea it isn't a probe,
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so the display, the app, target-temp alarms and Keep Warm all work natively.
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---
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## Step 0 — Prove the concept for $0 before buying anything
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**Do this first.** It validates every assumption in this repo in about ten
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minutes, using a multimeter and one resistor.
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### 0a. Ohm out a real Traeger probe
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With the probe at room temperature, measure all three pairs on the 3.5 mm plug:
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| Pair | Expected | Meaning |
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|---|---|---|
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| One pair reads **~1086 Ω** at 72 °F | PT1000 element | This is the sensor |
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| Another pair reads **~0 Ω** | bonded lead | 3-wire RTD, lead-compensated |
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| Third pair reads **~1086 Ω** | element + bonded lead | Consistent with 3-wire |
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If instead you see only two meaningful contacts and no near-zero pair, it's
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wired as a simple 2-wire RTD and the emulator gets simpler.
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Confirm it really is PT1000 by measuring at two known temperatures:
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| Bath | Temp | PT1000 should read |
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|---|---|---|
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| Ice water | 0 °C / 32 °F | **1000 Ω** |
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| Boiling water (sea level) | 100 °C / 212 °F | **1385 Ω** |
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Run `smokescreen table` for the full curve.
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> If these numbers don't match, **stop** — your grill isn't using the PT1000
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> curve this repo assumes, and `pt1000.py` needs new coefficients. Everything
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> else in the design still holds.
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### 0b. Fake a temperature with a plain resistor
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Put a fixed resistor across the element contacts of a 3.5 mm TRS plug (mirror
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whatever the real probe does with the third contact) and plug it into jack 1:
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| Resistor | Grill should display |
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|---|---|
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| 1.0 kΩ | ~32 °F |
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| 1.2 kΩ | ~127 °F |
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| 1.5 kΩ | ~264 °F |
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If the grill shows roughly these, **the entire approach is proven** — the rest
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is just making that resistor programmable. This experiment is worth more than
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any amount of protocol reversing.
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While you're here, note what the grill does with an **open** jack and a
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**shorted** jack. Those are its fault thresholds, and knowing them tells you
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whether the fail-cold policy actually trips a visible probe error.
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---
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## Bill of materials
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| Qty | Part | Notes |
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|---|---|---|
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| 1 | Raspberry Pi (any with I²C) | Zero 2 W is plenty |
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| 2 | **AD5272BRMZ-1** digital rheostat | 1 kΩ, 1024-step, I²C, ±1% end-to-end |
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| 2 | 1 kΩ 0.1% 25 ppm resistor | Sets the base of the window |
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| 2 | 3.5 mm TRS male plug | Or sacrifice a cheap replacement probe cable |
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| 1 | ADuM1250 + isolated DC-DC | Optional, strongly recommended — see Isolation |
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| — | GeeekPi nRF52840 dongle | For reading your BLE probes |
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### Why the AD5272 specifically
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The usual digipot fails here on two counts: 256 steps is ~4 Ω/step (≈1 °C, too
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coarse to hide), and typical end-to-end tolerance is ±20%, which is a ~50 °C
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error before you calibrate anything.
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The AD5272-1 gives **1024 steps across 1 kΩ ≈ 1 Ω/step ≈ 0.26 °C**, finer than
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the grill displays, with ±1% tolerance that calibration trims to nothing.
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Sizing check, from `smokescreen table`:
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```
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0 °C (32 °F) → 1000 Ω
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200 °C (392 °F) → 1759 Ω
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```
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A 759 Ω span fits inside the 1 kΩ rheostat with headroom to spare.
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---
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## Wiring, per channel
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```
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┌─────────── 3.5mm TRS plug → grill jack ───────────┐
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│ │
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tip ───────┴──[ R_fixed 1kΩ 0.1% ]──[ AD5272 A ] │
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│ │
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(W) wiper ───────────────┴─── sleeve
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│
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ring ─────────────────────────────────────┘ (mirror the real probe:
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bond to whichever contact
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measured ~0 Ω in step 0a)
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AD5272: SDA/SCL → Pi I²C, VDD → 3.3 V, GND → Pi GND
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ADDR strapped differently per channel (0x2F and 0x2C)
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```
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Use the AD5272 as a **rheostat** (two-terminal): tie terminal B to the wiper W,
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or leave B open per the datasheet's rheostat-mode figure. Do not wire it as a
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three-terminal divider.
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Both channels need **distinct I²C addresses** — the config loader rejects
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duplicates, because two rheostats answering to one address is a maddening bug
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to chase in the dark next to a hot grill.
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---
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## Isolation
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The grill controller is mains-powered and its probe jacks share a ground
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reference with its ADC. Tying the Pi's ground straight to it creates a ground
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loop that will, at best, skew your readings and, at worst, inject noise into the
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controller.
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The clean fix is an **ADuM1250 I²C isolator plus an isolated DC-DC** so the
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rheostat side floats with the grill and the Pi side floats on its own.
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At minimum, power the Pi from the same outlet as the grill, and check whether
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your step-0b resistor readings drift once the Pi's ground is connected.
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---
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## Calibration
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Per channel, once. This cancels resistor tolerance, wiper resistance, and
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contact resistance in one line fit.
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1. Wire up the channel, but plug it into your **multimeter**, not the grill.
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2. Command a low wiper code and measure across the plug:
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```bash
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.venv/bin/python -c "from smokescreen.outputs.rheostat import AD5272Rheostat; AD5272Rheostat(0x2f).set_code(0)"
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```
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3. Repeat at a high code (e.g. 1000).
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4. Solve the fit:
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```bash
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smokescreen calibrate --code-a 0 --ohms-a 1012.4 --code-b 1000 --ohms-b 2015.6
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```
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5. Paste the emitted `calibration:` block into `config.yaml`.
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Sanity-check the reported temperature range covers 32–400 °F. If it starts
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above ~40 °F, your fixed resistor is too large.
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---
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## Failure behaviour
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When a probe goes stale, disconnects, or reports something implausible, the
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emulator drives the channel to its **minimum** resistance — an obviously-cold
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reading.
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This direction is deliberate. A stuck-**hot** channel could convince the grill
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the food hit its target and trigger Keep Warm or a shutdown in the middle of a
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cook. A stuck-**cold** channel can only ever look wrong. Failures should be
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visible, never mistaken for success.
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The same fail-cold state is applied on clean shutdown.
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**This does not make the system safe to leave unattended.** It is a hobby
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project driving a fire with reverse-engineered numbers. Keep a real probe in
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the cook until you have several successful runs behind you.
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