Add ESPHome firmware templates, GND pour notes, v2 lessons learned
- firmware/: config + secrets templates committed, flashed config git-ignored - README: document the GND pour on F.Cu/B.Cu and the module keep-out rule areas - README: correct the calibration button wiring (S8 bCal_in, not an MCU GPIO) - README: v2 list - dedicated 5 V power input, decoupling, mounting holes, I2C pull-ups, schematic annotation, RoHS finish, project name, antenna keep-out
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@@ -23,20 +23,21 @@ running.
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```
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+5 V
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+-------------------------+---------------------------+
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| | |
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| +----------v----------+ |
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| | ESP32-C3 module |--- 3.3 V out --+|
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| | (onboard 3V3 LDO) | ||
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+--------------+ | ||
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| UART 9600 +----------+----------+ ||
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| | ||
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+-------------------------+
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| |
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| +----------v----------+
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| | ESP32-C3 module |--- 3.3 V out --+
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| | (onboard 3V3 LDO) | |
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+--------------+ | |
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| UART 9600 +----------+----------+ |
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| | |
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+-----v-------+ +------v-------+ v
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| Sensirion S8| | BME280 |<--- I2C (SDA/SCL) --+
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| Sensirion S8| | BME280 |<--- I2C (SDA/SCL) -+
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| NDIR CO2 | | T / RH / P |
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+-------------+ +--------------+
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SW1 (calibration button) -- RC debounce (1 k / 100 n) -- ESP32-C3 GPIO
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SW1 (calibration button) -- RC (1 k / 100 n) -- Sensirion S8 bCal_in (pin 8)
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5 V enters at the ESP32-C3 devkit's USB-C connector and is passed to the S8 via the 5 V net.
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```
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## Connections
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@@ -52,6 +53,10 @@ SW1 (calibration button) -- RC debounce (1 k / 100 n) -- ESP32-C3 GPIO
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| `/SCL` | pad 8 | — | pad 3 |
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| `bCal_in` | — | pad 8 (`bCal_in`) | — |
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The calibration button SW1 is wired to the **S8's own `bCal_in` input**, not to the MCU: pulling
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`bCal_in` low starts the sensor's background calibration. C1 (100 nF) and R1 (1 kΩ to GND) form the
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debounce/RC network on that line.
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Sensirion S8 pins `Alarm_OC`, `PWM_1kHz`, `DVCC_out` and `UART_R/T` are left unconnected.
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## Bill of materials
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@@ -105,28 +110,53 @@ All parts are through-hole or plug-in modules on pin headers; hand soldered with
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The Sensirion S8 requires a 5 V supply (its NDIR heater draws current pulses of up to ~150 mA) and
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about 30 s of preheat time before it reports valid CO₂ values.
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## Grounding and copper
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Both copper layers carry a **GND copper pour** (one zone on `F.Cu` + `B.Cu`, filled, thermal-relief
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connection to pads with 0.5 mm gap, 0.5 mm thermal bridge). Four rule-area keep-outs (pads and
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footprints excluded, copper pour allowed) mark the module areas, including the ESP32-C3 antenna
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region.
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## Firmware
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ESPHome, see [`firmware/`](firmware/) for the config and the Home Assistant wiring.
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S8 UART: 9600 baud, 8N1. BME280 I²C: standard address `0x76` (or `0x77` depending on the breakout).
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ESPHome on `esp32-c3-devkitm-1`, framework **esp-idf**. Config template:
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[`firmware/esp-sensair.yaml.example`](firmware/esp-sensair.yaml.example), secrets template:
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[`firmware/secrets.yaml.example`](firmware/secrets.yaml.example). The flashed config and
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`secrets.yaml` are git-ignored — **never commit live keys.**
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| Item | Value |
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|---|---|
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| I²C | `SDA = GPIO7`, `SCL = GPIO8`, bus scan enabled |
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| BME280 | I²C address `0x76`, temperature / pressure / humidity, 60 s interval |
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| Sensirion S8 | UART `TX = GPIO21`, `RX = GPIO20`, 9600 baud, 60 s interval |
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| Home Assistant | native ESPHome API with noise encryption, OTA updates |
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| Network | static IP, `fast_connect`, fallback hotspot + captive portal |
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## Known issues and lessons learned (v1 → v2)
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1. **No copper pours.** Neither layer has a ground plane. v2: solid GND pour on the bottom layer
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with stitching vias around the ESP32-C3 module.
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2. **No mounting holes.** The board is held by its connectors only. v2: four M2.5 holes.
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3. **No I²C pull-ups populated.** The design relies on the pull-ups of the BME280 breakout.
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1. **Power comes in through the ESP32-C3 devkit's USB-C 5 V pin.** The Sensirion S8 heater draws
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current pulses of up to ~150 mA, and that current travels through the module's USB connector, its
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5 V rail and its onboard regulator path. v2: a **dedicated 5 V power input** (2.54 mm header,
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screw terminal or DC barrel jack) wired straight to the 5 V net, independent of the module's
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USB-C, which then stays a programming/debug connection. Add to that: 220–470 µF bulk plus 100 nF
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at the S8 supply pins, reverse-polarity protection (series Schottky or P-MOSFET), a jumper to
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back-feed 5 V from the module's USB for programming-only setups, and a power-good LED. Before
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changing anything: measure the ripple and droop on the 5 V rail with an oscilloscope while the S8
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heater fires, so v2 fixes a quantified problem.
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2. **Single 100 nF decoupling capacitor** for the whole board. v2: 100 nF per supply pin, bulk
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capacitance at each sensor, and supply stitching vias into the ground pour.
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3. **No mounting holes.** The board is held by its connectors only. v2: four M2.5 holes.
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4. **No I²C pull-ups populated.** The design relies on the pull-ups of the BME280 breakout.
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v2: add 4.7 kΩ pull-up footprints on `SDA`/`SCL`.
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4. **Single 100 nF decoupling capacitor** for the whole board. v2: 100 nF per supply pin plus a
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10–100 µF bulk capacitor close to the S8 heater supply, which is the actual current-pulse source.
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5. **Schematic is not annotated** (references `U`, `U`, `U`), so KiCad BOM export is unusable.
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v2: annotate, set MPNs, and export the BOM from the schematic.
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6. **Leaded HASL** is not RoHS-compliant. v2: HASL lead-free, and ENIG for any impedance-controlled
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design.
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7. **Project name contains `+`.** It propagates into the Gerber file names (`esp+sensair-Front.gbr`),
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which some fabricator upload portals and drill parsers reject. v2: `esp-sensair`.
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8. **Antenna keep-out** for the ESP32-C3 module should be checked explicitly against the module
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datasheet (no copper, no silkscreen, no vias under the antenna area).
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8. **Antenna keep-out** for the ESP32-C3 module is defined as a rule area (pads and footprints
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excluded, copper pour allowed). v2: verify it against the module datasheet's antenna keep-out
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figure, and check the pour's clearance to the antenna region specifically.
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## Repository layout
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@@ -141,7 +171,7 @@ S8 UART: 9600 baud, 8N1. BME280 I²C: standard address `0x76` (or `0x77` dependi
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├── gerber/ Gerber X2, Excellon drills, job file, ready-to-upload zip
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├── bom/ bill of materials (CSV)
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├── docs/ schematic PDF, layer plots, 3D render
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├── firmware/ ESPHome configuration
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├── firmware/ ESPHome config templates, secrets templates, notes
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├── photos/ photographs of the assembled board
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├── LICENSE CERN-OHL-S-2.0 (hardware)
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└── LICENSE.MIT MIT (firmware config, BOM, docs)
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