ESP32-C3 CO₂ & Environment Sensor Board

A small environmental monitoring board: ESP32-C3 module with a Sensirion S8 NDIR CO₂ sensor (UART) and a BME280 temperature / humidity / pressure sensor (I²C). Flashed with ESPHome and integrated into Home Assistant.

Status: prototype v1 — designed in KiCad, fabricated at PCBWay, hand assembled, powered up and running.

Board size	50.05 × 40.05 mm
Layers	2
Assembly	through-hole / pin-header plug-in modules
Firmware	ESPHome (see firmware/)
Schematic	docs/esp-sensair-schematic.pdf
Fabrication data	gerber/ (Gerber X2 + Excellon + ODB job)

Block diagram

                              +5 V
                                |
      +-------------------------+---------------------------+
      |                         |                           |
      |              +----------v----------+                 |
      |              |   ESP32-C3 module   |--- 3.3 V out --+|
      |              | (onboard 3V3 LDO)   |                ||
      +--------------+                     |                ||
      |  UART 9600   +----------+----------+                ||
      |                        |                           ||
+-----v-------+         +------v-------+                    v
| Sensirion S8|         |    BME280    |<--- I2C (SDA/SCL) --+
|  NDIR CO2   |         |  T / RH / P  |
+-------------+         +--------------+

SW1 (calibration button) -- RC debounce (1 k / 100 n) -- ESP32-C3 GPIO

Connections

Signal	ESP32-C3 module	Sensirion S8	BME280
+5V	pad 11	pad 0 (heater supply)	—
GND	pad 12	pad 1	pads 0, 4
+3V3	pad 13 (module output)	—	pads 1, 5
/TX	pad 21	pad 6 (UART_RxD)	—
/RX	pad 20	pad 5 (UART_TxD)	—
/SDA	pad 7	—	pad 2
/SCL	pad 8	—	pad 3
bCal_in	—	pad 8 (bCal_in)	—

Sensirion S8 pins Alarm_OC, PWM_1kHz, DVCC_out and UART_R/T are left unconnected.

Bill of materials

Ref	Part	Package	Function
U1	ESP32-C3 Wi-Fi/BLE module (16 pads)	custom esp_sensair_stuff:ESP32-C3	MCU, radio, 3.3 V regulator on module
U2	Sensirion S8 NDIR CO₂ sensor	custom esp_sensair_stuff:Sensair S8	CO₂ measurement, UART, 5 V heater
U3	BME280 (plug-in breakout)	custom esp_sensair_stuff:BME280	temperature, relative humidity, pressure, I²C
SW1	Tactile push button 6 mm	Button_Switch_THT:SW_PUSH_6mm	S8 manual (background) calibration trigger
R1	1 kΩ	Resistor_THT:R_Axial_DIN0207_L6.3mm_D2.5mm_P7.62mm_Horizontal	button debounce, low-pass to GND
C1	100 nF	Capacitor_THT:C_Disc_D10.5mm_W5.0mm_P5.00mm	button debounce / supply decoupling

Machine-readable BOM: bom/esp-sensair-bom.csv

Fabrication

Order data used for the v1 prototype:

Parameter	Value
Fabricator	PCBWay
Layers	2
Board size (ordered)	40 × 50 mm
Board size (design)	40.05 × 50.05 mm
Board material	FR-4
Board thickness	1.6 mm
Finished copper weight	1 oz (35 µm)
Surface finish	HASL with lead
Design rules used	0.15 mm track / 0.15 mm clearance, via 0.7 mm / 0.3 mm drill
Drill sizes present	0.8 mm, 1.0 mm, 1.1 mm (39 plated holes, 4 vias)
Non-plated holes	none
Solder mask / silkscreen	as ordered (see gerber/)
Stencil	not ordered (THT assembly)
Qty / lead time / cost	fill in from your order confirmation

Fabrication files:

• gerber/esp-sensair.zip — Gerber X2 plot + drill files, ready to upload
• gerber/esp+sensair-job.gbrjob — Gerber job / stackup description

Note on the job file: the v1 project was started from a 4-layer KiCad stackup, so esp+sensair-job.gbrjob declares LayerNumber: 4 and labels the copper layers L1/L4 even though only the outer layers are used. The board was fabricated correctly as a 2-layer board. For the next revision: set the stackup to 2 layers in Board Setup → Stackup before exporting, so the job file is self-consistent for anyone re-ordering the board.

Assembly

All parts are through-hole or plug-in modules on pin headers; hand soldered with a soldering iron. The Sensirion S8 requires a 5 V supply (its NDIR heater draws current pulses of up to ~150 mA) and about 30 s of preheat time before it reports valid CO₂ values.

Firmware

ESPHome, see firmware/ for the config and the Home Assistant wiring. S8 UART: 9600 baud, 8N1. BME280 I²C: standard address 0x76 (or 0x77 depending on the breakout).

Known issues and lessons learned (v1 → v2)

1. No copper pours. Neither layer has a ground plane. v2: solid GND pour on the bottom layer with stitching vias around the ESP32-C3 module.
2. No mounting holes. The board is held by its connectors only. v2: four M2.5 holes.
3. No I²C pull-ups populated. The design relies on the pull-ups of the BME280 breakout. v2: add 4.7 kΩ pull-up footprints on SDA/SCL.
4. Single 100 nF decoupling capacitor for the whole board. v2: 100 nF per supply pin plus a 10–100 µF bulk capacitor close to the S8 heater supply, which is the actual current-pulse source.
5. Schematic is not annotated (references U, U, U), so KiCad BOM export is unusable. v2: annotate, set MPNs, and export the BOM from the schematic.
6. Leaded HASL is not RoHS-compliant. v2: HASL lead-free, and ENIG for any impedance-controlled design.
7. Project name contains +. It propagates into the Gerber file names (esp+sensair-Front.gbr), which some fabricator upload portals and drill parsers reject. v2: esp-sensair.
8. Antenna keep-out for the ESP32-C3 module should be checked explicitly against the module datasheet (no copper, no silkscreen, no vias under the antenna area).

Repository layout

.
├── esp+sensair.kicad_pro        KiCad project
├── esp+sensair.kicad_sch        schematic
├── esp+sensair.kicad_pcb        board layout
├── esp_sensair_stuff.kicad_sym  custom symbols (ESP32-C3, Sensirion S8, BME280)
├── esp_sensair_stuff.pretty/    custom footprints, drawn from component datasheets
├── fp-lib-table                 project-local footprint library table (${KIPRJMOD})
├── gerber/                      Gerber X2, Excellon drills, job file, ready-to-upload zip
├── bom/                         bill of materials (CSV)
├── docs/                        schematic PDF, layer plots, 3D render
├── firmware/                    ESPHome configuration
└── photos/                      photographs of the assembled board

Rebuilding the board

1. Open esp+sensair.kicad_pro in KiCad 10.
2. Run DRC; fix anything you change.
3. Export Gerbers/drill (the settings are stored in the project) and upload gerber/*.zip to your fabricator with the parameters from the fabrication table.
4. Order parts per BOM, solder through-hole parts and pin headers.
5. Flash the ESPHome configuration and add the device to Home Assistant.

License

Not chosen yet. Recommended for a published open-hardware module: CERN-OHL-S-2.0 for the hardware (schematic, layout, footprints) and MIT for the firmware configuration and scripts. A LICENSE file must be added before publishing.