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 (1 k / 100 n) -- Sensirion S8 bCal_in (pin 8)
   5 V enters at the ESP32-C3 devkit's USB-C connector and is passed to the S8 via the 5 V net.

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)	—

The calibration button SW1 is wired to the S8's own bCal_in input, not to the MCU: pulling bCal_in low starts the sensor's background calibration. C1 (100 nF) and R1 (1 kΩ to GND) form the debounce/RC network on that line.

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.

Grounding and copper

Both copper layers carry a GND copper pour (one zone on F.Cu + B.Cu, filled, thermal-relief connection to pads with 0.5 mm gap, 0.5 mm thermal bridge). Four rule-area keep-outs (pads and footprints excluded, copper pour allowed) mark the module areas, including the ESP32-C3 antenna region.

Firmware

ESPHome on esp32-c3-devkitm-1, framework esp-idf. Config template: firmware/esp-sensair.yaml.example, secrets template: firmware/secrets.yaml.example. The flashed config and secrets.yaml are git-ignored — never commit live keys.

Item	Value
I²C	SDA = GPIO7, SCL = GPIO8, bus scan enabled
BME280	I²C address 0x76, temperature / pressure / humidity, 60 s interval
Sensirion S8	UART TX = GPIO21, RX = GPIO20, 9600 baud, 60 s interval
Home Assistant	native ESPHome API with noise encryption, OTA updates
Network	static IP, fast_connect, fallback hotspot + captive portal

Known issues and lessons learned (v1 → v2)

1. Power comes in through the ESP32-C3 devkit's USB-C 5 V pin. The Sensirion S8 heater draws current pulses of up to ~150 mA, and that current travels through the module's USB connector, its 5 V rail and its onboard regulator path. v2: a dedicated 5 V power input (2.54 mm header, screw terminal or DC barrel jack) wired straight to the 5 V net, independent of the module's USB-C, which then stays a programming/debug connection. Add to that: 220–470 µF bulk plus 100 nF at the S8 supply pins, reverse-polarity protection (series Schottky or P-MOSFET), a jumper to back-feed 5 V from the module's USB for programming-only setups, and a power-good LED. Before changing anything: measure the ripple and droop on the 5 V rail with an oscilloscope while the S8 heater fires, so v2 fixes a quantified problem.
2. Single 100 nF decoupling capacitor for the whole board. v2: 100 nF per supply pin, bulk capacitance at each sensor, and supply stitching vias into the ground pour.
3. No mounting holes. The board is held by its connectors only. v2: four M2.5 holes.
4. 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.
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 is defined as a rule area (pads and footprints excluded, copper pour allowed). v2: verify it against the module datasheet's antenna keep-out figure, and check the pour's clearance to the antenna region specifically.

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 (${KIPRJMOD})
├── sym-lib-table                project-local symbol library table (${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 config templates, secrets templates, notes
├── photos/                      photographs of the assembled board
├── CHANGELOG.md                 release history
├── LICENSE                      CERN-OHL-S-2.0 (hardware)
└── LICENSE.MIT                  MIT (firmware config, BOM, docs)

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

Dual licensed, SPDX identifiers:

Scope	Licence	File
Hardware: schematic, layout, symbols, footprints, fabrication data	CERN-OHL-S-2.0 (CERN Open Hardware Licence v2, strongly reciprocal)	LICENSE
Firmware configuration, BOM, scripts, documentation	MIT	LICENSE.MIT

SPDX notices to keep in derived files:

SPDX-License-Identifier: CERN-OHL-S-2.0
SPDX-FileCopyrightText: 2026 Hannes Gehrmann

Making a board from these files and distributing it is explicitly allowed; if you modify the design and convey it, CERN-OHL-S requires you to publish the modified design under the same licence and to mark what you changed (sections 3.2 and 3.3).