Sensor modules

AXS-042 — SCD40 true-NDIR CO2 sensor PMOD module

Catalog #42, level L3 (I2C command protocol).

Level L3 ERC clean DRC clean Fab package 9 components

3D render of the AXS-042 PCB — Rendered from the PCB layout. No physical board has been manufactured.

Components#

Every placed component, with the reason it is on the board. Extracted directly from the schematic, so this cannot drift from the design.

Ref	Value	What it does on this board	Part number	Footprint
`C1`	100nF 16V X7R	VDD high-frequency decoupling	`GRM188R71C104KA01D` Murata	`C_0603_1608Metric`
`C2`	10uF 10V X5R	Emitter-pulse bulk reservoir (datasheet minimum)	`GRM21BR61A106KE19L` Murata	`C_0805_2012Metric`
`C3`	10uF 10V X5R	Second reservoir to hold droop below 0.1 V during pulses	`GRM21BR61A106KE19L` Murata	`C_0805_2012Metric`
`J1`	PMOD PLUG 2x6	PMOD peripheral plug (Digilent spec 1.2)	not selected generic	`PinHeader_2x06_P2.54mm_Horizontal`
`JP1`	SDA PULL-UP	Cut to remove the SDA pull-up when chaining modules	—	`SolderJumper-2_P1.3mm_Bridged_RoundedPad1.0x1.5mm`
`JP2`	SCL PULL-UP	Cut to remove the SCL pull-up when chaining modules	—	`SolderJumper-2_P1.3mm_Bridged_RoundedPad1.0x1.5mm`
`R1`	4.7k	I2C SDA pull-up	`RC0603FR-074K7L` Yageo Datasheet ↗	`R_0603_1608Metric`
`R2`	4.7k	I2C SCL pull-up	`RC0603FR-074K7L` Yageo Datasheet ↗	`R_0603_1608Metric`
`U1`	SCD40	Photoacoustic NDIR CO2 sensor with integrated T/RH	`SCD40-D-R2` Sensirion Datasheet ↗	`Sensirion_SCD4x-1EP_10.1x10.1mm_P1.25mm_EP4.8x4.8mm`

Signals#

Net labels used in the schematic, which are also the names the HDL top level should use.

3V3 GND SCL SCL_PU SDA SDA_PU

Production status#

Schematic ERC	0 violations
PCB layout	complete
DRC	0 violations
Fab package	exported (24 gerbers + drill + CPL + BOM)
Fabricated	no — design stage only

Source: boards/sensor-modules/axs-042-scd40

Catalog #42 (../README.md), level L3 (I2C command protocol).

Status: schematic generated, ERC clean (0/0), netlist reviewed. PCB layout not started. Not released.

What it teaches#

Sensirion's command-based I2C protocol: 16-bit commands, CRC-8 on every data word, millisecond command latencies, and periodic-measurement state — a step up in protocol discipline from register-style devices. Reports true CO2 ppm (photoacoustic NDIR), not an eCO2 estimate, plus temperature and humidity.

Design#

Sensirion SCD40-D-R2, fixed address 0x62, 3.3 V.
Emitter pulses draw >100 mA peaks: 2 × 10 µF reservoirs + 100 nF fitted at the sensor.
4.7 kΩ bus pull-ups behind cut jumpers JP1/JP2; bus repeated on 9/10.
Layout notes: keep the sensor opening unobstructed; no paste on the exposed-pad opening per Sensirion handling guidance.

Regenerate / remaining#

python3 generate_design.py — then PCB layout, HDL, BOM check, checklist.