Sensor modules

AXS-045 — MQ-series combustible-gas carrier PMOD module

Catalog #45 in the AruviX sensor line, level L3 (SPI ADC + 5 V domain handling).

Level L3 ERC clean DRC clean Fab package 15 components

3D render of the AXS-045 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	ADS7042 AVDD/DVDD decoupling (place at the pins)	`GRM188R71C104KA01D` Murata	`C_0603_1608Metric`
`C2`	1uF 16V X5R	ADS7042 reference/bulk decoupling	`GRM188R61C105KA93D` Murata	`C_0603_1608Metric`
`C3`	10nF 50V C0G	RC filter / charge reservoir at the ADC input	`GRM1885C1H103JA01D` Murata	`C_0603_1608Metric`
`C4`	100nF 16V X7R	MQ socket supply decoupling	`GRM188R71C104KA01D` Murata	`C_0603_1608Metric`
`C5`	10uF 10V X5R	Heater bulk reservoir on the 5 V feed	`GRM21BR61A106KE19L` Murata	`C_0805_2012Metric`
`D1`	BAT54S	Series Schottky pair clamping GAS_DO between GND-0.3 V and 3V3+0.3 V	`BAT54S,215` Nexperia Datasheet ↗	`SOT-23`
`J1`	PMOD PLUG 2x6	PMOD peripheral plug (Digilent spec 1.2)	not selected generic	`PinHeader_2x06_P2.54mm_Horizontal`
`J2`	MQ-2 BREAKOUT SOCKET	Socket for a common MQ-2 breakout: 1 VCC(5V), 2 GND, 3 DO, 4 AO	not selected generic	`PinSocket_1x04_P2.54mm_Vertical`
`J3`	EXT 5V IN	External 5 V for the MQ heater: 1 +5V, 2 GND (common with PMOD GND)	not selected Phoenix Contact	`TerminalBlock_Phoenix_MKDS-1,5-2-5.08_1x02_P5.08mm_Horizontal`
`R1`	10k	AO divider top: 10k/18k scales 5 V full-scale to 3.21 V	`RC0603FR-0710KL` Yageo Datasheet ↗	`R_0603_1608Metric`
`R2`	18k	AO divider bottom (5 V x 18/28 = 3.21 V < 3.3 V AVDD)	`RC0603FR-0718KL` Yageo Datasheet ↗	`R_0603_1608Metric`
`R3`	1k	RC anti-alias / charge-bucket source resistor into the SAR input	`RC0603FR-071KL` Yageo Datasheet ↗	`R_0603_1608Metric`
`R4`	10k	DO series limiter into the BAT54S clamp (5 V pull-up on cheap breakouts)	`RC0603FR-0710KL` Yageo Datasheet ↗	`R_0603_1608Metric`
`R5`	100R	Series protection on the ADC-driven data line	`RC0603FR-07100RL` Yageo Datasheet ↗	`R_0603_1608Metric`
`U1`	ADS7042	12-bit 1 MSPS SAR ADC, SPI, AVDD as reference	`ADS7042IDCUR` Texas Instruments Datasheet ↗	`VSSOP-8_2.3x2mm_P0.5mm`

Signals#

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

3V3 ADC_CS_N ADC_SDO EXT_5V GAS_AO GAS_DIV GAS_DO GAS_DO_RAW GAS_FILT GND MISO SCK

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-045-gas

Catalog #45 in the AruviX sensor line (../README.md), level L3 (SPI ADC + 5 V domain handling).

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

What it teaches#

Reading a genuinely awkward legacy sensor safely. The MQ-2 breakout is a 5 V part with a 0–5 V analog output and a comparator digital output that pulls to 5 V on cheap boards. The carrier demonstrates three standard defenses: an external supply domain, a resistive divider into an ADC, and a Schottky rail clamp — then digitizes with the same ADS7042 SPI pattern as AXS-040.

The 5 V requirement (read this first)#

The MQ heater requires external 5 V on the J3 screw terminal (~150 mA continuous — MQ sensors run hot by design and need minutes of warm-up before readings settle). J3 ground is common with the PMOD ground plane. Never feed 5 V into any PMOD pin.

Design#

J2 = 1×4 pin socket for a common MQ-2 comparator breakout: 1 VCC (5 V), 2 GND, 3 DO, 4 AO. Breakout pin orders vary — verify against your board's silkscreen before fitting.
AO path: 10 k / 18 k divider (5 V × 18/28 = 3.21 V < 3.3 V full scale), then RC 1 k + 10 nF into the ADS7042 AINP; AINM to GND. Multiply the ADC reading by 28/18 in fabric to recover the 0–5 V breakout level.
DO path (danger handled): DO pulls up to VCC = 5 V on cheap breakouts. It passes a 10 k series resistor (R4) into a BAT54S series-Schottky pair (COM = GAS_DO, A→GND, K→3V3), clamping the PMOD-side net to the 3.3 V rails. Net GAS_DO lands on PMOD pin 2. Do not bypass R4/D1.
100 Ω series on the ADC-driven SDO line.
ADC decoupling 100 nF + 1 µF (3V3); socket decoupling 100 nF + 10 µF (EXT_5V).
PMOD Type 2 SPI for the ADC, DO as a GPIO input in the MOSI position.

PMOD pin	1	2	3	4	5	6	7	8	9	10	11	12
Net	~CS	GAS_DO	MISO	SCK	GND	3V3	NC	NC	NC	NC	GND	3V3

Part substitutions vs. the module spec sketch: the spec explored a zener clamp (BZT52 3V6 / 3V3 + large series R) before settling on 10 k series + BAT54S to 3V3/GND, which clamps harder with less leakage spread; that is what is implemented. The KiCad Diode:BAT54S symbol's stock footprint is SOT-23 (BAT54S,215 is a SOT-23 part — a SOD-323 footprint would not fit this 3-pin device).

Regenerate#

python3 generate_design.py

Remaining for release#

[ ] PCB layout (heater current path, clamp next to the PMOD edge), DRC, fab
[ ] Pin the Phoenix terminal orderable part (MPN is TBD:)
[ ] Verify MQ-2 breakout pin order against a purchased board
[ ] hdl/: SPI master + ADS7042 unpack, warm-up timer, threshold logic, testbench, ECP5 demo
[ ] BOM export and availability check, design-review checklist