incutec-OpenRX/AGENTS.md

OpenRX

Open source ExpressLRS receiver family. Four board variants share an ESP32-C3 core, a TLV75533 3.3 V LDO and the ExpressLRS unified firmware; they differ in radio IC, frequency band, RF front end and antenna interface. Consumer specs (dimensions, telemetry power, per-variant feature table) live in the README; this file is the technical write-up.

Repo

Maintainer	@bastian2001
Status	See the status-* topic on the repo. Never written here.
Designed in	KiCad 10
Layout	Multi-variant: one KiCad project per variant, each directory exactly one level below repo root, which is what makes the shared library and 3D model paths resolve
Variants	OpenRX-Lite/, OpenRX-Lite-UFL/, OpenRX-Mono/, OpenRX-Gemini/, each with OpenRX-<Variant>.kicad_pro, .kicad_sch, .kicad_pcb, .kicad_dru
Local library	shared/libs/OpenRX-Shared.pretty/ and .3dshapes/, nickname OpenRX-Shared, referenced via ${KIPRJMOD}/../shared/libs/. The sym-lib-tables still name OpenRX-Shared.kicad_sym, deleted from the repo: symbols exist only embedded in the design files and carry an LCSC property for JLCPCB BOM export; passives and some packages use stock KiCad footprints resolved through their embedded copies
Shared library	OpenDrone-hw/KiCad-Library, catalogue only; every library this repo uses is local to the repo
Firmware targets	shared/elrs-targets/: per-variant ExpressLRS hardware JSON plus targets_entries.json
Fab	OpenRX-<Variant>/fab/ holds the committed release sets; production/ working exports are gitignored, regenerable with the Fabrication Toolkit (fabrication-toolkit-options.json per variant)
Board setup	Four boards, each 6 layers, 1.0 mm. Line standard: 0.09 mm clearance and track, via 0.35 on 0.20 drill
Archive	Five superseded designs live in git history, removed from the tip 2026-08-14, last at archive/legacy-projects/. shared/sheets/ is a legacy sheet no variant instantiates
License	CERN-OHL-S-2.0

Rules

Identical in every OpenDrone board repo. Do not edit here; edit the template.

• Never text-edit .kicad_sch, .kicad_pcb or .kicad_dru. Use KiCad, or kicad-skip / the pcbnew API for scripted changes. .kicad_pro is JSON and may be edited directly for metadata.
• Metadata yes, connections no. An agent may write BOM and documentation fields (MPN, Manufacturer, LCSC, Cost, Datasheet, text variables). An agent may not change nets, wiring, routing, placement, footprint assignment, or any value that changes the circuit.
• Close KiCad before any write to a KiCad file. KiCad caches library tables at process start and overwrites files on save.
• Reuse before you draw. Check KiCad-Library and its PARTS-USED.md first. If the part is there we have already sourced, footprinted and shipped it: copy the symbol and footprint into this repo's lib library and use it. Draw a new part only when the library has nothing that fits, and import it with easyeda2kicad from its LCSC number.
• One person holds a board layout at a time. KiCad files do not merge. Say on Discord that you are taking it. See CONTRIBUTING.md.
• ERC and DRC clean before every pull request. Commands below.

Environment

# schematic and board checks, per variant touched
kicad-cli sch erc --exit-code-violations OpenRX-<Variant>/OpenRX-<Variant>.kicad_sch
kicad-cli pcb drc --schematic-parity --refill-zones --exit-code-violations OpenRX-<Variant>/OpenRX-<Variant>.kicad_pcb

# netlist, for scripted analysis
kicad-cli sch export netlist --format kicadsexpr -o /tmp/<variant>.net OpenRX-<Variant>/OpenRX-<Variant>.kicad_sch

On macOS kicad-cli is at /Applications/KiCad/KiCad.app/Contents/MacOS/kicad-cli, and pcbnew imports only under KiCad's bundled Python. Shared scripts (renders, STEP export, packaging art) live in OpenDrone-Scripts.

The design files are KiCad 10 format (version 20260306). Current kicad-skip reads them fine (symbol fields, nets) but is not write-safe on this format: an in-place metadata write reformats the entire file and can drop tokens it does not parse. Until kicad-skip supports it, treat kicad-skip as read-only here and make scripted edits through the pcbnew API or the KiCad GUI.

Architecture

Every variant is the same core: an ESP32-C3 (U1, QFN-32 5 x 5 mm) runs the ExpressLRS unified firmware, talks CRSF over its UART0, and drives a WS2812B RGB status LED (D1). The 2450AT18A100E ceramic chip antenna (AE1, net WIFI) is the ESP32-C3's own Wi-Fi antenna for OTA flashing and configuration on every variant; the ELRS link antenna interface is per variant (RF chains below). Clocks: 40 MHz crystal (X1) for the ESP32-C3 on all variants; the radio TCXO is 52 MHz on the SX1281 variants and 32 MHz on the LR1121 variants.

Sheets: Lite and Lite-UFL each carry their own copy of esp32c3_sx1281_lite.kicad_sch, identical apart from the antenna termination; Mono uses esp32c3_lr1121_mono.kicad_sch. Gemini is hierarchical: OpenRX-Gemini.kicad_sch instantiates esp32-c3.kicad_sch, clock.kicad_sch and lr1121.kicad_sch twice (esp32c3_lr1121_gemini.kicad_sch in that directory is a legacy flat sheet, not in the hierarchy).

RF chains:

• Lite: SX1281 (U3) RFIO -> 2450FM07D0034T (FL1) -> 47948-0001 chip antenna (AE2). Lite-UFL: identical circuit, terminating in a U.FL (J1) instead; the filter output and U.FL are both 50 ohm. No PA/LNA, no RF switch, no sub-GHz on either.
• Mono, single LR1121 dual-band. 2.4 GHz: LR1121 RFIO_HF -> FL1 -> RFX2401C (U4) PA/LNA -> SKY13373 (U5) -> J1 U.FL. Sub-GHz TX: LR1121 RFO_HP_LF -> IPD (T1) TX_HP -> SKY13373 -> J1. Sub-GHz RX: J1 -> SKY13373 -> IPD RX -> LR1121 RFI_P/N_LF. The IPD's TX_LP port is unconnected: ELRS never uses the LR1121 low-power PA.
• Gemini, dual LR1121, two copies of the Mono chain: radio 1 (U3, U4, U5, T1, FL1) feeds J1, radio 2 (U6, U7, U8, T2, FL2) feeds J2. In dual-band modes the firmware never swaps radios: radio 1 is always sub-GHz (J1 takes the 900 MHz antenna), radio 2 always 2.4 GHz (J2). The single 32 MHz TCXO in clock.kicad_sch feeds both radios and is powered from 3.3 V, both VTCXO pins unconnected; the fabricated boards predate that supply change and power it from radio 2's (U6) VTCXO pin, so there U6 must initialise first or neither radio has a clock.

On the LR1121 variants the front end is driven by the radio's own DIOs, not ESP32-C3 GPIOs: DIO5 = RFX2401C RXEN, DIO6 = RFX2401C TXEN, DIO7 = SKY13373 V1, DIO8 = SKY13373 V2. Wiring is symmetric on Gemini, so one switch table serves both radios (decode under Firmware).

Known mismatch, kept as-is: 2450FM07D0034T pin 1 is 40 ohm, designed for SX128x, while LR1121 RFIO_HF is 50 ohm. That gives ~19 dB return loss (VSWR 1.25) and 0.05 dB mismatch loss: negligible next to the filter's own 14 dB typical return loss.

Key parts

LCSC numbers from the schematic symbols and the verified fab/ BOM sets. The SKY13373 carries no LCSC field in either; its number was checked against LCSC directly.

Function	Ref	Part	LCSC	Fitted on
MCU	U1	ESP32-C3		all
3.3 V LDO	U2	TLV75533PDQNR	C2861882	all
2.4 GHz radio	U3	SX1281IMLTRT	C2151551	Lite, Lite-UFL
Dual-band radio	U3, U6	LR1121IMLTRT	C7498014	Mono; Gemini x2
PA/LNA	U4, U7	RFX2401C	C19213	Mono; Gemini x2
RF switch	U5, U8	SKY13373-460LF	C150853	Mono; Gemini x2
Sub-GHz balun/IPD	T1, T2	0900PC16J0042001E	C19842466	Mono; Gemini x2
2.4 GHz band-pass filter	FL1, FL2	2450FM07D0034T	C2651081	all; Gemini x2
Wi-Fi antenna	AE1	2450AT18A100E	C89334	all
ELRS chip antenna	AE2	47948-0001	C152351	Lite
ELRS U.FL	J1, J2	U.FL-R-SMT-1(80)	C88374	Lite-UFL, Mono; Gemini x2
40 MHz crystal	X1	CJ17-400001010B20	C2875272	all
52 MHz TCXO	OSC1	OW7EL89CENUNFAYLC-52M	C22434896	Lite, Lite-UFL
32 MHz TCXO	OSC1	OW7EL89CENUYO3YLC-32M	C22381772	Mono, Gemini
Status LED	D1	XL-1010RGBC-WS2812B	C5349953	all
BOOT button	U9	TS2306A	C2976675	Gemini

Sourcing: the 0900PC16J0042001E has no LCSC stock, consign it from DigiKey. Watch SX1281 stock for volume runs.

Power

5V pad (TP3)
└── TLV75533PDQNR (U2), 3.3 V, 500 mA
    ├── ESP32-C3 (U1), WS2812B (D1)
    ├── radio(s) and TCXO
    └── RF front end: RFX2401C, SKY13373 (Mono, Gemini)

Connectors and I/O

Solder pads carry the external interface. Pad to net mapping from the schematic netlists:

Pad	Net	ESP32-C3	Function
RX (TP1)	U0RXD	GPIO 20	CRSF / serial in to RX
TX (TP2)	U0TXD	GPIO 21	CRSF / serial out / telemetry
5V (TP3)	+5V	-	5 V supply in to the LDO
GND (TP4)	GND	-	Ground
BOOT (TP5)	BOOT	GPIO 9	Pull low at power-up for UART download mode

On Lite, Lite-UFL and Mono, BOOT is the TP5 solder pad and there is no physical switch. On Gemini the tactile button (U9) sits on BOOT alongside a smaller 1.5 mm test pad (TP5); the other four pads remain.

GPIO assignments, from the target JSON in shared/elrs-targets/:

Function	Lite / Lite-UFL	Mono	Gemini
Serial RX / TX	20 / 21	20 / 21	20 / 21
Radio SCK / MOSI / MISO	6 / 4 / 5	6 / 4 / 5	6 / 4 / 5
Radio NSS / RST	7 / 2	7 / 2	0 / 2
Radio BUSY / DIO1	3 / 1	3 / 1	3 / 1
Radio 2 NSS / RST / BUSY / DIO1	-	-	7 / 10 / 8 / 18
RF switch control	-	LR1121 DIO5-DIO8	same, per radio
Status LED	8 (GRB)	8 (GRB)	19 (GRB)
BOOT / button	9	9	9 (button)

Firmware

The ExpressLRS hardware-target definitions live in this repo (shared/elrs-targets/, with targets_entries.json prepared for upstream submission; not yet merged into ExpressLRS/targets). The referenced unified firmware images exist upstream:

Variant	Product name	ELRS firmware target	Platform	Upload
Lite	OpenRX Lite 2.4GHz RX	Unified_ESP32C3_2400_RX	esp32-c3	UART, Wi-Fi, Betaflight
Lite-UFL	OpenRX Lite-UFL 2.4GHz RX	Unified_ESP32C3_2400_RX	esp32-c3	UART, Wi-Fi, Betaflight
Mono	OpenRX Mono Dual Band RX	Unified_ESP32C3_LR1121_RX	esp32-c3	UART, Wi-Fi, Betaflight
Gemini	OpenRX Gemini XrossBand RX	Unified_ESP32C3_LR1121_RX	esp32-c3	UART, Wi-Fi, Betaflight

Minimum ExpressLRS version 3.5.0 (min_version in targets_entries.json). Transmit power per variant is in the README table; the authoritative values are power_values in the per-variant target JSON.

Stock unified firmware runs the SX1281 variants unmodified. Mono and Gemini require an ExpressLRS fork branch: TCXO enable via SetTcxoMode on both, and on Gemini also radio-1 second reset (NRESET on strapping pin GPIO 2) and software chip-select for radio-1 NSS on GPIO 0. Mono and Gemini share one binary; radio_nss_2 in the Gemini JSON enables dual-radio mode. Both set radio_dcdc: true and radio_rfsw_ctrl: [15, 0, 12, 8, 8, 6, 0, 5]: each byte is a bitmask passed to SetDioAsRfSwitch, bit0 = DIO5 through bit3 = DIO8. V1/V2 = 0/0 shuts the SKY13373 down and disconnects the antenna.

Index	Mode	Value	DIO5 (RXEN)	DIO6 (TXEN)	DIO7 (V1)	DIO8 (V2)
0	Enable	15	on	on	on	on
1	Standby	0	0	0	0	0
2	Sub-GHz RX	12	0	0	1	1
3	Sub-GHz TX LP	8	0	0	0	1
4	Sub-GHz TX HP	8	0	0	0	1
5	2.4 GHz TX	6	0	1	1	0
6	unused	0	-	-	-	-
7	2.4 GHz RX	5	1	0	1	0

Revisions

Rev	Date	Change
rev2	2026-08-14	Fab sets re-exported and verified against board and schematic (Lite_rev2, Lite-UFL_rev2, Mono_rev2, Gemini_rev2). Lite-UFL J1 carries LCSC C88374 on board and schematic, previously missing from the BOM. E6 substitute STEP models rebuilt from the trusted wrl geometry; schematic PDFs in exports/schematics/ regenerated.
	2026-08-05	OSHWA certification (BE000030 to BE000033). Lite/Lite-UFL ELRS target pin remap, Mono/Gemini target updates, shared KiCad-Library submodule wired (2026-08-04). Layout rework (clock 3.3 V supply, enlarged pads, Lite/Lite-UFL and Mono outlines +1.0 mm) landed after the validated build and has not been fabricated.
rev1	2026-06-10	Combined OpenRX-all fabrication set ordered at JLCPCB (gerbers, BOM, CPL in OpenRX-Gemini/).
	2026-06-07	Single-source-of-truth docs pass, standardized board renders.
	2026-03-23	Initial repo, 6-receiver lineup; later reduced to the four current variants (retired designs in git history).