OpenRX-Mono

Imported from OpenDrone-hw/OpenRX-Mono (AGENTS.md). Install upstream with npx skills add OpenDrone-hw/OpenRX-Mono. Copyright stays with the author.

OpenRX Mono

Open-source dual-band ExpressLRS receiver. An ESP32-C3 controls one LR1121; 2.4 GHz and sub-GHz RF paths share one U.FL antenna through an RF switch. A separate ceramic antenna serves the ESP32-C3 Wi-Fi interface.

Architecture

The ESP32-C3 (U1) runs ExpressLRS, communicates over UART0/CRSF, and drives the WS2812B status LED (D1). It controls the LR1121 (U3) over SPI. At 2.4 GHz, RFIO_HF passes through FL1, the RFX2401C PA/LNA (U4), and SKY13373 switch (U5) to U.FL J1. Sub-GHz transmit uses the LR1121 high-power output through T1 and U5; receive returns through U5 and T1 to RFI_P/N_LF. T1's low-power TX port is unused. LR1121 DIO5/DIO6 drive U4 RXEN/TXEN and DIO7/DIO8 drive U5 V1/V2.

Power

5V pad (TP3)
└── TLV75533PDQNR (U2), 3.3 V
    ├── ESP32-C3 (U1) and status LED (D1)
    ├── LR1121 (U3) and 32 MHz TCXO (OSC1)
    └── RFX2401C (U4) and SKY13373 (U5)

Key parts

Connectors and I/O

Radio SPI is SCK/MOSI/MISO on GPIO6/4/5; NSS/RST/BUSY/DIO1 are GPIO7/2/3/1. The RGB status LED is GPIO8 in GRB order.

Layout rules

Preserve both controlled-impedance RF paths, the ground-via fences around the front end, and the Wi-Fi antenna keepout. Keep the LR1121, IPD, PA/LNA, RF switch and U.FL interconnects short; do not disturb the exposed-pad via arrays.

Firmware

ExpressLRS target Unified_ESP32C3_LR1121_RX, platform esp32-c3, minimum version 3.5.0. Upload methods are UART, Wi-Fi and Betaflight passthrough. The authoritative GPIO, RF-switch table and power values are in firmware/OpenRX Mono LR1121.json.

Repo

Parts and datasheets

• Per-repository part index: hardware/OpenRX-Mono.kicad_sch and its listed sub-sheet are authoritative for what this board fits. Export the netlist with the command below when a script-readable board index is needed; do not maintain a second hand-written BOM.
• Proven shared parts: hardware/KiCad-Library/PARTS-USED.md is the catalogue index. Its Boards column identifies every repository using each LCSC part; filter it for OpenRX-Mono to get this repository's proven shared-part view.
• Exact datasheets: hardware/KiCad-Library/datasheet/manifest.json maps shared symbols to the committed PDFs and their SHA-256 hashes. The PDFs live in hardware/KiCad-Library/datasheet/, and symbol links resolve there through OPENDRONE_LIB.
• Local-only parts: inspect hardware/lib.pretty/ and hardware/lib.3dshapes/, then verify supplier fields in the board schematic. Do not duplicate a shared part or datasheet.

Environment

kicad-cli sch erc hardware/OpenRX-Mono.kicad_sch
kicad-cli pcb drc --schematic-parity --refill-zones hardware/OpenRX-Mono.kicad_pcb
kicad-cli sch export netlist --format kicadsexpr -o /tmp/OpenRX-Mono.net hardware/OpenRX-Mono.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. Reusable scripts for renders, STEP export, and packaging art come from Incutec hardware tooling. The OpenDrone release standard is RELEASES.md. Board-specific scripts, where a board has any, live in hardware/tools/.

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 the OpenDrone library and its PARTS-USED.md first. If the part is there we have already sourced, footprinted and shipped it, and its symbol links to the exact committed datasheet: place it from OpenDrone. Draw a new part into lib only when the catalogue has nothing that fits, imported with easyeda2kicad from its LCSC number. Pulling a newer catalogue is a deliberate, reviewed commit: git submodule update --remote hardware/KiCad-Library, then DRC.
• 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.
• Run ERC and DRC before every pull request. Existing approved findings may remain; a new type or increased count must be reviewed before merge. Commands are in Environment above.

Revisions

By task

Board-specific paths are in Environment above. KPY is KiCad's bundled Python named there.

• Check the design: run the ERC and DRC commands in Environment before every pull request.
• Add a part: place it from the OpenDrone library if hardware/KiCad-Library/PARTS-USED.md lists it; otherwise import it into lib with $KPY <hardware-tooling>/hardware/kicad/import_part.py (read --help first), KiCad closed.
• Render the board for the README: $KPY <hardware-tooling>/hardware/kicad/render_board.py hardware/OpenRX-Mono.kicad_pcb --outdir images, KiCad closed.
• Analyse the netlist: export it with the netlist command in Environment, then read it with a script; never hand-write a second BOM.
• Update the shared library: git submodule update --remote hardware/KiCad-Library, run DRC, commit as its own reviewed change.