NS ELRS RX MICRO

Nano 2.4 GHz ExpressLRS Diversity PA/LNA Receiver

An ultra-compact (18×24 mm), open-hardware 2.4 GHz ExpressLRS receiver featuring true antenna diversity, a high-power FEM (+20 dBm telemetry), integrated Wi-Fi ceramic antenna, and paneled breakaway rails for low-cost JLCPCB PCBA.

Key Specs • Visuals • Architecture • Pinout • PCB Stackup • Firmware Setup • BOM • License

1. Hero Visuals & 3D Views

Figure 1: Production 3D Top Render of the Nano 2.4 GHz ExpressLRS Diversity PA/LNA Receiver

🔍 Detailed 3D Perspective Comparison (Front & Back Views)

Front Perspective (Core Silicon & RF Frontend)	Back Perspective (Passives, Testpads & Ground Plane)
Top Side (L1): ESP32-C3FH4, Semtech SX1280 Transceiver, Skyworks SE2431L PA/LNA FEM, 2450FM07D0034 Bandpass Filter, dual U.FL IPEX jacks, Johanson 2450AT ceramic chip antenna, status LED, crystals, and JST-GH connector.	Bottom Side (L4): Solid thermal relief stitching vias, dedicated USB test pads (USBD+/USBD-), BOOT strapping testpad, and low-impedance RF ground shield flood.

2. Key Specifications & Features

The NorthernStudios Nano 2.4 GHz ExpressLRS Diversity Receiver is engineered for long-range, high-reliability UAV control links, telemetry backhaul, and high-interference drone racing / freestyle environments. Combining high-efficiency RISC-V computing with Semtech's SX1280 transceiver and Skyworks' SE2431L front-end module, it provides industrial-grade packet reception sensitivity, +20 dBm (100 mW) telemetry uplink, and seamless over-the-air firmware management.

Executive Specification Matrix

3. Schematic & Architecture

System Schematic

The schematic is captured in KiCad 8/9 with complete hierarchical decoupling, controlled RF coplanar waveguides, and low-noise power distribution.

Figure 2: Complete Electrical Schematic — ESP32-C3FH4 Core, SX1280 2.4 GHz Transceiver, SE2431L Front-End Module, and Johanson RF Network

Architecture & Signal Flow

The receiver features two independent 2.4 GHz RF paths:

1. LoRa / FLRC Flight Control Path: Routes bidirectional RF signals between dual external U.FL dipole/immortal-T antennas, through the SE2431L SP3T switch, through the internal PA/LNA, through the matched 2450FM07D0034 bandpass filter, and into the Semtech SX1280 transceiver.
2. Wi-Fi WebUI Configuration Path: Connects the ESP32-C3's internal Wi-Fi/Bluetooth RF transceiver (LNA_IN) directly to an onboard Johanson 2450AT ceramic chip antenna through a precision 50Ω Grounded Coplanar Waveguide (GCPW), isolating telemetry link tuning from configuration management.

flowchart LR
    subgraph Antennas ["Antenna System"]
        A1["U.FL Antenna 1 (ANT1)"]
        A2["U.FL Antenna 2 (ANT2)"]
        AC["Johanson 2450AT Chip Antenna"]
    end

    subgraph FrontEnd ["RF Front-End (FEM)"]
        SW{"SE2431L SP3T Switch"}
        PA["Power Amp (+22 dB)"]
        LNA["Low Noise Amp (+12.5 dB)"]
    end

    subgraph Filtering ["RF Conditioning"]
        BPF["Johanson 2450FM07D0034 Filter"]
    end

    subgraph Silicon ["Core Transceiver & Processing"]
        SX["Semtech SX1280 Transceiver"]
        ESP["Espressif ESP32-C3FH4 (RISC-V)"]
    end

    subgraph Host ["Flight Controller Interface"]
        FC["Flight Controller (CRSF UART)"]

ASCII Architecture Reference

+---------------------------------------------------------------------------------------+
|                                    ANTENNA SYSTEM                                     |
|    [U.FL Port 1]                     [U.FL Port 2]            [Johanson 2450AT Chip]  |
|          |                                 |                             |            |
|          +----------------+----------------+                             |            |
|                           | (50Ω GCPW)                                   | (50Ω GCPW) |
|                           v                                              |            |
|       +---------------------------------------+                          |            |
|       |         Skyworks SE2431L FEM          |                          |            |
|       |  [ANT Switch] <-> [PA / +22dB]        |                          |            |
|       |               <-> [LNA / +12.5dB]     |                          |            |
|       +-------------------+-------------------+                          |            |
|                           ^                                              |            |
|                           | (Tx / Rx Enable & Ant Sel)                   |            |
|                           v                                              |            |
|       +---------------------------------------+                          |            |
|       |     Johanson 2450FM07D0034 Filter     |                          |            |
|       +-------------------+-------------------+                          |            |
|                           | (Matched Differential)                       |            |
|                           v                                              |            |
|       +---------------------------------------+                          |            |
|       |       Semtech SX1280 Transceiver      |                          |            |
|       |        2.4 GHz LoRa / FLRC / FSK      |                          |            |
|       +-------------------+-------------------+                          |            |

4. Hardware Configuration & Pinout

The pin assignment is optimized for optimal trace routing on 4-layer PCBs, preserving signal integrity along high-speed SPI lines while isolating sensitive radio triggers from high-frequency clock harmonics.

ExpressLRS Hardware Pin Mapping

External Connector & Pad Pinout

       +---------------------------------------------+
       | [ANT1]                               [ANT2] |
       |                                             |
       |  (1) 5V   [ 4.5V - 5.5V DC Input ]          |
       |  (2) GND  [ Power & Signal Ground ]         |  Active PCB Area:
       |  (3) TX   [ CRSF Telemetry to FC RX ]       |  16.0 x 22.0 mm
       |  (4) RX   [ CRSF Control from FC TX ]       |
       |                                             |
       | [JST-GH] (1:GND, 2:5V, 3:TX, 4:RX)          |
       | [TP_BOOT] [TP_USBD-] [TP_USBD+]             |
       +---------------------------------------------+

• Castellated Solder Pads (Edge J3–J6): Standard 2.54 mm (0.1") pitch for direct wire soldering or header pins.
• Micro-Connector (J7): JST-GH 4-pin horizontal locking receptacle (1.25 mm pitch) matching modern flight controllers (Matek, SpeedyBee, Holybro, BetaFPV).
• Test Points: Dedicated pads for BOOT (J10/J11), USBD- (TP1), and USBD+ (TP2) for debugging or manual recovery.

5. PCB Fabrication & Stackup Requirements

The board utilizes a 4-layer controlled-impedance stackup specifically tailored for the JLCPCB JLC04161H-3313 or JLC04161H-7628 process.

Figure 3: 4-Layer Controlled-Impedance Stackup (0.1 mm Prepreg / 1.6 mm Total Board Thickness)

Layer Stack Architecture

RF Coplanar Waveguide Specifications

• 50Ω GCPW Trace Width ($W$): 0.18 mm (7.1 mil)
• Trace-to-Ground Clearance ($S$): 0.15 mm (5.9 mil)
• Dielectric Height ($H$): 0.10 mm (Prepreg to Layer 2 GND)
• Ground Via Pitch: Continuous via stitching with $\le 1.0\text{ mm}$ center-to-center spacing along all RF microstrips to suppress substrate resonance.

Panel & Breakaway Frame Handling

The receiver is fabricated in a standardized 70 × 70 mm manufacturing panel frame containing:

• Tooling Holes: 4× 3.2 mm non-plated holes located at panel corners for automated high-speed SMT feeders.
• Optical Fiducials: 3× 1.0 mm round copper fiducials with 2.0 mm soldermask openings for machine vision alignment.
• Breakaway Tabs: 0.6 mm perforated mouse bites with center drill holes on each breakaway edge.

[!TIP] Post-Reflow Breakaway Removal Instructions:

1. Score the mouse bite tabs lightly using an engineer's carbide scribe or hobby hobby knife on both sides.
2. Support the active receiver body flat against a bench surface and bend the outer rails downward with gentle, even pressure.
3. Clip remaining nibs flush using precision flush cutters (e.g. Plato 170).
4. Lightly dress the castellated edges with 400-grit abrasive paper to ensure no copper burrs short against the edge ground plane.

6. Flashing & Firmware Setup

Method 1: Initial UART Bootloader Flash (First-Time Commissioning)

1. Connect the receiver to a 3.3V USB-to-UART adapter (CP2102, FT232RL, or CH340):
   • 5V $\rightarrow$ 5V (or external clean 5V supply)
   • GND $\rightarrow$ GND
   • TX $\rightarrow$ Adapter RX
   • RX $\rightarrow$ Adapter TX
2. Engage Bootloader: Bridge the BOOT test pad (GPIO09) to GND using tweezers or a jumper wire, then connect USB power to the adapter. Release the bridge after power is applied.
3. Open ExpressLRS Configurator (v1.6.0+):
   • Target: Generic target using ESP32-C3 (or choose unified target)
   • Device: Generic ESP32-C3 2.4GHz RX
   • Flashing Method: UART
   • Set your Binding Phrase, regulatory domain (FCC / LBT), and Wi-Fi credentials.
4. Click Build & Flash. Once flashing is complete, power-cycle the board.

Method 2: Wi-Fi Over-The-Air (OTA) Updating (Recommended)

1. Power the receiver via flight controller or a 5V bench supply.
2. Leave your radio transmitter powered off. After 60 seconds, the receiver will automatically switch into Wi-Fi Access Point mode.
3. The onboard status LED (D1) will begin rapidly blinking (5 Hz).
4. Connect your PC or smartphone to the Wi-Fi network:
   • SSID: ExpressLRS RX
   • Password: expresslrs
5. Navigate your browser to http://10.0.0.1 (or http://elrs_rx.local).
6. Upload the updated .bin firmware file or modify runtime configuration values directly via the WebUI.

Custom Target Definition (hardware.json)

To load the hardware pinout dynamically on unified ExpressLRS firmware builds, navigate to the Hardware tab in the WebUI and paste the following target definition:

{
  "target": "NS_ELRS_RX_MICRO_2400",
  "name": "NorthernStudios Nano 2.4GHz Diversity RX",
  "brand": "NorthernStudios",
  "radio": "sx1280",
  "mcu": "esp32c3",
  "serial_rx": 21,
  "serial_tx": 20,
  "radio_busy": 3,
  "radio_dio1": 5,
  "radio_miso": 7,
  "radio_mosi": 12,
  "radio_nss": 13,
  "radio_rst": 2,
  "radio_sck": 6,
  "power_txen": 4,
  "ant_ctrl": 10,
  "led": 8,
  "led_inverted": true,
  "power_min": 0,
  "power_high": 3,
  "power_max": 3,
  "power_default": 0,
  "power_control": 1,
  "power_values": [-18, -13, -9, -5, -2, 0, 3, 6, 10, 13]
}

7. Bill of Materials (BOM) & Sourcing

Every active IC, passive, and connector has been selected for high performance, standard commercial availability, and turnkey automated assembly via JLCPCB SMT / LCSC.

Core Active Silicon & RF Front-End

Crystals, Connectors & Passives

8. License & Regulatory Compliance

Open Hardware License

This project is licensed under the CERN Open Hardware Licence Version 2 – Strongly Reciprocal (CERN-OHL-S-v2).

• Hardware Design Files: You may modify, manufacture, distribute, and sell this hardware, provided that any derivative designs or modifications are released under the exact same CERN-OHL-S-v2 terms.
• Documentation & Media: All accompanying schematics, 3D renders, diagrams, and written documentation are released under CC BY-SA 4.0.

Regulatory Notice & Amateur Radio Compliance

[!WARNING] Radio Frequency Operating Compliance: This receiver is an open-source development device operating within the 2.400 GHz – 2.4835 GHz ISM band. Telemetry uplink operations can emit up to +20 dBm (100 mW) EIRP.

• Amateur Radio Licensing: Depending on your jurisdiction, transmitting at elevated telemetry power levels (+20 dBm) may require a valid Amateur Radio (Ham) license.
• European Union (CE / RED): Users operating within CE-compliant jurisdictions must configure ExpressLRS to use LBT (Listen-Before-Talk) mode with EIRP restricted to $\le 10\text{ dBm}$ (10 mW) or $\le 20\text{ dBm}$ with active LBT duty-cycling.
• United States (FCC Part 15 / Part 97): Operation must comply with FCC rules governing digital transmission systems within the 2.4 GHz band. The authors and contributors assume no liability for non-compliant RF transmission or unauthorized frequency usage.

9. Contributing & Acknowledgments

Contributions, issue reports, and pull requests are welcomed!

• For hardware bug reports, impedance recalculations, or BOM alternatives, please open an issue or pull request against this repository.
• For firmware features and telemetry protocol questions, please consult the upstream ExpressLRS Project.

Acknowledgments

• ExpressLRS Team: For developing the world's most performant open-source RC radio link.
• Semtech Corporation: For the exceptional SX1280 2.4 GHz LoRa/FLRC silicon.
• Espressif Systems: For the ESP32-C3 RISC-V SoC architecture.
• Skyworks Solutions: For the SE2431L high-efficiency RF front-end module.
• KiCad Development Community: For providing the world-class open-source EDA suite.