# Show HN: A USB-C LTE modem that gives my computer a phone line > Source: > Published: 2026-10-31 15:56:41+00:00 A 4-layer board that gives a computer its own phone line: LTE data, SMS and voice calls, with the call audio on USB. Plug in a nano-SIM and one USB-C cable, and the computer can answer a call through your headset, hand it to a script or an AI agent, or send a text. The board was designed by [Koppar](https://koppar.ai/?ref=gh-lte) from a one-page spec ([`spec.txt`](https://github.com/Koppar-AI/koppar-lte-modem/blob/main/spec.txt)), then reviewed part by part against the datasheets. One USB-C cable, and the computer sees three ordinary devices through the on-board USB hub: | Device | From | For | |---|---|---| | Network adapter | Quectel EC25-EUX (U8) | LTE data, with 3G and 2G fallback (European bands) | | Serial ports | EC25-EUX | AT commands: caller ID ( `+CLIP` ), texts (`AT+CMGS` ), answer (`ATA` ) | | USB sound card | RP2040 (U5) | Call audio: the module's 1.8 V PCM interface, level-shifted, as a USB microphone and speaker | There is no battery, screen or speaker. Three LEDs show NET (green), RING (amber) and CALL (blue). Power is 5 V from USB-C, a 3.8 V / 3 A buck for the module's 2 A transmit bursts, and 3.3 V for the hub, the RP2040 and its flash. | Top | Back | |---|---| | Path | What it is | |---|---| | [`spec.txt`](https://github.com/Koppar-AI/koppar-lte-modem/blob/main/spec.txt) | The spec the board was designed from. | | [`board/`](https://github.com/Koppar-AI/koppar-lte-modem/blob/main/board) | KiCad 10 project: schematic, board, rules, the module footprint, symbol and 3D model. | | [`board/fab/`](https://github.com/Koppar-AI/koppar-lte-modem/blob/main/board/fab) | Gerbers and drill files ( `gerbers.zip` ), IPC-2581, schematic PDF. | | [`board/fab/jlc/`](https://github.com/Koppar-AI/koppar-lte-modem/blob/main/board/fab/jlc) | JLCPCB assembly BOM and placement files, with the part choices checked by hand. | | [`REVIEW.md`](https://github.com/Koppar-AI/koppar-lte-modem/blob/main/REVIEW.md) | The design review: what passed, what changed and why, ordering notes, what is still open. | | [`firmware/`](https://github.com/Koppar-AI/koppar-lte-modem/blob/main/firmware) | The RP2040 sound-card firmware (after bring-up). | **Board.** Upload `board/fab/gerbers.zip` to JLCPCB: 4 layers, 1.6 mm, stackup JLC04161H-3313 (the antenna feeds are 49 Ω on it). Every via is 0.45 / 0.2 mm and none sits in a pad, so pick *Min via hole size 0.2mm/(0.3/0.35mm)* and *Via covering: Plugged*: no via-in-pad, no extra options. **Assembly.** Use `board/fab/jlc/BOM-JLCPCB.csv` and `CPL-JLCPCB.csv`. Glance at the placement preview before confirming, especially the LEDs and the SIM socket (the card slot is at the board edge). [`REVIEW.md`](https://github.com/Koppar-AI/koppar-lte-modem/blob/main/REVIEW.md#ordering-notes-checked-2026-10-30-against-the-live-jlcpcb-catalogue) lists the parts the files correct. **Parts you source yourself.** These are not on JLCPCB's assembly line, or are not on the board at all: | Part | What to get | Where | |---|---|---| | U8, the LTE module | Quectel EC25-EUX, ordering code **EC25EUXGR-128-SGNS** (EC25EUXGA-128-SGNS is end-of-life; same LGA-144 footprint). It must be the EUX: the North American data-only variants have no voice. | [Techship](https://techship.com/product/quectel-ec25-eux-lte-cat-4-lga) ,[DigiKey](https://www.digikey.com/en/products/detail/quectel/EC25EUXGR-128-SGNS/15516833) ,[Octopart](https://octopart.com/part/quectel/EC25EUXGR-128-SGNS) (stock changes, check the suffix). Deselect U8 at assembly and solder it with hot air or a stencil, or consign it. | | 2 antennas | LTE, 698 to 2690 MHz, U.FL (I-PEX MHF I), 50 Ω: one on MAIN (J3), one on DIV (J4) | e.g. [Taoglas FXUB63](https://www.mouser.com/new/taoglas/taoglas-fxub63/) or[Taoglas FXUB62.07.0150C](https://www.digikey.com/en/products/detail/taoglas-limited/FXUB62-07-0150C/16610012) | | SIM | A nano-SIM with a voice plan | Your carrier | | Cable and port | USB-C on both ends, from a port that offers 3 A at 5 V | The 5.1 kΩ CC resistors make the board a sink; they negotiate nothing, so a USB-A port or a 1.5 A source will brown out in a 2G burst | Open `board/koppar-lte-modem.kicad_pro` in KiCad 10. The module's footprint, symbol and 3D model are in the project (`board/footprints`, `board/custom_syms`, `board/models`), wired in by the project's library tables. KiCad's library has no 3D model for two of the footprints, so the 3D viewer shows them without a body: | Part | Footprint | 3D model | |---|---|---| | J1, USB-C receptacle | `Connector_USB:USB_C_Receptacle_HRO_TYPE-C-31-M-12` | [LCSC C165948](https://www.lcsc.com/product-detail/C165948.html) (EasyEDA model) | | J2, nano-SIM socket | `Connector_JAE:JAE_SIM_Card_SF72S006` | [LCSC C2977286](https://www.lcsc.com/product-detail/C2977286.html) (EasyEDA model) | [easyeda2kicad](https://github.com/uPesy/easyeda2kicad.py) converts either one to a `.wrl` you can attach to the footprint. The renders above use them. There is no BOOTSEL button. A blank RP2040 boots into its USB bootloader anyway, and the back has three labelled pad groups for the rest: - **BOOT + GND** : short them while plugging in and the RP2040 comes up as a USB drive for a new`.uf2` , even after a firmware that crashes before USB starts. - **SWD (CLK, DIO, GND)** : a Pico Debug Probe or any CMSIS-DAP probe. - **USB_BOOT + 1V8** : short them while plugging in and the EC25 starts in emergency download mode, for a modem update that went wrong. The modem does not switch itself on: the RP2040 drives its PWRKEY (GPIO22) and RESET_N (GPIO23). Until the firmware runs, the computer sees only the hub and the RP2040. The first version (rev A) had an antenna feed at 35 Ω that hopped layers, a buck 60 mm from its inductor, and a SIM socket facing the wrong way. All three are fixed here, along with the parts list and the assembly rotations. [`REVIEW.md`](https://github.com/Koppar-AI/koppar-lte-modem/blob/main/REVIEW.md) has every finding with its numbers. | Board | Rev B: 0 DRC errors, 0 unconnected, KiCad 10. Being ordered. | | Next | Once the boards are built: bring-up, AT commands, the sound-card firmware, the first call and text. | VoLTE is optional on the EC25-EUX. Where 2G and 3G are switched off, calls need a module firmware with VoLTE and a carrier that allows it; ask the seller. This board started as a page of text. [Koppar](https://koppar.ai/?ref=gh-lte) takes a spec like [`spec.txt`](https://github.com/Koppar-AI/koppar-lte-modem/blob/main/spec.txt) to a schematic, a routed board, a BOM at live prices and Gerbers. It also takes a KiCad board you already have, or works from your own agent over MCP. CERN Open Hardware Licence Version 2, Permissive ([`LICENSE`](https://github.com/Koppar-AI/koppar-lte-modem/blob/main/LICENSE)).