Koppar-AI LTE Modem Open Hardware Project

The Koppar-AI LTE modem project provides a hardware solution for integrating a cellular phone line directly into a computer. By utilizing a Quectel EC25-EUX module and an RP2040 microcontroller, the device presents itself to a host computer as a network adapter, a set of serial ports for AT commands, and a USB sound card for call audio. This allows users to handle calls and texts via a headset or through automated scripts and AI agents.

Hardware Architecture

The board is a 4-layer design that functions as a USB-C peripheral. It relies on a USB hub to expose three distinct interfaces to the host system:

  • Network Adapter: Powered by the Quectel EC25-EUX, providing LTE data with 3G/2G fallback.
  • Serial Ports: Used for issuing AT commands to manage SMS, caller ID, and call answering.
  • USB Sound Card: Managed by the RP2040, which bridges the modem's 1.8V PCM audio interface to the USB host, allowing the computer to treat the phone line as a standard audio input/output device.

The power delivery system includes a 3.8V buck converter specifically sized to handle the 2A current spikes required during GSM transmit bursts, ensuring stability without an onboard battery.

Design and Development

The project was developed using a "spec-first" approach, where the initial requirements were defined in a single text file before being translated into a schematic and PCB layout. The repository includes the complete KiCad 10 project, manufacturing files (Gerbers), and assembly instructions for JLCPCB.

Key design iterations were documented in a review process, which identified and corrected several critical issues from the initial revision, including:

  • Antenna feed impedance mismatches.
  • Improper placement of the buck converter relative to its inductor.
  • Incorrect orientation of the SIM card socket.

Community Feedback and Considerations

Discussion surrounding the project has highlighted both the utility and the technical challenges of the implementation. Users noted that the reliance on the Quectel EC25-EUX module introduces a dependency on VoLTE support, which is critical as carriers worldwide phase out 2G and 3G networks.

Regarding the design process, some community members expressed skepticism about the "AI-generated" nature of the hardware, questioning its viability without physical validation. The author, @abijahkaj, clarified the development workflow:

"I wrote the spec as one page of text, and Koppar, the tool I'm building, generated the schematic and the board. Then I reviewed it against the datasheets. The review found a 35 Ω antenna feed that hopped layers, a buck 60 mm from its inductor, and a SIM socket facing into the board."

Future Outlook

The project is currently in the pre-assembly phase. Future development milestones include the physical bring-up of the boards, finalization of the RP2040 sound-card firmware, and testing of the end-to-end audio path. The design is released under the CERN Open Hardware Licence Version 2 (Permissive).

Sources