The four-layer hardware design combines a Quectel EC25-EUX module and a Raspberry Pi RP2040 microcontroller to deliver data, SMS, and voice functionality to any host PC. Once connected to a powered port with an active nano-SIM, the device exposes a sound card, network adapter, and serial command interfaces via an internal USB hub. This architecture eliminates the need for separate displays or batteries by drawing up to 3 amps of 5-volt host power to support cellular transmission bursts. The RP2040 chip handles the necessary audio level-shifting and modem power sequencing while rear test pads allow for emergency recovery.
Key points
The design routes bidirectional voice call audio directly into the computer for use with…: The design routes bidirectional voice call audio directly into the computer for use with headsets or automated software.
Koppar published full KiCad schematics and Gerber fabrication archives to allow builders…: Koppar published full KiCad schematics and Gerber fabrication archives to allow builders to assemble the open-source hardware.
Sourcing the Quectel LTE module, dual cellular antennas, and an active voice-enabled nano…: Sourcing the Quectel LTE module, dual cellular antennas, and an active voice-enabled nano-SIM is left to individual builders.
Sources checked by our analysis pipeline: 1
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 from a one-page
spec (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…
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