Citronics built a router based on the Fairphone 2 mainboard

Belgium-company Citronics has designed a router based on the mainboard of the Fairphone 2 smartphone, connecting the Qualcomm Snapdragon 801 “system-on-module” to a carrier board with Ethernet, USB ports, and other connectors, while leveraging 4G LTE, WiFi, and Bluetooth connectivity from the phone’s core board.

Citronics calls this type of hardware “Circular Microcomputers,” which refers to small computers focusing on sustainability and circular economy principles using parts from discarded devices like old smartphones. There’s actually more than one router, as Citronics designed its own development kit, and also partnered with other companies to design custom gateways based on the Fairphone 2 mainboard. Let’s have a look at the devkit first.

Citronics devkit:

Fairphone 2 mainboard
- SoC – Qualcomm Snapdragon 801 (MSM8974AB) quad core Krait 400 processor @ up to 2.26 GHz with Adreno 330 GPU
- System Memory – 2 GB LPDDR3
- Storage – 32GB eMMC flash
- Connectivity – 2G/3G/4G LTE, dual-band WiFi 5 up to 433 Mbps, Bluetooth 4.0 LE; Note: antennas not provided
- Various expansion connectors
Storage – SD card slot (not shown on photos)
Networking – 10/100M Ethernet port
USB
- 4x USB 2.0 Type-A ports
- 1x USB Type-C port for power only?
Expansion – 40-pin Raspberry Pi-compatible GPIO header with UART, SPI, I2C
Power Supply – 5V to 60V via USB-C PD, PoE
Power Consumption – 2 to 20 Watts
Dimensions – 12.5 x 6.5 x 1.8cm
Weight – 99 grams

Fairphone 2 core module Citronics router devkit — Fairphone 2 core module/mainboard (left) and Citronics router devkit (right)

The devkit comes preloaded with Alpine Linux OS. There’s not that much to see on the website, but we are told that “a comprehensive wiki site to support developers is under construction. It will be online at delivery time”. Delivery time means March 2025, so later this month.

I was also informed by email that Spin42 worked on Buildroot support with the kernel being a fork of mainline Linux. Debian is also being worked on, but nothing has been released just yet.

Destore Citronics Fairphone 2 industrial gateway — Destore gateway

Besides the devkit, Citronics can also provide custom microcontrollers based on the Fairphone 2 mainboard and one example is a residential heating optimization box by Destore whose electronics are shown above. Other partnerships led to the creation of sustainable computing platforms for machine vision classes at UCLouvain university and monitoring and control gateways for Karno (again for heating control). The company also developed a solution with 25 synchronized displays based on Fairphone 2 boards currently used by artists and companies.

Samples of the Citronics DevKit can be pre-ordered for 150 Euros including VAT through a Google Doc form. The company stresses that the kit is for professional customers aiming at prototyping and R&D activities, and NOT intended for average consumers. Antennas are not provided, so you’d have to handle that yourself. Delivered are expected to start this month (March 2025).

Thanks to Zoobab for the tip.

Jean-Luc Aufranc (CNXSoft)

Jean-Luc started CNX Software in 2010 as a part-time endeavor, before quitting his job as a software engineering manager, and starting to write daily news, and reviews full time later in 2011.