Raspberry Pi developing project

2.1 Mountainous-Area Edge Endpoint Physically, it is a mailbox-style cabinet with a mail slot, integrating the following internally: Portable Wi-Fi scanner: After a letter is fed through the entrance, it is scanned automatically. Reference selections include the Brother DS-940DWe (built-in battery, Wi-Fi, duplex scanning, approx. 15 pages/min) or the Epson WorkForce DS-80W (built-in battery, can scan approx. 300 pages, 4 sec/page). Prefer models that support automatic document feeding and Wi-Fi Direct, so that they can communicate directly with the Raspberry Pi on the same local network. Raspberry Pi control board: Equipped with a touchscreen and running OpenWrt or a custom Linux system, it performs three functions: ① receiving images pushed by the scanner and uploading them to the server; ② serving as a 5G/4G-to-Wi-Fi hotspot for the scanner and printer to connect to; ③ running a local print agent that polls the server for print jobs and drives the printer. 5G access can be implemented by pairing the Raspberry Pi with a SIM8202G-M2 HAT or a Quectel RG520N-EB-class module, forwarding the mobile network as a local Wi-Fi hotspot. Portable Wi-Fi printer: Reference selection includes the Epson WorkForce WF-110W (built-in battery, approx. 50 pages, supports Wi-Fi Direct and Epson Connect remote printing, weight 1.6 kg). The printer connects via Wi-Fi to the Raspberry Pi hotspot, and the local agent dispatches print jobs to it. UPS/power bank: Provides backup power for the Raspberry Pi, scanner, printer, and 5G module. Unstable power supply is a common constraint in mountainous areas; the UPS should at least ensure that the two short operations—“scan and upload” and “pickup and print”—are not interrupted. 2.2 Cloud Server Responsible for letter storage, status management, reply distribution, print job queues, device heartbeat monitoring, and operation auditing. At its core is a letter-centric state machine; all endpoint-side behaviors are triggers for state transitions. 2.3 Urban Reply Endpoint Urban high school and college students log in via the Web or a lightweight App, view the list of letters awaiting replies assigned to them, read the scanned copies, and write replies online (with optional text input or handwritten photo upload). After a reply is submitted, the server sets the corresponding letter’s status to “replied” and generates a pending print job.