Hacker Turns $20 4G Hotspot into a Dedicated Texting Device

The Modding Process

A recent Show HN post details how a developer transformed a common, inexpensive 4G wireless hotspot into a dedicated device for sending and receiving SMS messages. The project, born out of a desire for a reliable, offline texting solution, leverages a $20 TP-Link M7350 hotspot and a bit of custom firmware.

The core of the project involves replacing the hotspot's stock firmware with OpenWrt, a popular Linux distribution for embedded devices. This allows for significant customization and control over the device's hardware, which is typically locked down by the carrier. The developer explains that the M7350, while no longer sold new, can be found on the used market for around $20. This low entry cost makes the project accessible to hobbyists and developers interested in exploring cellular hardware hacking.

Once OpenWrt is installed, the real work begins. The developer needed to interface with the Quectel EC25 cellular modem inside the hotspot. This modem, common in many cellular devices, supports AT commands for controlling its various functions, including SMS. The challenge lies in exposing these commands through a user-friendly interface and ensuring reliable communication.

The project details the steps taken to achieve this, including setting up the necessary drivers and software packages within OpenWrt. Crucially, the developer had to figure out how to pass AT commands directly to the modem. This often involves understanding the serial port configurations and ensuring the correct permissions are set for user-level access.

The goal was to create a device that could send and receive SMS messages independently of a connected computer or smartphone. This means the hotspot itself needs to manage the cellular connection and the SMS protocol. The developer achieved this by writing custom scripts that poll the modem for incoming messages and send outgoing messages based on specific triggers or commands.

One of the key hurdles in such projects is dealing with the proprietary nature of cellular modems and their firmware. Carriers often implement restrictions that prevent users from accessing certain functionalities or flashing custom software. By successfully installing OpenWrt and gaining direct access to the modem's AT command interface, this project bypasses those limitations.

Leveraging AT Commands for SMS

AT commands are a standard set of commands used to control modems. They are typically sent over a serial connection. For SMS, commands like `AT+CMGF=1` set the modem to text mode, `AT+CMGS` initiates sending a message, and `AT+CMGL` lists stored messages. The developer's custom software acts as an intermediary, translating user requests into these AT commands and processing the modem's responses.

The implementation involves a loop that continuously checks the modem for new SMS messages. When a message arrives, the software parses it and can then trigger various actions. For outgoing messages, the software can be configured to send messages to specific numbers with predefined content. This opens up possibilities for automated alerts or remote control systems that rely on SMS for communication.

The choice of OpenWrt is significant. It provides a stable Linux environment with a package manager, making it easier to install and configure the necessary software for modem control and network management. Furthermore, OpenWrt's web interface can be extended to provide a basic control panel for the device, allowing users to monitor its status, view message logs, and even compose new messages without needing to SSH into the device.

The developer shared snippets of the code, showcasing how they interact with the modem's serial port (e.g., `/dev/ttyUSB0` or similar). This level of detail is invaluable for others looking to replicate or build upon the project. It highlights the practical challenges and solutions involved in low-level hardware manipulation.

Why This Approach Matters

The implications of this project extend beyond a simple hobbyist endeavor. In an era where many communication channels are increasingly reliant on internet connectivity and proprietary cloud services, having a dedicated, low-cost SMS device offers several advantages. SMS is a resilient communication method that often works even when internet services are down or unreliable. This makes it suitable for critical alerts, remote monitoring, or communication in areas with poor internet infrastructure.

Consider situations where a robust, always-on texting device is needed. This could range from industrial monitoring systems that need to send alerts via SMS when a sensor breaches a threshold, to emergency communication kits that require a fallback method when data networks fail. Traditional cell phones, while capable of SMS, are often complex, require active SIM plans with data, and are not designed for continuous, automated operation.

This hack offers a more focused, cost-effective solution. By repurposing a cheap hotspot, the developer created a device that is essentially a programmable SMS gateway. The $20 price point is particularly compelling, as it makes such a solution economically viable for a wide range of applications where a dedicated commercial SMS gateway might be too expensive or overkill.

The project also speaks to the broader trend of manufacturers locking down hardware. Many consumer electronics, including routers and hotspots, come with proprietary firmware that limits user modification. Projects like this demonstrate that with enough technical skill and persistence, these limitations can often be overcome, unlocking new functionalities and extending the lifespan of devices.

The surprising detail here is not the low cost of the hardware, but the relative ease with which a common consumer device's core cellular modem functionality can be repurposed. The open-source nature of OpenWrt and the ubiquity of AT command interfaces for modems make this kind of hack feasible, provided one is willing to dive into the command line and serial communication.

Future Possibilities and Considerations

The developer mentions that the project is ongoing and has potential for further development. One could imagine integrating this device with other IoT platforms or home automation systems. For instance, it could act as a bridge, allowing systems to send alerts via SMS when certain events occur, or receive commands via SMS to trigger actions.

Security is a critical consideration. Exposing a device to send and receive SMS messages means it could potentially be used for spamming or other malicious activities if not properly secured. The developer would need to implement robust authentication and authorization mechanisms to prevent unauthorized use. This might involve IP-based access controls for commands, or even a simple web interface with a password.

Another aspect is power consumption. While hotspots are designed to be relatively power-efficient, continuous polling of the modem for messages could impact battery life if the device is meant to be portable. Optimizing the polling interval and using modem sleep modes would be important for extending operational time.

The availability of compatible hotspots is also a factor. While the M7350 is a good candidate, finding similar cheap hotspots with accessible internal hardware and modems that support standard AT commands would be key for others wanting to replicate this. The process of flashing OpenWrt can also vary significantly between devices, sometimes requiring hardware modifications or specific flashing tools.

Ultimately, this project serves as an inspiring example of what can be achieved with a deep understanding of embedded systems, Linux, and cellular communication protocols. It transforms a seemingly simple consumer gadget into a versatile tool for custom communication solutions.