Cellular SIM Tester — Digi WR21

Overview

This tool validates whether a cellular SIM card is properly activated. Running as a Python script on a Digi WR21 embedded router, it identifies the SIM’s vendor or carrier, then systematically attempts each known APN associated with that carrier. Status and results are shown on an attached 2-line USB/Serial LCD text display.

Hardware Interface

A RGB rotary encoder is connected via I²C to provide a simple user interface, allowing navigation and confirmation without a keyboard. The encoder and LCD share the same I²C bus, presenting a unified interface to the host Python application.

I²C Driver Fork

The open-source driver for the 2-line LCD display originally supported I²C in slave mode only. This project forks that driver and extends it to support both host and slave modes simultaneously, while remaining fully backwards compatible with existing code. This enabled the rotary encoder to be hosted on the same driver, giving the Python application a single path to read input from the encoder and write output to the display.

Use Case

SIM provisioning validation can be tedious when carrier APNs vary and activation status is opaque. This device automates the guesswork — insert the SIM, and the tester walks through the known APN list for that carrier until a data connection is confirmed or all options are exhausted.