Reverse engineering · Hardware · Firmware
Automatic flasher for BK7231N modules
A Python + Arduino Mega tool that automates flashing open firmware (OpenBeken) onto BK7231N WiFi modules by controlling their power with a transistor.
- Year
- 2026
- Status
- Workbench tool
- Role
- Hardware and software design
- Repository
- Repository coming soon
01 · Problem
To flash open firmware onto a BK7231N you must reset it exactly when the tool asks for it. The module on this board does not expose the enable pin (CEN), so doing it by hand —pulling wires at the right moment— is unreliable.
02 · Approach
An Arduino Mega cuts the module ground with an NPN transistor to power-cycle it on command. It can work alongside a CH341A adapter set to 3.3 V or act as the serial bridge itself, with a voltage divider on TX. A Python script orchestrates everything with ltchiptool.
03 · Result
Two working modes (two ports, or the Arduino as a bridge), power-on, power-off and download-mode reset commands, reading the flash to a file as a backup, and a console interface with per-phase progress.
04 · Stack
- Python
- pyserial
- ltchiptool
- Arduino Mega 2560
- CH341A
- OpenBeken
The trick: reset by cutting ground
Arduino pin 7 ──[1 kΩ]──► NPN base (2N2222 / BC547) collector → BK7231N module GND emitter → Arduino GND
pin 7 HIGH → module has ground → onpin 7 LOW → no ground → offThe Arduino listens for simple serial commands (POWERON, POWEROFF, DOWNLOAD, RESET, BRIDGE) and, in bridge mode, passes data transparently between the PC and the module.
Non-negotiable precautions
- Always 3.3 V. The module does not tolerate 5 V: the Arduino TX goes through a 1 kΩ / 2 kΩ divider, and the CH341A must be set to 3.3 V.
- Do not touch the SPI bus of the internal flash or its supply: only the board’s VBAT, GND, RX and TX are used.
- Backup first. The script can read the whole flash to a file before writing anything.
When the chip is beyond saving, the other way out is to replace it: that is how the ESP32-C3 smart switch was born.