Sonoff S20 firmware for CatBus

This is firmware for the Sonoff S20 IoT wall switch, to be controlled by CatBus.

Inside the Sonoff S20 is an ESP8266 connected to a button and a relay, and the serial pins are hooked up. It is thus fairly easy to take apart and reflash with something else, in this case MicroPython.

The firmware:

• Uses the front button to toggle the switch on and off.
• Listens to an MQTT topic to turn the switch on and off.
• Publishes the state of the button to an MQTT topic when the button is pushed.

Flashing

1. Install the latest version of esptool from PIP. Don't use the version from APT, it's too out-of-date.

2. Erase the existing flash with $ esptool.py --port /dev/ttyAMA0 erase_flash, and hard reset the device.

3. Write the MicroPython firmware with $ esptool.py --port /dev/ttyAMA0 --baud 406900 write_flash 0 micropython.bin, and hard reset the device.

4. Connect to it with $ screen /dev/ttyAMA0 115200.

5. Press C-b to bring up the MicroPython prompt, and run import webrepl_setup, enabling it at boot. This allows you to update the firmware over WiFi.

6. Disable the WiFi Access Point (AP):

   import network
   ap_if = network.WLAN(network.AP_IF)
   ap_if.active(False)

7. Enable the WiFi client:

   import network
   sta_if = network.WLAN(network.STA_IF)
   sta_if.active(True)
   sta_if.scan()
   sta_if.connect('<essid>', '<password>')
   sta_if.isconnected()
   sta_if.ifconfig()

8. Copy main.py and catbus.py using webrepl_cli:

   $ python3 webrepl/webrepl_cli.py -p <password> main.py 192.168.16.42:/main.py
   $ python3 webrepl/webrepl_cli.py -p <password> catbus_config.py 192.168.16.42:/catbus_config.py

9. Run the catbus_config setup utility:

   import catbus_config
   catbus_config.edit()

10. Reboot the switch by unplugging it, waiting for around 10 seconds for the capacitors to drain, and plugging it back in.

Links

• Tasmota is an all-in-one firmware for such devices.
• MicroPython docs for ESP8266.

Snippets

To get the device's MAC address, run:

import network
import ubinascii

ubinascii.hexlify(network.WLAN().config('mac'), ':').decode()