This guide was written using Sonoff Mini v1.0 (MPN:IM190416001 on the back of the box)

This will be the steps we make:
Original Firmware → Tasmota Firmware → ESPHome Firmware

• power on Sonoff Mini.
• Download eWeLink app, configure the sonoff as you normally would (hold the button for 5s to enter pairing mode)
• Upgrade its firmware to the latest version (currently 3.6.0)

• Put the Sonoff in DIY Mode following the steps below (found here):

  • Long press the button for 5 seconds to enter pairing mode, then press another 5 seconds to ender Compatible Pairing Mode (AP). The LED indicator should blink continuously.
  • From a mobile device, connect to a WiFi named like ITEAD-XXXXXXXX with password 12345678
  • On the same device, open a browser and go to http://10.10.7.1/, this will open the WiFi settings of the Sonoff
  • Here click WIFI SETTING, insert SSID and Password of your real WiFi connection where you want to connect the Sonoff and press SAVE.

  Note: the first time you do this will probably fail. It will connect your sonoff to your wifi but the LED on the Sonoff will be ON without blinking and will not be in DIY mode. Just repeat the steps from the first (long press button etc) and everything will be good.

• Check if your Sonoff is in DIY mode by opening a terminal window on your PC and running these 2 commands (macOS or Linux only, sorry windows):

  export SONOFF_IP=<your_sonoff_ip> # replace this with your Sonoff IP
  
  curl -XPOST --header "Content-Type: application/json" --data-raw '{"deviceid": "", "data": {}}' http://$SONOFF_IP:8081/zeroconf/info

  It should respond with a json with some data.

• Unlock OTA with this command:

  curl -XPOST --header "Content-Type: application/json" --data-raw '{"deviceid": "", "data": {}}' http://$SONOFF_IP:8081/zeroconf/ota_unlock

• Flash Tasmota Firmware:

  Before going on note one thing: Sonoff Mini supports a MAXIMUM of 508KB of firmware size, do NOT flash anything bigger.

  You will find the lite firmware version on sonoff-ota.aelius.com, go there and copy the SHA256 string of that firmware and put it inside the HASH variable in the first command below.

  export HASH=<sha256_firmware_hash>
  
  curl -XPOST --data "{\"deviceid\":\"\",\"data\":{\"downloadUrl\": \"http://sonoff-ota.aelius.com/tasmota-latest-lite.bin\", \"sha256sum\": \"$HASH\"} }" http://$SONOFF_IP:8081/zeroconf/ota_flash

• Pick your mobile device and connect to a WiFi network named like tasmota_XXXXXX-#### without any password (taken from Tasmota initial configuration)

• Open a browser and go to http://192.168.4.1 (it could open automatically for you) and insert again your real WiFi Credentials, then hit Save.

• When done, go to your PC and open a browser on your Sonoff IP, it should open the Tasmota Web UI.

To keep things simple, we will use ESPHome CLI, so you will need Python installed on your PC.

• Install ESPHome CLI on your PC with these 2 commands:

  pip3 install wheel
  pip3 install esphome

• Create a new folder where you will store (locally) your ESPHome firmwares and config files. In my case I made it under /home/<user>/Projects/esphome
• Inside that folder create a start.sh file, with this content:

  esphome dashboard ./config/

• Inside that same folder, create a new config/ folder, used by the command above

Remember to backup your config files, those are the "source" of your firmwares containing their configuration, they cannot be recovered from an existing flashed device. It would be a good idea to make a private git repo to store them.

• Inside the config/ folder create a secrets.yaml file that will contain sensitive informations like WiFi SSID, Passwords, API keys etc, and insert this content inside it (insert your actual WiFi data, and some new password for api and ota):

  # Your Wi-Fi SSID and password
  wifi_ssid: "My WiFi Name"
  wifi_password: "my_wifi_password"
  api_password: "invent_a_password"
  ota_password: "invent_a_password"

• Again, in the config/ folder create a sonoffmini.yaml that will be your sonoff configuration for your firmware, and insert this (remember to customize device_name, device_ip and device_gateway if needed):

  Config for Sonoff Mini used as a lamp, see more here.
  Just a tip: decide your device IP that you want BEFORE doing any flashing, and then put it in here, because changing IP afterwards is a bit annoying. Remember to assign that IP to your Sonoff inside your home Router.

  substitutions:
      device_name: sonoffmini
      device_ip: 192.168.1.x
      device_gateway: 192.168.1.1
  
  esphome:
      name: ${device_name}
      platform: ESP8266
      board: esp01_1m
  
  wifi:
      ssid: !secret wifi_ssid
      password: !secret wifi_password
      manual_ip:
          static_ip: ${device_ip}
          gateway: ${device_gateway}
          subnet: 255.255.255.0
      # Enable fallback hotspot (captive portal) in case wifi connection fails
      ap:
          ssid: "ESPHOME"
          password: "12345678"
  
  logger:
      
  api:
      reboot_timeout: 15min
      password: !secret api_password
  
  ota:
      password: !secret ota_password
  
      # the web_server & sensor components can be removed without affecting core functionaility.
  
  web_server:
      port: 80
  
  sensor:
      - platform: wifi_signal
          name: ${device_name} Wifi Signal Strength
          update_interval: 60s
      - platform: uptime
          name: ${device_name} Uptime
      #######################################
      # Device specific Config Begins Below #
      #######################################
  
  binary_sensor:
      # the 7 lines below define the reset button
      - platform: gpio
          pin: GPIO00
          id: reset
          internal: true  # hides reset switch from HomeAssistant
          filters:
          - invert:
          - delayed_off: 10ms
      # the 3 lines below toggle the main relay on press of reset button
          on_press:
          - light.toggle:
              id: light_id
  
      # the 13 lines below toggle the main relay on command
      - platform: gpio
          name: relay_toggle
          internal: true  # hides relay toggle from HomeAssistant
          pin: GPIO04
          id: gpio_light_id
          on_press:
          then:
              - light.toggle:
                  id: light_id
          on_release:
          then:
              - light.toggle:
                  id: light_id
  
      # the 2 lines below create a status entity in HomeAssistant.
      - platform: status
          name: ${device_name} Status
  
  status_led:
      pin:
          number: GPIO13
          inverted: true
  
  output:
      # the 3 lines below control the main relay
      - platform: gpio
          pin: GPIO12
          id: main_light_relay  
      # the 3 lines below control the Blue LED
      - platform: esp8266_pwm
          id: blue_led
          pin: GPIO13
          inverted: True
  
  light:
      # the 4 lines below define the main relay as a light
      - platform: binary
          name: ${device_name}_light # you can enter a custom name to appear in HomeAsistant here.
          output: main_light_relay  
          id: light_id
      # the 4 lines below define the Blue LED light on Sonoff Mini, to expose in HomeAssistant remove line "internal: true"
      - platform: monochromatic
          name: ${device_name}_blueled
          output: blue_led
          internal: true # hides the Blue LED from Homeassistant

• Make start.sh executable and run it:

  chmod +x start.sh
  ./start.sh

  This will start a Web UI to manage your ESPHome configs and install them on your devices on http://localhost:6052
• Here you should see your config created before and your secrets under the secrets section
• We now need to create the firmware in .bin format to manually flash it inside our Sonoff
  • In the ESPHome Web UI, on your sonoffmini config "box" click the bottom right "3-dots" button, and click "Install"
  • On the window that opens up, click Manual Download > Modern Format
  • After it finishes it will download a sonoffmini-factory.bin file. That, finally, is our ESPHome firmware.
• In your browser open your Sonoff IP to access Tasmota, click Firmware Upgrade, under "Upgrade by file upload" upload your sonoffmini-factory.bin and click Start upgrade
• Once finished you should see the ESPHome Web interface opening a browser tab on your Sonoff IP

• Open the Home Assistant ESPHome Integration page
• Click the blue "Add integration" button to open your Home Assistant dashboard
• Here proceed with the configuration, using as Host your Sonoff IP, and leaving the Port as it is.

Done! Now you should have your Sonoff between the devices configured in your Home Assistant ready to be used.