Flashing Sonoff S31 With Flipper Zero (No USB Dongle Needed!)

Table of Contents

A step-by-step guide on flashing the Sonoff S31 smart plug with Tasmota, using only a Flipper Zero (no USB-to-Serial dongle, no soldering required).

I was having challenges with my old Wemo devices. They were constantly dropping connections and I wanted to get away from cloud dependencies. After some research, I came across the Sonoff S31 — a smart plug available in two variants: Zigbee and Wi-Fi.

What really caught my attention was the Wi-Fi version: it could be flashed with custom firmware like Tasmota or ESPHome, making it fully local and perfect for my Home Assistant setup. Sold!

But I still had one big question:
How can I flash these devices without a USB-to-Serial dongle?

Looking at my desk, I noticed my Flipper Zero. Could I use its GPIO pins to flash the Sonoff? Challenge accepted.

Opening the Sonoff S31

Opening the device is straightforward. Use a knife or screwdriver to pop off the gray cover, then remove three screws. That reveals the board inside:

Sonoff Opened

On the PCB, you’ll see the flashing header pins. For our purposes, we only need VCC, RX, TX, and GND:

Sonoff Pins

Wiring the Flipper Zero

The Flipper Zero provides both 5V and 3.3V outputs.
⚠️ Important: the Sonoff S31 expects 3.3V, so make sure to use the right pin.

On the Flipper Zero, connect:

  • 3V3 → VCC
  • TX → RX
  • RX → TX
  • GND → GND

Notice the TX/RX inversion: your Flipper’s TX must go to the Sonoff’s RX, and vice versa.

Here’s what my setup looked like:

Flipper wiring

To avoid soldering, I used test hook clips (Amazon link) instead of pin headers. That way I didn’t need to modify the board permanently.

Setting Flipper Zero to USB-UART Bridge

Next step is to expose the Flipper as a USB-to-Serial adapter:

  1. On Flipper, go to GPIO → USB-UART Bridge
  2. This creates a COM port on your computer that we’ll use to flash firmware.

USB UART

When everything is wired, hold the power button on the Sonoff while plugging it in — this enables boot mode.

Boot Mode

Flashing Tasmota

Now comes the fun part: flashing. Head to Tasmota’s web installer.

  1. Click Connect
  2. Select the Flipper’s USB device
  3. Choose the Tasmota (English) build
  4. Wait while it flashes

Tasmota Select
Choose Device
Installing

Wrapping Up

And that’s it! The Sonoff S31 is now running Tasmota. You can connect to its Wi-Fi AP, configure it, and integrate directly into Home Assistant — all without relying on untrusted cloud services.

No USB dongle, no soldering, no nonsense. Just a Flipper Zero + Sonoff S31 + Tasmota = Local Smart Plug Freedom.

Enjoy your new offline-ready smart plugs!

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