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So we all know that SonOff devices are basically an ESPxx device with a mains PSU and a relay. Pretty much all of the discussions of flashing include very sensible warnings about not trying to involve mains power when also connecting a 3v3 serial port and a computer all together.

It seems that the boards are well designed from a 'low voltage' electrical safety point of view, reasonable clearances and PCB cutouts, etc.

At least the TH-10 has a non-mains port (for the sensor input), so maybe it is properly double insulated.

On the SonOff Mini, I see the antenna wire comes with a 'strong electricity' warning, so I assume this is either live, or only trivially insulated.

If I add additional GPIO connections to a SonOff device, should I assume they are potentially mains potential, and treat as such (using earthed enclosure and 600V insulation), or can I safely expose these terminals for human contact?

Are all SonOff devices equal in this respect, or are some less unsafe than others (regardless of the certification, there is always some potential for failure).

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Absolutely not. The SonOff Mini does not have a transformer power supply, and the 'switch' contacts are at half mains potential when working. This has the unfortunate side effect that if the antenna is not well insulated, a short to the antenna seems to be able to destroy the device (I assume it was a ground short, but it was in the ceiling void). Replacing R9 did not restore it, nor did powering from 3v3.

From a couple of tests (using 240v supply):

  • Sonoff Basic R3: High impedance ~70v on the mcu side, no voltage on 'low impedance' measurement.

  • Sonoff Mini : Low impedance 110v

  • Sonoff RF: No voltage 'low impedance', slightly less high impedance.

So SonOff mini really does have mains voltage on all terminals, the others seem to be isolated to some indeterminate safety standard.

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