Low voltage modification to isolate SONOFF ZBMINI ZigBee mini smart light switch

I have a bathroom exhaust fan-light combo that uses a simple low voltage switch loop to control the fan portion.

I was not able to find an inwall (box) style Zigbee or Zwave relay module that had a simple relay/switch isolated from the 120V line and neutral. I did find a couple of raw relay boards that supported Zigbee but did not feel conformable using them with an enclosure.

I ended up buying one of the SONOFF ZBMINI ZigBee mini smart light switch module almost by mistake. I’m pretty sure one of the Q&A’s said it was isolated. When it arrived I quickly noticed it was not what I wanted. I figured for $14 I might as well take a closer look.

To my surprise, the needed modifications looked to be pretty straightforward. I started by desoldering the main pins that connect the line terminals together and are that are shared with the relay: See Image #1 below (I was only able to attach one media item because this was my first post to the forum)

Next, I cut the trace with a utility knife. I started with the top trace but still had continuity. The trace is doubled up on the backside of the board so I had to cut all the way through the PCB See Image #2 below

Now the second terminal block was disconnected from the relay. By cutting this small trace I was able to completely isolate the main contacts of the relay from the drive circuit See Image #3 below

Last, I added a small jumper wire to connect the second terminal block back into the control circuit. I also resoldered the first terminal back onto the board See Image #4 below

That was it! The final touch was to grab my kid’s paints and color code the terminals to make installation a little easier. While I was writing this post I noticed that I could have reversed the color code on the first two terminals. The L and N terminals are being fed into a rectifier circuit it is probably not a big deal. Everything worked as when wired per my color-coding.

See Image #5 & #6 below

L & N: Line and Natural 120V wires that power the control circuits
1 & 2: Relay controlled terminals (used to turn on/off the fan in my install)
S & S: Optional wall switch to enable local controller.

Everything wired up pretty easily and I was able to simply turn the power back on and paired it with Zigbee2mqtt. Just one simple push of the pairing button!

1 Like

Whilst that does work I would be extremely concerned about the insufficient LV to ELV isolation creepage distances. And A-N isolation creepage distances.

See this article for more: https://www.powerctc.com/en/node/4757