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Modify SonOff Basic R2 WiFi for 12V DC supply/switching

In the observatory, as part of the "Power Project" I have installed 4 x SonOff R3 Wifi units, these are 4 Channel WiFi connected relay units that I can switch remotely using Google Home etc. and use them to power up mount, camera and other devices.

I have a couple of devices in the observatory, that I was wanting to be able to control, but have discrete wifi relays due to their positioning. One of these for example is the shutter controller for the dome. It occasionally needs to be power cycles when the comms lockup to the rotator controller, and a single channel WiFi relay would be ideal for this. I could simply put it in series with the power supply switch off the battery to the controller.

So, I was looking for a similar single channel unit, that I could use, but unfortunately, I could only find the SonOff Basic R2, and these are designed to use and switch an incoming AC Supply.

After doing some digging on Youtube, I did find a video that showed a modification for 12v, but these were on an older version of the PCB design.

The units I have are PCB Revision 1.3, and did manage to track down the circuit diagram on the SonOff website, and found this at;

The power circuit diagram is in Section 7.0, and is illustration 1.

Having looked at the board, and looked at the circuit diagram I was able to modify this to operate off a 12Vdc supply and switch that supply on the output terminals.

The modified circuit diagram, with the addition of a 7805 regulator can be seen below;

To facilitate the modifications, I carried out the following;

  1. Remove the inductor L1

  2. Remove the diode D3

Using a 7805 regulator I connected this as follows;

  1. 7805 Input to the upstream side of the removed inductor L1 pad, '2' on illustration 1.

  2. 7805 Output to the downstream side of the removed diode D3 pad, labelled 'C' on illustration 1.

  3. 7805 Common to the 0V (-ve) side of the input bridge rectifier BR1, pin 2

  4. 7805 Common to the 0V (-ve) side of the transformer , labelled 'P2 1',at the joint where it connects to C17 '2'

I attached the regulator to the side of the relay using hot glue, just to keep things in place to make re-assembly easier. Everything fits perfectly inside the original case without further changes.

If you do make the modification, remember to relabel the terminals, L becomes +12V and N becomes -ve.

WARNING: Making modifications to the original manufacturers device will invalidate your warranty.

You undertake this modification at your own risk. Faults may result in damage or fire.

Note: I have also posted the images/schematic change over on my Astrophotography related Github at the following repository;

The post modification images of the PCB changes can be seen below;

As you can see in the third image, I used the airgap to pass the regulator common to the underside of the board.

So there you have it, one modified SonOff Basic R2 for use with 12V DC.

One point to note, is that I have not put any heatsink on the regulator, I do not anticipate it to get very hot, as the pcb circuit is pulling very little power, and the output load does not pass through this regulator, but is switch through the relay via the thick PCB trace and the thick blue wire.


I ran the unit, with the relay energised for over 24hours, and there was no issue with temperature on the 7805. It was warm to the touch, but not hot to the point where you couldn't keep your finger on it.

I suspect it would probably be okay, but guess it will get a little bit warmer with the case closed up, and then when it is running in the observatory in the summer months, where ambient temperatures could get above 35'C, I decided to take the 7805 out of the case, onto a DIY aluminium heatsink. So something to consider if you are doing you own mod, depending on the load and ambients.

You can see the added heatsink below. The unit will be out of the way in the observatory, so not really bothered by how it looks, as nothing can get caught on it or see it.

Until the next time,

Clear Skies!

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