What is Reddit's opinion of BestMounts - Low Voltage Mounting Bracket 2 Gang Multipurpose Drywall Mounting Wall Plate Bracket (Dual Gang 6 Pack, Orange)?

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This is the only electrical code compliant method I can come up with. I have the same need, with an older house, built on a concrete slab.

1. Put a 2 gang old work low voltage bracket and mount it next to an existing receptacle.
2. Tear out the receptacle box, and replace it with an old work box that has a 1/2" knockout hole drilled in the side towards the low voltage hole. This may require replacing with a 2 gang box, if you cannot neatly get the old box out.
3. Get a doorbell UL/ETL listed transformer that mounts on a 1/2" knockout. You have to pick what voltage you want and what price, but there are a lot availble. Attach the low voltage leads long enough to extend out of the wall. Reach through the low voltage hole and insert the 120 volt leads into the side of the receptacle box, and imstall the lock nut.
4. Connect the 120 volt transformer leads to the receptacle supply.

You can now get a dc power supply for your device that will work from the low voltage AC. You may need another bracket higher up the wall for sensors, etc.