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What things will I need to consider for hardwiring a planned electronics project into the wall?

The plan is a DIY smart thermostat. An LCD touchscreen panel will be flush with the wall. Attached to the back will be the electronics, including a miniature computer (a Raspberry Pi) and an interface board of my own design. The interface board will include a board-mounted 120V-to-DC power supply, connections for line voltage, and connections for 24VAC furnace wiring.

I imagine hiding the wiring and tucking the electronics in a 3 gang box and mounting the assembly to the box with something like this: https://www.thingiverse.com/thing:2749782 -- basically the LCD panel+electronics assembly will be screwed to an adapter plate that slides into slots in another adapter plate that is screwed to the box. Then the touchscreen covers the box and everything else is out of sight and out of reach.

Here are some questions I know I have:

• The assembly would very easily be removed from the box (just slide it up against the force of gravity). Great for servicing wiring, but is that too easy? What if someone bumps into it and it falls out and the wiring gets yanked out and then there is 120VAC dangling within easy reach?


• Am I required to enclose the high-voltage parts of the interface board? The board-mounted power supply might be a class II device, but would be a through-hole component meaning the under-side of the interface board would not be finger-friendly. But the under-side of the interface board would normally be out of finger's reach.


• Do I need to ground everything?


• Is the board-mounted power supply required to be a class II device?


• Am I required to include a fuse in the design? Or is that a matter of judgment and engineering? For example, the electronics would have to let a lot of magic smoke out to trip the circuit breaker. Even if the circuit breaker were 1A that's 120W, an order of magnitude larger than the planned power usage. So my judgment says to include a fuse, but I don't know if it's required by code.

Does anything else jump out at you?

Not only will you need to get your local inspector's approval, in writing (and the inspector will defer to Underwriter's Laboratories or other NRTL, so we're talking about getting a UL listing for your one-off) ...

... But all your thermostat wiring must now be re-done in Class I wiring methods Because you are intermixing thermostat control power with mains. You can no longer enjoy the exemptions provided for low-voltage systems such as phone, doorbell, burglar alarm, everyone else's thermostat, etc. You'll need to rewire it in Romex or better, with proper junction boxes instead of informal splices.

Have you noticed something about every single small electronics product of the last 20 years? They all come with wall warts etc. You know why? Because that is how you pass muster with Underwriter's Laboratories and the various other NRTL's (Nationally Recognized Testing Labs). There are still standards to meet, but it is much more achievable when your product uses an external wall-wart that is already UL listed.

if you do any competitor research, how does Nest get 120V power? Ecobee? Ring doorbell? They don't. They use the 24 volts AC that is already present in the thermostat circuit between the R and C wire. That means they sidestep the hard part of UL listing.

So you have precisely 3 options:

• set things up so your project is powered off the 24VAC thermostat transformer


• use a wall wart to make a different low voltage, and feed that up via spare thermostat wires


• go through the rigmarole of getting your homebrew project UL listed

If there is a existing C wire for the thermostat (that you could power a commercial smart thermostat with) you can use a non-insulated 24AC to DC converter. Using a bridge rectifier and a switch-mode power supply (plus capacitors as needed). The transformer at the furnace be protected against short circuits.

This also immediately means you don't have to worry about any high voltage wiring.

Commercial thermostats tend to have a fixed mounting plate where the wires screw into and the thermostat itself slots into that and there are blade connectors that make the connection to the wire. That way when removing there is no chance of yanking the wires.

That kind of connection doesn't look that hard to add to your wall mount.