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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?

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    Way too much for a single question. Please break it up, and remove any opinion-based components (such as the ease of removal thing). They're off topic. – isherwood Jan 16 at 16:41
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    But why. You're talking about $300 worth of stuff instead of a $100 thermostat that might cost zero dollars after a government rebate. – Mazura Jan 16 at 19:21
  • Also, make sure you have a mechanical back up in case your buggy software locks up the device while you're gone and the house freezes in winter. – whatsisname Jan 16 at 22:08
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    @Mazura But simply buying one is not nearly as fun – bta Jan 17 at 0:58
  • It's all fun and games until... you're out of town; wife's in labor, and the power's out because the freaking switch on the wall doesn't work. Also, she says it smells like burning plastic... – Mazura Jan 17 at 4:43
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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
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    It's good to find out how dumb an idea is before actually implementing it. Thanks for the insights – Matt Thomas Jan 17 at 4:13
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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.

  • This is the right direction. I can't imagine OP really needs 120v for anything thermostat or computer related. If the 24AC power supply for the A/C isn't providing enough power, it can be upgraded (but probably not needed). Once you're down to 24VAC it gets simple. – JPhi1618 Jan 16 at 18:31
  • @JPhi1618 Actually, I was kind of concerned about peak current draw through the 40VA transformer and blowing a fuse. I thought it might have been simpler to skip all those worries by just tapping into a beefier power source. It actually could be a little cheaper, too, just in terms of component costs: 120V AC-DC converters are much more common than 24V. – Matt Thomas Jan 17 at 4:35
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    @MattThomas well, another option is to use a pair of unused wires in the thermostat cable. They often run more wires than needed, so putting 12vdc with your own transformer could be possible. – JPhi1618 Jan 17 at 4:47
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    @MattThomas you can also replace the transformer with a beefier one, up to what the existing wires can handle. buck regulators are also pretty easy to get. – ratchet freak Jan 17 at 10:05

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