Are there any “reasonably safe” options to power a Spark Core inside a single gang wall box from 120? It’s partly a connector problem in that there’s plenty of space for a 120v->5v transformer/supply, but I’m not sure how to do it most safely and simply.
This is a great question, and I’ve been toying with ideas on how to solve this as well. Especially since I would like to have the Spark Core control some existing 3-way switches as well. That said, it gets a little tight in a 1 or 2 gang box with just the wiring and switches/outlets. Also most of them are metal so that doesn’t help…
Something I just thought of that might be a nice solution without having to make a custom wall switch or outlet… remove the wall outlet or switch plate, figure out which side of the electrical box is secured to the wall stud… then cut away the drywall on the opposite side large enough to add another electrical outlet. However instead of using a metal electrical outlet box, use the plastic ones that they sell and secure it to the first outlet box with some short bolts or better yet rivets. You can then buy an outlet cover or switch plate that is a double or triple or whatever you need. I haven’t looked but I’m sure they have different configurations that don’t require 2 or 3 switches or outlets, but possibly have just one switch and one side completely covered flat. Or use covers for those Leviton switch plates and 3D print an insert cover for the Core side (which would be the right side of this picture):
Then to actually run the power, something that can wirenut to AC power and provide a 5V 1A output would easily fit in the plastic electrical box with the Core and whatever else you need. A module like this might be pretty good as well, I’ve never tried one:
http://www.ebay.com/itm/5V-700mA-AC-DC-Power-Supply-Buck-Converter-Step-Down-Module-/371064465501
Get’s cheaper the more you buy:
http://www.ebay.com/itm/10pcs-5V-700mA-AC-DC-Power-Supply-Buck-Converter-Step-Down-Module-New-/400762335085?tfrom=371064465501&tpos=top&ttype=coupon&talgo=undefined
So the UL508A and UL1950 standards would be really good to read up on. It can be done safely but there are lots of thorny issues here, especially if you expect your home owner’s insurance to be in effect if there are problems.
Good points @bko. I’ve designed products to those standards and still have not read them cover to cover. It’s also just about impossible to interpret the standards yourself such that you can design something that would pass UL standards. You have to do your best, and then put it in the hands of someone at UL and see what they uncover, while being ready at the drop of a hat to make quick minor changes.
Also, unless you already have access to them for free, I can’t see anyone spending the money to buy a UL standard… best bet is to use all UL rated power supplies, fire retardant enclosures, non-pvc jacked wire (silicone or xlpe), use fuses and play it safe.
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@BDub et al, might I remind you of the Electrical Construction Code(s) which govern the (minimum) safety standards for electrical wiring in a residence. Any modifications to existing outlets and (possibly) wiring should not be taken likely and increases the risk of injury or fire. I recommend doing bench testing using a “simulation” of the electrical boxes and circuit using a standard 3 prong plug to provide power. I am doing that for developing a pool filter timer based on a Core and an SSR.
I am always surprised at how much “stuff” can be put in a single gang box (eg. touch dimmer) when it is carefully designed. That usually means a custom PCB but it is possible. Just remember, it may “only” be 120V but it can kill you. 
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The other option is something like this: http://www.leviton.com/OA_HTML/ProductDetail.jsp?partnumber=1306§ion=42484&minisite=10251 plus an off the shelf miniature USB charger. The apple one is probably a good value given the size/quality/cost. I think that’d fit in a “reasonable” single gang box if I could avoid a bunch of bundled up romex.
I basically want to experiment with the spark controlling my hue lights. I’ve run some prototypes using batteries but ideally the switch sits in the light switch location and runs off the 120v AC line so it doesn’t need batteries. I found some AC->DC converters like this (http://www.farnell.com/datasheets/606238.pdf) so is this all I would need? Would I have issues with heat? Would the AC transformer interfere with the spark?