I want to change only ONE of the switches to a smart switch. I am confused about what I need to buy. Can I just use one of these smart switches? What confuses me is - how does the cover plate work? Do I need to change both to smart switches or do they make a cover plate with one of each? How do I find that on the Internet, i.e. which search terms to use?
The large rectangular opening is called "Decora"
There are only 3 popular shapes:
- Small slot: the traditional switch, like you have now.
- Two ovals: for traditional receptacles (also dual switches)
- Large rectangle aka "Decora": which covers your smart switch, AND all Decora switches, AND all Decora receptacles, including all GFCI and AFCI receptacles and switches.
The big three. "blank" not pictured. There are others, but they're rare.
Any competent hardware store will stock 2-gang cover plates of every possible combination, including "1 side blank" and "both sides blank". Typically in at least 3 colors!
They also make plain switches in Decora, so if all else fails, you could replace the other switch with a "Decora plain switch", wired exactly the same. And then you could use a 2-gang "Decora both sides" cover plate. Those are so common Walmart stocks them.
graphic courtesy Kyle Switch Plates source of graphic
Yes, you will need a new cover plate. They make them in all sorts of combinations.
Here is an example: Decorator/Toggle plate
You'll need to open it up to see exactly what is going on in the box and confirm that a neutral is present if it's needed for the smart switch. Flip breakers until all controlled lights turn off, then remove the plastic cover plate and screws at the top and bottom of each switch under the coverplate. Carefully pull the switches out so you can see what's going on in the box and take a picture that clearly shows all wires to add to your question.
Your smart switch probably needs line and neutral, which are usually present in a 2 gang switch box. Assuming the required conductors are present, one or both switches can be switched out for a smart switch.
You have old style toggle cover plate, which can be replaced with decora for the smart switch by replacing the other switch with a generic decora switch. Your hardware store may also carry combo plates with 1 toggle slot and one decora slot. Also note/measure the dimensions of the box, where you live and what splices are present in the box. The smart switch you want to put in may be oversized and not acceptable in the box, so you need to check. Method of calculating box volume varies by region.
When you screw everything back together, take care to align the two straps so that the cover plate sits straight and level with the switches traight, level and evenly spaced.
Oh hold on. Just checked your link and I've never heard of "Gosund" so it's probably rebranded off brand stuff, and anything electrical on Amazon requires a critical eye. You need to check what certification marks are required in your region and make sure that any parts you want to install bear those markings. For example, Canada allows electrical parts with markings from either Canadian Standards Association(CSA) or Underwriter's Labs(UL).
Furthermore, if you are buying from Amazon, Ebay or Aliexpress, you should not assume that a device that has the correct markings is legitimately marked.
You want a combo cover, unfortunately there is no single term that describes that shape. Decora is brand name term works pretty good, but you can also use terms rocker, decorator, or gfci combo covers.
Since you provided a link to Amazon I would caution that their pricing sometimes preys on the inattentive or uninformed buyer. You can get a effectively the same cover for $14.99 or $1.27. The $1.27 is probably the biggest vendor in the business (and owner of the Decora brand).