Hi @jmuraca,
Thanks for the clarification! – Indeed, the solution to your requirement is pretty simple and should be obvious, but that’s one of the common observations here about SmartThings and the mobile Apps – the system is not intuitive to a LOT of people! 
If you have created a Device Type and Device Instance (handler for your Arduino) with capability.switch
and/or capability.contactSensor
(ref: https://graph.api.smartthings.com/ide/doc/capabilities), then you are half-way done.
Now any existing or new SmartApp that allows you to select Device(s) of these capability(s) will be able to utilize your Thing. There may be some good references in the SmartThings online documentation, but let me try to describe a couple scenarios here. (Please feel welcome to write me (@tgauchat) a Private Message and we can even arrange a little time to walk through these over the phone or screen share…).
To control a light or switch, you could use the “Lights & Switches Shortcuts” page off of the main Dashboard.
Select the gear
icon and you will be presented with a list of existing shortcuts, and at the bottom of the list there is an option to Add new, or Auto import additional Things (this need not include your Arduino Thing, though, because the Arduino is a trigger, not a target):

The same page can be used to select from among the lights and switches shortcuts (or a group shortcut) and attach a SmartApp to them, from a selection of simple SmartApps that activate based on various conditions (such as open/close
of a .contactSensor
, or attach a non-default .switch
to the light or group.


The alternative way is to add and configure a SmartApp directly from the Dashboard “+” Page: 
But you actually have to explore quite a few pages of “Alerts”, “Actions”, and “Mores” before you might even settle on the exact same basic switch to light connection you reached via the Dashboard - Lights & Switches Shortcuts I described above.
A random example SmartApp that gives you a little extra functionality is “Smart Light Timer”…

Note that this specific SmartApp is hardcoded to only accept motion detectors (capability.motion
) and contact sensors (capability.contactSensor
) as triggers:
I hope this very brief overview helps, though it is difficult to demonstrate the design and functionality of SmartThings via a few words and screenshots… You can dig around the online documentation and help pages where there are more examples, details, and videos even.
But as I said … I’m more than happy to walk through this with you. There are various bits that might not work as expected depending on your overall setup.
Cheers!
…CP (Terry).