home automation - What's a compact way to control a light switch with Arduino without interacting with high voltage?
you can glue an LED controlled by an MCU into the sensor hole of a dusk to dawn sensor. these are available in socket and outlet shapes, are UL certified, and require no messy wiring. I've been able to switch an outlet model by duct-taping an ESP8266's built-in LED against the window; can it get any simpler?

Here's a working example's parts, exploded for easy identification.


You can spot an inline socket-to-outlet adapter, a dusk-to-dawn sensor, a bulb, a cheap nodeMCU ESP8266 devboard, a short micro usb cable, a USB wall wort, and an RGB LED module attached to dupont wires. I'm wiling to bet anyone here can figure out how these pieces fit together to make a $10 "smart bulb". You can get most of these parts at the local home center, and everything shown on amazon/ebay.

Of course you won't get a lot of respect from EEs, but you will get the job done quickly and safely, and the opto-isolated controls work with any platform. I have cheaper/smaller build i've moved onto, but i still have several of these switch builds in-place for years and they work great. 

I would recommend getting a dusk-to-dawn switch that is labeled as being compatible with LED bulbs; they tend to use a lower-"bleed" IGBT instead of the older/cheaper diac/triac combo which causes some low-energy bulbs to flicker when off.