Cast the Lumos spell by saying clearly and loud "Lumos"! Of course, you can switch the light off by saying "Nox", but surely you already knew that spell.
You can choose among 5 different wands for your interface: Harry's, Hermione's, Ron's, Dumbledore's or You-Know-Who's.
The voice recognition service only works with internet connection. If you don't have internet, don't worry, there's a small button you can use to turn on and off the light (boring...).
Last but not least, remember that happiness can be found even in the darkest of times, if one only remembers to turn on the light.