My downstairs loo is not the brightest of places. I would like the light to come on automatically when you go in there so it doesn't appear too dim.
One obvious way is with a motion sensor. But I've heard people complain that it senses people walk past the door even when closed (I guess it can see through the gap) and turns the light on when it's not wanted. As the downstairs loo is right off the main hallway with lots of to and fro traffic, this could become annoying.
A separate motion sensor (PIR) can be wired into the light switch and you use your existing light. Or you can get lights with motion sensors built in, such as www.lights.co.uk/round-aras-led-ceiling-lamp-sensor-matt-nickel.html, but they all seem to be too big for a small room at 30cm and more. The one I've linked to is the smallest I could find, but still at 26cm in diameter.
Another way would be a sensor on the door which turns on the light for, say, five minutes enough for a person to do their thing and leave. But the door sensors I have seen are for wardrobes etc which turn on the light when you open the door and turn it off again when the door is closed plunging the person inside into darkness.
If anyone has done this and has a recommendation, I'd love to hear it.