If this is 24/7 then you would just a scene to Scenes - Scripts To Turn OFF and set it to 50%.
If not 24/7 then you would use night lights and night glow.
or if is is different again you could use my 
Smart Light blueprint and use your motion sensor in night light condition with a trigger to turn your lights on at 50%. Then when they are on at 50% when motion is detected set your night lights to 100% and use the If lights are ON, adjust the lights when crossing over with Use cross over time delay from night lights to normal lights in night light control.
With what you have asked I have no idea what you trying to do so I given you some options that are general.
Blacky 