I want to trigger an action with a touchless sensor or, even better, a distance sensor (to detect presence). But the sensor should be behind a solid plastic case of a few millimeters. For example, the two circles of the Ultrasonic Sensors have to be covered. Is there a sensor that can do that?
I prefer a long distance detection, about 100cm, but if it is not possible, a few centimeters can work as well (hand wave).
Whats the application? The distance need to be in centimeters? A PIR could be suitable?
Check out ,VL6180X for 100mm and VL53L0X for 1m range.
The application is smart touchless switch. I want that it click on a switch when someone enters the room (or at least hand wave gesture).
The PIR can work for my application, but can the sensor works behind a solid plastic case?
Thanks! Can they work behind a solid plastic case?
Search for RCWL-0516.
Looks interesting. I will give it a try. Does it work even if it covered by a solid plastic case?
RCW, yes.
Amazing! I will give it a try. Thank you!
I prefer a long distance detection, about 100cm
The RCWL-0516 is a microwave motion detector, and will respond to movement anywhere in a reasonably large room (several meters in any direction). Even through walls!
yes but not sure about the thickness .. I could check tomorrow.
Through walls is problematic... I want to detect presence in a specific room, not entire house 
The range is limited, but wood walls do not stop microwave signals. The microwave motion sensors are dirt cheap, so experiment!
Microwave or PIR or ultrasound can't do presence.
They can only detect motion.
Leo..
I tried 1mm plexiglass over VL6180X and it wors great. I don`t have thicker piece of plastic to try, but I guess it sould be fine.
Something strange, I just tried APDS9930 behind the same piece of plexiglass and it`s not working...