Hi Alfred,
First I would like to thank you for your quick response/reply regarding my project. I took your advice and kept searching, both on internet and this forum. I bought some components and an “experimental card” (no soldering needed yet). I got the fade-in and fade-out working and I’ve also added a 555 timer circuit with an adjustable resistor to control the timing (how long I want my LEDs to light up my staircase). Last weekend I tuned up everything just the way I want it. Since then I’ve been struggling with the TRIGGER 
The actual circuit that will trigger my 555 timer circuit to turn the LEDs on. There are numreous inspiration projects on the internet but I can’t figure out if these will do the job. They are called: IR beam breaker, ultrasonic barrier, IR transmitter and receiver, laser etc etc. I would like to find the perfect (simple and more or less cheap) trigger circuit that could do the job in both daylight and at night time, as soon as someone takes the first step, either on the bottom floor OR from upstairs, it will trigger the 555 timer circuit.
I appreciate any guidance towards my goal. I’ve ordered the last piece of railing (upstairs) from the “wood firm” and together with these LEDs – this project, my mission will be complete 
Best Regards,
Peter