I found the remaining inductor and soldered it onto the board. And I also added the connector, connected it to a LED and a power supply and switched on. And see! It works! Without any fiddling or whatever
And immediately I had to play around with it and combine it with a PIR sensor. The nice thing is that the circuits are almost fully compatible. The ouput of the HC-SR501 can be directly connected to the PWM input of the LED driver. And both run on the 12V power supply. Unfortunately the PIR will only work up to 20V so my planned 24V will be a bit too much for it.