Finally, I managed to work again on the prototype. The zeroing of the handle seems to be a Problem. I did not find much Details on all the other Projects which attempted this. My Approach was now with a Hall sensor and a small magnet. I was surprised how precise it can work on a distance of 5mm. It works also quite good on a larger distance. 

Now I have to make new PCB and replace all my handles with the new PCB. Let's see how this will work...

You find the Video of the test here: