Because Lego is not quite strong enough to tighten the belt perfectly, I replaced the construction with a turnbuckle and a simple custom made pulley attached to it. With this, I am able to dial in the tension very precise, so that it has no slip but also is not wearing the belt to much. To prevent the belt from being twisted by the new tightener, I secured it with some electrical tape.

Here is also a picture of the other side with the dc-motor and its gear.

To be enable to integrate this project into my home automation, I attached an ESP 8266 to the arduino.
It is connected via serial and currently just hangs from the wires under the arduino but i plan to move it somewhere else in the room, to attach more sensors for my home automation to it.

As you can see, there is an voltage regulator on the board, but it seems to be broken and just produces 19V (the supply voltage from the ac-adapter), so i still use an USB adapter.