@PeterH - I tried it already with this idea, but unfortunately the container is too small...maybe i try with small float switches ?
I was thinking of something like this
Fluidmaster Toilet Cistern Bottom Entry Float Valve 400UK074, which could be fitted in a small container. It wouldn't need to be installed directly into the lower container - if it's more convenient, you could add a separate container with one of these valves, positioned so that the water level is at the required level for your container, and connect the two via a link pipe or siphon.