Hi, Branko

You could place two auto adhesive copper sheets on two facing sides of your plastic tank : you built a capacitor; the fluid level will make the capacitance vary with the level.

Use this " capacitor" as the capacitor of a typical RCTime function capacitor, and it's in the pocket.

if needed, a CD4060 could also be used to get an easier-to measure period.

Another trickier solution is to use the capacitor in the RC Clock circuit of the Pic ...

Alain