The present disclosure relates to a sensor for a capacitive determination of a filling level of a cartridge filled with a liquid substance. The sensor comprises a planar flexible foil arrangeable to an outer circumference of a tubular shaped barrel of the cartridge. The foil has at least a sensing zone, a communication zone and at least a first electrode. The foil further comprises least a second electrode located in the sensing zone, a processor electrically connected with the at least first and second electrodes, and an antenna located in the communication zone. The antenna is electrically connected with the processor.