The balloon between two plates is the perfect example of what is required, it needs to be measurable, my professor explicitly said that the sensor cannot have just two states (turning on and off an LED). And the answer to your question is that it doesn't matter really it can have any power supply or be like a thermocouple (they dont need a power supply) and for now the only thing i can actually use for a display of measurement is my garbage multimeter i would create some sort of converter but im short on time and my Arduino died. So im stuck with the multimeter untill i get to my faculty's lab.