Does anyone know how I easy can get 2.5V to 5V? the 2.5V comes from the output of a 4 bit magnitude comparator but is too low as input for an inverter, so the inverter always got a high output.
You're asking the wrong question. The question should be "Why is my comparator output voltage so low?". See my reply in your other thread. You are overloading the comparator with the relay (the relay requires more current than the comparator can provide). You need drivers for the relays.