Activating a relay when certain voltage received (Water injection system)
Use a comparator. Connect the middle pin of your potentiometer to the negative input and the sensor output to the positive input. Connect the potentiometer between 5V and ground.
The comparator should have a schmitt trigger input, to avoid toggeling of the relay.







simulate this circuit &ndash; Schematic created using CircuitLab

You will propably have to put a FET at the output of the comparator to drive the current of the relay. And don't forget to put a flyback diode over to relay to avoid destroying your FET.