I'm very new to this. Please bear with me. I want a simple circuit which beeps when something is moved apart farther than a certain distance. I'm thinking of a sensor and something that's sensed (sensee?). They would be about 3 inches apart, if they are moved farther apart than that I need a beep. I need this to be somewhat configurable, between 2 and 4 inches. My initial thought is NFC, but I don't want to use a smart phone, and ideally I want something really fundamental and simple, with a passive sensee (no power needed on that) and very low power draw on the sensor and beeper circuit.
What are my options?