My requirement is to design a simple, cost-effective AC electric field sensor. It needs to be small in size and work within a specific frequency range meaning I would specify a frequency and it needs to sense if a field within that frequency range is present in the area.

I don't have much knowledge about field sensors and would appreciate recommendations on what to read about this.

I would also like to know if such a sensor could be made programmable as in to change the threshold frequencies.