Here is a experimental circuit to sense current flowing through a wire by putting a electric cord near the top of a miniature power transformer that acts like a sensor and then turning on the appliance and that small signal gets amplified to the point where it can be rectified and then amplified in DC to turn on a LED, at the output.

No connections to the AC cord just magnetic flux cutting through the transformer windings and amplifying the signal to a usable output voltage..

This was fun to design...

 

would it have been simpler with an opamp?

 

Probably, but sometimes the fun is using a specific design approach, sorta like audiophiles building amps with tube circuits.