I need to switch a circuit based on whether another circuit (a millivolt circuit without meaningful voltage) is open or closed but I am not familiar with any component that can achieve this. A multimeter has a continuity checker that can tell if the circuit is open or closed but how would I trigger a switch based on this?

I need to switch a circuit based on whether another circuit (a millivolt circuit without meaningful voltage) is open or closed but I am not familiar with any component that can achieve this. A multimeter has a continuity checker that can tell if the circuit is open or closed but how would I trigger a switch based on this?

Thanks!

A multimeter tests continuity by passing a current through a circuit(probe tips and the test materiel) and measuring the voltage across a series resistor. You can do the same.
Paul