Detecting external dc voltage/current

aloha,

I am having an external 12V DC circuit with Closed/Open states. I am tying to figure out what is the best way to detect Closed/Open states using particle without altering the circuit too much (drawing much current or increase the existing circuit resistance of 4kΩ by more than 10%)

cheers!

Opto-isolators and current sensors come to mind.

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