You're right I've never used one. That's why I'm asking here.
"An inductive sensor is an electronic proximity sensor, which detects metallic objects without touching them" I read that statement from Wikipedia and thought an inductive sensor might work. Can you explain why it won't work?
The piezo disc would work with just wire hitting the frame, but the weight is about 10lbs of steel (an old window sash weight). I don't think I want that hitting anything. Any other options for non-contact detection of steel?