Hey All,

I am working on a project. I have a pulley that moves a string with a cart across a gap (backwards and forwards). I want to know when that cart enters and exits that gap. What sensor would I use to detect when something passes it? This is a road crossing model I am building. And I want to be able to track dynamically when the cart enters and exits the road space. The road will be roughly 6in wide to give you an idea of dimensions. I am just starting into the world of Arduinos and so I apologize if this is a real dumb question. But when I search online, I get a bunch of random "kits" with all sorts of sensors. I cannot think of a better way to describe what I am looking for to Google. Cheers!