Tracking a real life object

I am having trouble coming up with a way to do this. I want to track an object of some sort that is small enough to tie to a rope and track the location of that object. Only the x location of the object is really important. I was thinking a small led light or a band that could be put around the rope and you would track that with something but I can't think of a way to implement it. I know what I need, but not the tool that would do it.

I essentially want to use a real life objects position as the controller for the game.

I thought of tying a wii sensor to it and using a wiimote to kind of reverse track the movement but the sensor would be too bulky and I would have to look into how people have managed to interface the wiimote with a pc.

Then there is the problem of interfacing whatever I am using with unity, so it would be best if it had a usb connection and a decent library to go along with it for using the data it kicks out.

Anyone have any ideas for this? Something from a robotics kit? Any ideas on where I could go to ask this kind of question again if you don't know yourself?