Automatic drawbridge(or lift bridge) system..Help!!

I'm making a drawbridge which has a mechanism to detect the arrival of ships and open up..and when there is no ship traffic, it automatically closes down and allows road transport..
Need some help regarding this..Any help will do..
I think i kind of have an idea about how to implement it in large-scale ( u know, in an actual drawbridge) but i really really need help in making the working model...