It can be as simple as a contact closure. We like to put a video window on the garage and gate controls page so that you can watch them open or close. If you cant see them...to know when/if you should stop them then you might want to think about motion sensors that send the door back up or stop it from opening (like if your dog made it in the garage and you dont want him to run out).
Avid Stamp Collector - I really love 39 Cent Stamps