Yes you still need 120 vac for the screen motor. What you have is a 12v source ( the projector ) which triggers a 12v relay in the screen. When the 12v is present, the screen unrolls, when the 12v is not present, the screen rolls up. Even if your screen does not have a 12v trigger, it is rather easy to make on up. Just a relay with a 2 position contact or 2 relays ( one NO and one NC ) Just make sure that the contacts on the relay are rated to carry the current that the screen motor will draw.