One thing I see often with DIY'ers is not knowing that most stud finders need to be calibrated when placed on the surface you want to find framing. Press it against the wall, hold it still, push the button and when you here "beep" then you can move it across. Sometimes you need to calibrate it in a different spot as you can't calibrate it over a stud.

I was just teaching a guy this at work the other day, he was watching me install some cabinets in a classroom and was admiring my Franklin stud finder, saying how his doesn't work. I pulled out my cheap 20 dollar one, and showed him how they need to he calibrated. Most stud finders work fine if you aren't going over plaster, double drywall, or a similar situation.

You could also use toggles of the projector isn't overly heavy. Or toggle a finished board onto the ceiling, round over the edges and paint or stain, then attach projector to that. A few toggles spaced apart over the board should leave nothing to worry about.