I have 2 from Foy's Pigeon Supplies. (inside mount with light sensor) You can put it on a timer if you like, but the light sensor is adjustable. It's expensive, but well worth every penny. In over 3 years, it has never failed on me. I change the batteries once a year and that's it. (no power needed, just batteries)
http://www.foyspigeonsupplies.com/electronic-doorkeeper
http://www.foyspigeonsupplies.com/electronic-doorkeeper