Instructables
Picture of Dorm Room Doorbell
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The idea behind this project is that you can build a simple "doorbell" for your  dorm room that uses light instead of sound to alert you (and not your RA!) of when you have a guest. 

The best way to build this project is to build a simple light circuit using 2 AA battery's and a battery holder, connected to a flashlight bulb. Then hot glue the bulb to the top of the battery pack and Velcro the pack to the inside of your door. Just run the two wires that lead to the switch through the hinge of the door (loosely so it does not pinch too much when the door closes) and place the switch on the outside of the door. Then fashion a fancy doorbell cover from some cardboard!