I am fairly new to circuits, (I am an applied physics major, I have one year left until I get my bachelors) and I thought it would be really cool to make something that shines a light or something if someone crosses past a sensor. For example, if I wanted to know if someone walks into my shed outside, I could set up one of these and if someone does walk in the door a light in my house would illuminate. My idea (which is probably pretty naive) is the following.. I saw these "light to frequency" sensors at radio shack, I figured I would shine an infrared laser on it and calibrate it to sound an alarm whenever that signal is disrupted. As far as knowing HOW to do this, I do not know. Is this project feasible for someone with my experience? Where can I get the proper sensors? Does anyone have any circuit diagrams applied to something like this I can study? Thanks!