What type of sensor is used for a dell inspiron 1150 monitor?

I think I have a problem with the sensor on my dell inspiron 1150. It acts as though it is asleep (the monitor is just shut off) This happened shortly after taking apart the laptop and putting it back together. I don't know where or what kind of sensor is used in the dell inspiron 1150 to tell it when its closed or open. Where is this?|||Simplest is a micro switch that gets hit when the screen is closed down. If there is no visible tiny opening above the keyboard area where a small protrusion on the screen would mate when closed, the switch could be a magnetic type microswitch.