car parking/conveyors

am designing an automated car parking..using lift and array of conveyors
the car transfers from the lift to its space with the help of conveyors...which are on lift and on the parking lot
i need to stop the conveyor as soon as the car competely transfers to its slot
which sensor should i use...to know tht the car has been shifted from the lift to its slot...

am looking for tht particular kind of sensor tht we usually see in movie...laser diode kind...a straight thin beam
what is tht sensor actually called...and are there any other sensors tht i can use??

using microcontroller to controll it

Ultrasonic sensors (e.g., those operating at 500KHz or more) can be very good at accurate postioning. Check out Hyde Park or Telemechanique.
See: http://www.sesensors.com
John