I made a simple LDR / LED setup for an elevator, where the current location (upper floor or lower floor) is shown via an LED that is located above either elevator door. A laser was installed in the elevator shaft at the lower level. A light dependent resistor sensor was placed opposite the laser.
When the laser is shining on the LDR, the upper floor LED lights up (as the lift is not on the lower floor). When the last is not shining on the LDR, the lower floor LED lights up (as the lift is on the lower floor).
A potentiometer sets the threshold at which the LDR triggers the LED changes. The setup was based around a Freeduino RBBB PCB and components for a quick and simple installation.