In article ,
"dennis@home" writes:
How about magnets and reed switches (you may need a relay or two too).

Put a couple of magnets on the screen where you want it to stop and use the
reed to break the connection when the magnet gets there.

One way which is used for this sort of thing is to monitor the
motor current, which typically increases when the transport
hits an endpoint, top or bottom. It also has the advantage
that the motor drive is cut if the thing jams mid-way.
You need a limit timer too, incase that fails due to some
sort of gearbox failure or the current sense not operating
correctly. The current sense might need overriding for
the first half second or so when the motor starts up, or to
incorporate a half second delay in the current sense which
can be more useful if the transport presents a varying load
on the motor.

--
Andrew Gabriel