http://www.arcfn.com/2010/03/detecting- … no-ir.html
Use TSOP4038, TSOP58038 or an equivalent fixed-gain IR receiver (a common receiver used with remote controls is not preferred as they require “off” periods in the signal). ~$1. Instead of an IR led you can use a decently bright 20ma red LED so you can see the beam. That should give you four feet of distance easily.
To switch the motor on/off it would be simplest to just use a [relay. You’re only moving the conveyor in one direction so a motor controller would be overkill.](http://www.seeedstudio.com/depot/electronic-brick-5v-relay-module-digital-p-479.html?cPath=156_160)