The QTR-1A reflectance sensor carries a single infrared LED and phototransistor pair in an inexpensive, tiny 0.5" x 0.3" module that can be mounted almost anywhere and is great for edge detection and line following applications. This sensor is sold in packs of two units.
This 5mm Water-clear Phototransistor will pick up visible and infrared light and conduct current. When light reaches this high sensitive/fast response time component it's much like current reaching the base of a transistor which allows current to flow from collector to emitter.
This Melexis US5881LUA (flat TO-92 package) magnetic sensor features a low sensitivity, unipolar design, meaning that it won't be affected by earth's magnetic field, and only responds to a change in one polarity. This particular model responds only to a South-pole magnetic field.