This is a typical application circuit for a switch with an LED and phototransistor sensor. In this design when no object is in the slot the phototransistor is on and the output voltage level is in the low state. To reverse this, the Rc resistor would be placed on the emitter side of the phototransistor. Rc should be sufficient to provide the appropriate switching levels for the required switching speeds. The appropriate LED resistor (RF) should be defined to provide a forward current (IF) equivalent to how the device is tested in the electrical characteristics table on the data sheet. OPTEK can assist in providing the best circuit for a given application.