How to use photo interrupter to switch a higher voltage circuit
Your switch needs to pass at least 3mA when closed and withstand 6.6V when open, and its output needs to be isolated from Ground. The obvious choice is an optocoupler.

Depending on how much output current the photo-interrupter can provide, you might get away with just a sensitive optocoupler or you might need another transistor to drive it. Using a driver transistor the circuit could look like this:-







simulate this circuit &ndash; Schematic created using CircuitLab

In this configuration the optocoupler is turned on when the beam is interrupted. Since the 4N25 can have a CTR (Current Transfer Ratio) of as little as 20%, R4 is sized to put ~16mA through the optocoupler LED.