Nice generic circuit for a IRLED / phototransistor pair. I was refering to the poster with a hangup with voltages, In semiconductors, voltages are usually ratings between pins, not the voltage they are capable of switching. I'd set the resistor for the LED (IRLED) to a value to limit the current to close to its maximum, and then suck it and see using a DMM for the resistor on the phototransistor side. It will vary on many different factors, especially the optical path between the two devices. Increasing the resistor on the phototransistor will increase its high output level with the above circuit (when the LED is off or obscured), but its a trade off with noise and the low level when the LED is On or the optical path is open .