today im just checking with another groups members that using phototransistor with IR led in their project. I try to use white paper facing on the IR LED and its was reflected to the phototransistor. But photodiode cant.
A phototransistor is more sensitive (but much slower - not something that will affect your use of it though) than a photodiode. You can view a phototransistor (sensitivity-wise) as a diode followed by a transistor, so the sensitivity is greater by a factor of the transistors h_FE (current gain).
The op-amp circuit can
be made to reflect the IR with a piece of paper, even with a photodiode
... But not until you actually get your hands off
the keyboard and on