Detect fast movement using photodiode

[mackrackit]

That means I just connect the IR LED continously to 5V with series resistor , no need to modulate the IR Tx?

Correct.
As long as you do a little shielding for ambient light. Basically you can look at the modulation used in IR remote control applications as a way of electronically shielding.

Ty it. Take a photodiode, put a piece of heat shrink or something else around it so only the end can receive light. Connect it to a meter instead of the PIC, have sitting in a shadow. Hit the end of the photodiode with IR or some other light source and test.

You can also reverse the photodiode connection and have it detect light instead of dark.