He is talking about remote controls for TV's, cable boxes, etc. They use IR modulated with a 38 kHz squarewave, that pulses on and off in several bursts per second with a different pattern for each button. This works very well in a family room, but is *way* too slow to detect a "really fast object". The problem is that the standard remote control receiver part, which is an IR phototransistor, amplitude detector, bandpass filter, envelope detector, and output driver all in one little module, is purpose-built for one job that is not speed-dependent. However, if you replicate the parts you need, tweaked for detection speed, you can get what you want. A bullet chronograph can detect speeds above Mach 2 with optical sensors. Not right for your job, but it indicates that what you want to do is possible.
Give us more details. Object size and speed. distance from transmitter to detector, power supplies available, etc. The basic approach is the same - modulate the IR beam so the detector can differentiate it from sunlight, ambient light variations from wind in trees, etc; bandpass filter the signal from the phototransistor so the receiver is seeing energy only from your transmitter; amplitude-detect that signal to detect a dip caused by the object; use that signal for whatever. With a higher modulation freq, such as 100-200 kHz, the delay through the receiver electronics should be short enough not to mess up the timing.
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