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Help with IR motion sensor

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I'm trying to buld this


Left = transmitter
Right = receiver

This circuit has a transmitter and a receiver. So from what I understand, the interruption in a modulated pulsating beam that is transmitted by an infrared diode and received by an infrared transistor sets off the alarm. So what I am looking at is two different circuits. Can't these two be integrated into one circuit?
So I absolutely must have the circuit with the Photodiode right? If they cant be integrated, how must the receiver be placed with respect to the transmitter?

What are some uses of a Phototransistor?

--thank you 8)

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If the circuits were integrated side-by-side and pointing in the same direction then they could be beaming and receiving the IR from a mirror on the other side of a corridor to be monitored. An intrusion would break the beam and signal the alarm.
If the intruder had his own mirror or modulated IR beam then he could easily not be detected. ;D

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Thanks for the link Shahriar. It seems they have integrated the emitter and receiver circuits.

D2 picks-up the infra-red beam generated by D1 and reflected by the surface placed in front of it.

All what this circuit does is detect an object as it moves toward it, in this case the car right? If thats so, then this circuit would do just fine for experimental purposes. A vehicle is relatively wide though. Would it detect a person just as good as the car?
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