Infrared motion detector circuit

Here is the circuit diagram of an infrared motion detector that can be used to sense intrusions.Infra red rays reflected from a static object will be in one phase, and the rays reflected from a moving object will  be in another phase.The circuit uses this principle to sense the motion.

The IC1 (NE 555)  is wired as an astable multivibrator .The IR diode connected at the
output of this IC produces infrared beams of frequency 5Khz.These beams are picked by the photo transistor Q1 .At normal condition ie; when there is no intrusion the output pin (7) of IC2 will be low.When there is an intrusion the phase of the reflected waveforms has a difference in phase and this phase difference will be picked by the IC2.Now the pin 7 of the IC 2 goes high to indicate the intrusion.An LED or a buzzer can be connected at the output of the IC to indicate the intrusion.