help me for "ULTRASONIC MOTION DETECTOR".

Ultrasonic motion detectors emit and receive ultrasonic sound energy in a region of interest.Object motion within the region of interest and in the range of the ultrasonic motion sensor is detected and an alarm signal is produced.The range of these detectors differs from the designed range whenever the actual atmospheric sound propogation conditions vary from the atmospheric conditions assumed in the design .Ultrasonic motion detectors are commonly used for automatic door openers and security alarms. ultrasonic beams are easily blocked by thin materials, including paper and as a result False triggering is easly caused by reflections from blowing curtains, pets, and flying insects.

Circuit description
The circuit uses a matched pair of 40 kHz transducer elements to detect movement up to 22 feet away. Crystal locked circuit for maximum stability. An LED is switched on for movement indication. The transmitter sends out a steady ultrasonic signal at 40kHz( wavelength ~= 6 mm).
The reflected sound is detected by ultrasonic receiver. The
signal is then amplified by amplifiers.An amplifier is selfbiasing. The time constant of the first amplifier is set to let the 40kHz signal through. Between
the first & second amplifier there is a negative peak
detector consisting two diodes which follows the envelope of
the 40kHz signal. If there is no movement the envelope is
just a straight line. The time constant ofsecond amplifier is much slower so that it will follow this envelope. When
there is no movement neither diodes can conduct. If the signal falls or rises the diodes conducts which the output to go high. A low pass filter screens out unwanted spurious signals. Specific details about the circuit are available in this link:
http://electronickitskit/complete/surv/k49.pdf