This is a ultrasonic motion detector (or movement detector) circuit. The circuit claimed has a high movement sensitivity. Even air moving (hot air rising, wind blowing) will trigger it when the trimpot is set near the most sensitive position.
The transmitter sends out a steady ultrasonic tone at 40kHz. At this frequency the wavelength is about 6 mm. Any reflected sound is detected by ultrasonic receiver. The signal is then amplified by IC1:A and IC1:B. IC1:A is selfbiasing via C2 & R5. 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 (diode D1 & R8) which follows the envelope of the 40kHz signal. If there is no movement the envelope is just a straight line. The time constant of IC1:B is much slower so that it will follow this envelope. All the amplifiers are AC coupled to prevent DC bias problems.
Then the signal is fed through a window detector IC1:C which detects both positive and negative pulses. When there is no movement the potential at pin 7 sits at half the supply potential and neither D2 or D3 can conduct. The potential at pin 8 is low. If the signal rises D3 conducts causing the output to go high. If the signal falls then D2 conducts which also causes the output to go high. Thus the name window detector circuit because it detects potentials which move both below and above a given range.
A low pass filter screens out unwanted spurious signals, then an amp IC1:D set up as a monostable flip flop converts any signal that gets through the filter into a substantial pulse to turn on the BC639. This turns on the LED and provides a Signal Out to drive a separate relay or any other device you may wish to signal to. The time constant of the monostable flip-flop is about half a second and is set by C8 & R10. D4 is used to separate the charge & discharge time constants. It lets the circuit switch on immediately movement is detected but allows about a 1/2 second delay for the reset.
The circuit available in kit, you can order the kit at kitsrus.com or electronickits.com. If you want to built the ultrasonic motion detector by yourself, just download the PDF manual document for detail explanation and parts list for this circuit: