Hi Crazyman,
C1 shorts the received signal on the schematic but the pcb has it correctly grounded.
It looks like the circuit was designed to use a very old opamp for IC1 that needed C3. Now it uses a 741 opamp that does not need C3.
Use a switch between points 7 and 8 to latch the output when it is turned on. When it is turned off then the beeper or relay at the output will activate for each movement the circuit detects.
I don't think it will be sensitive and I don't think it will detect somebody wearing a sweater or a coat. Unless they move in front of a close wall.
My ultrasonic rangefinder "sees" my hand just fine out to about 1-1.5 meters, but detects harder and flatter objects better at a distance. It can see a wall up to just under 3 meters away, and appears to be limited mostly by the timeouts I set in the code.
I don't know about your circuit; mine is just a simple 8051 uC using opposing output pins to drive a piezo transducer plus an op-amp receiver to detect the bounces.