Ultrasonic sensor without micro controller
I am not sure that your polarities and values are correct but the idea I get from analyzing the circuit, is that it triggers the transmitter, turns on the echo line and waits for the receiver to detect the echo to turn off the echo line.  The counter starts counting at the rise of the echo line and stops at the fall of the same line.  So essentially you have a count proportional to the time it takes the signal to propagate and its echo to return.  If you "calibrate" this, you have a distance measuring device, or a type of alarm.