simple ultrasonic detector

Hello I’m trying to build an ultrasonic obstacle detector which will be fixed to know distance and only produce a output when obstacle is occur.
I have the K3502 ultrasonic parking sensor, but the k3502 gives different values (buzzer sound) when the distance between the vehicle and the obstacle is increased.
Can anybody help me to build a simple detector or a way to modify the K3502 to work as simple ultrasonic detector.

Data Sheet. The schematic is on page 19.