Use of Ultrasonic sensor for measuring distance........

How do I use HC-SR04 for only detecting obstacle ?
Because when I am trying to do so I am making the trig pin high but when I am getting obstacle in front of that I am not getting echo pin high ?
Is there any different logic ?

i was suffering a browser and find this link
http://homemaderobo.blogspot.com/2012/08/ultrasonic-sensor-interfacing-with-8051.html
it use same logic don't know where the problem is going in hardware or in software

So how do I use it as an Obstacle detector ? Basis of this logic ...

@ akshaydusane
are you want to detect the obstacle or want to find out the distance of obstacle
if you want to simply a detection then you can use ultrasonic switch
that output will be high or low
high mean detected low mean nothing in front of sensors

I want to simply detect the obstacle using HC-SR04 sensor...
what is ultrasonic switch ?

@ akshaydusane
ultrasonic switch is a simple circuit
it uses ultrasonic transmitter and receiver to detect is anything is in front of sensor or not

I think this dicussion has already discussed. the operation of this sensor is same as Parallax PING)) check out the following thread where everything is explained in detailed.

http://www.8051projects.net/t27437/8051-discussion-forum/pulse-generator-using-p89c51rd2.htm#post_27849

@Ajay Bhargav
No Sir I dont want to find distance only detect obstacle using this sensor only..

well then its much easier. if you are not getting echo then problem is with duration of pulse. Make sure you follow datasheet.

What should be the duration of pulse according to you ?