Originally Posted by ahmedragia21

just like that O -------------------X <-- object
i really wanna do it to detect a human passing near that sensor then do an alarm

You could try a laser pointer with a remote 2.5cm prism, detect loss of return beam.
That is an interrupted laser pointer beam.

Originally Posted by ahmedragia21

Hi, Im trying to sense if there is an object at 100m ? can infrared do it or ultrasound ??

Consider this:
You have two telescopes of some sort ... maybe spotting scopes .... probably whatever you can afford. Each scope is placed on a line several meters apart, and sighted in on the target .... 100 meters away. When you have sighted and focused each scope on the target, you remove, affix, ... or somehow place ... a phototransistor or some other optically sensitive IC at the focus point of each telescope. .... You might describe this as a 'parallax' method. The final step would be to enable an AND logic gate to alert you when both scopes had sensed the same target at the same time.

The success of this method would depend on whether you could get the target object to create enough of a contrast signal in the phototransistors to switch logic states.....
Some additional improvisation and testing might be necessary... not certain how far apart the two scopes would have to be separated.
You wouldn't want to have a false positive .... if an object nearer than the target was lengthy enough to trigger the alarm.

A Charge Coupled Device type of sensor, as used in cameras or astronomy, might have a better chance of providing a signal indicating that the target is
present. However, a CCD would require a more complicated interface circuit.