if any obstruction comes in between the IR led and IR 
receiver than you can use this signal by connectig any 
terminals to your comparater i.e. what you want a voltage 
or zero output at the output

proxmity sensor is nothing but landmine sensor,generally we
use metal detectors to find landmines,so many principles are
present one of those is,whenever u place metal close to
detector, because of the difference b/w coefficents of
detector and desired metal some flux will produce and that
allows to pass current through the circuit,and produce
output.if u connect buzzer to o/p then it will make sound
whenever u place metal in front of detector.