The way it works is, the cap normally has both ends at positive rail. When hall switch turns on, it pulls the one end of the cap to ground.

It gives a negative going pulse at R1,R3 junction as the cap begins it's charge, and then after tou time, the pulse at R1,R3 junction goes high.

It will stay in this state until another high to low transition of the hall switch.

Just have to select correct values of RC so that it doesn't interfere with other timing in the system. i.e. with 0.01uF and 4K7 should give approx. 32uSec.(if I'm not mistaken)