Connections as shown for 555 timer give output only if trigger pulse width is greater than time constant (t2 = 1.1RtCt) of mono MVBR circuit. Q1 is normally off. Pin 2 of 555 is then high. At start of trigger pulse, output at point C is low. Positive trigger drives Q1 on for time determined by R1C1, feeding negative-going pulse to trigger pin 2. Timer then acts as normal mono, driving Q2 on for time t2. If input pulse is still high at end of t2, it appears at output D since Q2 is now off. Output pulse width is thus equal to input trigger width less 1.1RtCt. For greater accuracy, insert delay between point A and R4 equal to inherent propagation delay of timer.-S. Sarpangal, Build a Pulse-Width Detector with a 555 Timer, EDN Magazine, Oct. 5, 1977, p 93 and 96.