R71 being a
also ensures the fast ON/OFF switching of Q2
. . . if Q2 so decides
C1 , C2 shape switch ON overshoot , ringing and bypass R3 , R2 for even faster switching
D3 , D4 terminate C1 (somewhat/occasionally reduce the effect of parasitic capacitances in Q2)
R3 , R2 - in principle - set the speed/threshold/sensitivity of the Q2
R1 sets the speed/stability/sensitivity of the Q2
R3 , R1 along with R4 , D2 - can be used to control ON-state current
D5 might occasionally boost the switch OFF as well as not due
forward recovery time ← the least also applies to D3 , D4
Q1 (as a reverse diode to the LED - D1) could be justified at high current , high frequency switching + the BJT has quite large parasitic capacitances itself , but if the low capacitance protection diode is used the surge bypass capacitor may also be justified in parallel to D1 , D.Q1
▲▲ the schematic is provided to give one some basic idea of pulse shaping - not that all of it would be a must in all occasions