"He is not detecting a threshold, but a period or duration of an incoming trigger pulse."
He is actually detecting a THRESHOLD (more accurately the presence of a signal or voltage of 5v or 12v) and producing a pulse of 1.5mS duration. The duration of the pulse does not matter. The circuit I have created produces an output of 1.5mS, no matter how long the input signal is present.
The poster had a problem with a positive going pulse, just as I did, using a 555. That's why I suggested using a Schmitt gate.