MOTM

Here are some more thoughts.

Since you're using a differentiator to detect rapid voltage changes, you are
only going to extract _triggers_ from your input. With a handful of extra
parts, you can create a gate output as well.

There are ways of using a 555 timer chip as a 'pulse width' circuit. You
could have the triggers 'trip' the 555, which will then stay high for a time
and then fall a specified period later, which can be set with a
potentiometer. This would give you gates with the same 'on-time.' The 555 is
used as the basis of a lot of gate delay circuits.