As my CD4013s were in the mail this week end, I took time to finalize this idea and remove the transistor flip flop (which also works)
briefly, a gate occurs at the set pin which produce a change of the state at Q output. Becoming high, C3 starts charging through the attack pot. When the voltage across C3 reaches the threshold voltage of IC3 comparator, the output of IC3 goes high, creates a trigger for the 555 to start a pulse at its third pin. The duration of that pulse is determined by the HOLD pot and C8 value ( 1.1(RxC) ).
Once the pulse ends, it is detected by D13 and T3. Normally, T3 is open putting the reset pin of the CD4013 to ground. However, when the pulse is detected T3 is turns off quickly and the reset pin goes high allwoing the flip flop to reset, and C3 to discharges through the decay pot.
D8 is here to maintain the voltage at 10V and avoid it to go higher during the “hold” period (we want it to stay still). I remain concerned about the zener dissipation when the attack pot is at its lowest value but it seems to work good so far…A solution, athough a tinny bit complexer, could be to use another transistor to block the current through when C3 voltage reaches 10V (via IC3 comparator).
IC2B is another comparator. When C3 go lower than 0.7V (reference voltage of R7/D9) its output goes high creating another high pluse to the set pin, restarting another sequence.
The interesting part (I think) is that the attack/decay and the hold period are independant of each other. One can be very long and the other even longer or a short attack and very long hold period (as a minute long gate) followed by a slower decay .The hold period is available at one output for whatever odd reason you may have (I don’t know which one, I haven’t got a modular…).
For the fancy-tunning-looking modular there’re two LEDs for signaling the state of the output voltage (attack/decay and hold period).
Now, as every diyer knows, here comes the inevitable wish to add things to the design. Maybe using the other half of the CD4013 to create the selector for the normal/Re-trigger with a dual package LED ? I quite dislike those cheapo blue meachanical switch and I always found that a momentary switch is always more nice to use (but not everytime pratical to incorporate). Or the former transistor flip flop could do that task because as common sense may point out, the other half of CD4013 can be used for another envelope…A module with four of those could be handy !!