[sdiy] My scheme and some questions on a A-SR envelope shaper

[Ishaan Dalal]

For my barebones synth project, I hit upon an idea to make a rudimentary
ADSR (without the decay), using the following scheme: Use the base square
wave of the note to re/trigger one half of a 4538 dual monostable, which
charges a capacitor through a diode and appropriate resistor. The control
voltage from the R-C junction goes to my VCA. After the attack, the tone is
sustained for as long as the key is pressed. The moment it is released, the
cap discharges through another diode-R combo back into the monostable
output, giving the release.

What I would like to do is to add a sustain "time-out", where even if the
key was kept pressed longer than n seconds, the A-SR would automatically
begin the release. The easiest way would be to use the other monostable on
the 4538 to time the attack+sustain, however, this would not release if the
key was pressed only momentarily but go on for the full duration. I could
probably do it with some flip-flops and using the resets of the monostables,
but I'm strapped for space, and if possible would like to just use the other
monostable on the 4538, along with simple logic (read: constructed with
diodes/resistors and/or transistors), that releases the note after n
seconds, even if the key is still pressed.

Any ideas?

Cheers,
Ishaan