OK, posting the classic Thomas Henry Design of a 4016 and 555 based ADSR. This appeared in Polyphony (PAIA's DIY and Synth mag) March-April 82; I have also seen it in at least on of Tom Henry's design books. I suggest buying either the back issue or one of TH's books, the article has a quite detailed description of how the ADSR works.
I spoke to Jon Simonton before he died (sad thing, but still, I am glad to have gotten to speak to him and get to know him a tiny bit while he was alive, he was super friendly and enthusiastic); John gave me permission to post this schematic.
Eagle schematic is here and the eagle BRD file is here. See the PNG file for the schematic here. Wiring diagram is here.
About the layout and circuit:
The circuit as designed by TH requires 2 signals to fire it: a Gate signal (0V to maybe 1V or more? Needs to be a diode drop above ground....) and simultaneously, a single inverted TRIGGER pulse to allow the 555 to start its cycle. So TH's design, as per his original article in POLYPHONY, expects an inverted trigger signal and a positive going gate signal at the same time. This seemed a bit odd to me when I was first laying out the board since I don't have gear or seen a lot of Midi to CV that generates these signals simultaneously or even asynchronously; for instance, on my kenton Midi to 1V/oct CV the gate and trigger signals are all GND to positive V.
But dealing with this design feature (???) was easy enough, Just tie Q3's collector to the "input side" of C11.
I also added a second resistor/NPN to the circuit you see here to accommodate AND signals (something I needed for my own particular build) but you probably won't need that, so feel free to leave Q3 and R1 out.
Here is a gerber of TH's design. There is a mistake in it that is on me! Pin 7 and 8 of the 4016 need to be tied to ground, but the gerber has the 4016 V-- tied to -15 which won't work. To fix this, if you do make a PCB from the gerber, cut the trace to V-- on the 4016 and jump the pin to ground. but instead of modding, I'd suggest just grabbing the SRC and BRD, getting Eagle Lite, and recreating the gerber. The schematic above corrects this mistake. Otherwise as far as I know this circuit is 100% good to go, I have built this and it makes a nice steady ADSR signal when provided a 0-5V gate.
one final note: I revised the board a bit (REV5) to accommodate a positive going trigger signal. Schematic here, board here. I haven't tested this revision yet but my mod to TH's design is super simple, I inverted a positive going a trigger before it hits cap C11. I'd guess if you built it it'd work but of course as a very wise tech once told me: never trust anything but your Fluke.