This module is the spiritual successor to my old Kirschmann ADSR module which, whilst being very useful and an interesting use of CMOS ICs, required a fairly hefty trigger signal in order to work.
This is an updated envelope generator which uses a 555 timer configured as a Schmitt trigger (see: https://www.nutsvolts.com/magazine/article/555-monostable-circuits - figure 15 in particular) from which the envelope is generated. The net result is something that not only triggers at around 1V but also has far fewer parts, particularly ICs, than the Kirschmann design. I'm going to call that a win, and it isn't dissimilar to the design that Doepfer use in their EG modules, albeit without the bells and whistles.
In addition, I've 'borrowed' a couple of features from a Rene Schmitz design, specifically: the resistor R13 is optional, but adding it will give the input a bit of hysteresis (the degree of which is governed by the values of R13 - 1MΩ is what I use) which is handy if you're using a slowly changing waveform as a trigger. Secondly, there's a buffer between the sustain and decay pots which removes any interdependence between the two. It also uses up an otherwise uncommitted op-amp.
There's a range switch too - envelope values are governed by the control pots and the values of the two capacitors. As built, the two capacitor values are 330nF (good for percussive patches) and 2.2uF (good for pads etc.) - if you're prepared to get down and dirty with a soldering iron then you can easily replace these.
The module is supplied with a 5HP (~25mm) panel which is laser-cut from 3mm opaque black perspex and engraved appropriately. Mounting hardware (M3x10 machine screws) and a power cable are also provided.