envelope generator based on "The Fastest Envelope in the West"
by René Schmitz, https://www.schmitzbits.de/adsr.html

the modifications to the original circuit are: feed 5v into the control pin to clamp the output to 0-5V, use 100k/1k in/out impedances, add a front panel led that matches the gate input, and add a manual trigger switch to the front panel.