enevelope generator with LM556 timer based on "The Fastest Envelope in the West"
by René Schmitz, https://www.schmitzbits.de/adsr.html

the modifications to the original circuit are to make use of the dual 556 timer,
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
