I'm designing an ADSR circuit where I use a schmitt trigger to store the state of the envelope and figuring out the resistor values to get the threshold voltages I wanted was a bit slow, so I made this little tool for playing around with them: https://tools.sandelinos.me/opamp-schmitt-trigger-playground
Just put in the output saturation voltages of your opamp and mess around with the resistor values (the arrow buttons jump through E12 standard values) until you get thresholds that you like.









