Envelope follower with gate output
Top knob adjust input level (signal sent to follower), first half is attenuating the signal from 0 to 1x, second half is amplifying it up to 8x (final block's output is trimmed to max 1). Middle knob controls follower's reaction time (lag). Bottom knob sets threshold value for gate - if follower's output is above threshold's value, gate is on. Three lamps on right show follower's output level, bottom lamp shows gate output.