Opacity slider for Adobe Illustrator

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1. Slider potentiometer.
2. Teensy PCB.
3. Patch them together and upload the firmware.
4. Open Ai, go to Edit → Keyboard Shortcuts, search for the ‘Opacity’ and assign the keys.
5. Use midiStroke for the MIDI-to-keystrokes conversion. Add MidiPipe if you need to use more than one controller for your setup.
6. Done. Unfortunately it is discrete (0% → 10% → 20% → …) and a bit sluggish (but so the Palette Gear is) but worth trying if you like having physical controllers for the stuff like this.

Firmware:
http://theamateurmaker.com/simple-midi-controller-simple-knob-slider/
midiStroke:
http://www.charlie-roberts.com/midiStroke/
midiPipe:
http://www.subtlesoft.square7.net/MidiPipe.html