< IFD:DUPL
https://www.arduino.cc/en/Tutorial/AnalogInput
https://www.arduino.cc/en/Tutorial/Fading
http://www.toptechboy.com/arduino/lesson-11-arduino-circuit-to-dim-led-with-potentiometer/
https://www.arduino.cc/en/Reference/servo
https://forum.arduino.cc/index.php?topic=384423.0
https://www.arduino.cc/en/Tutorial/Sweep
https://www.arduino.cc/en/Tutorial/Knob
https://oscarliang.com/using-potentiometer-to-control-piezo-speaker-for-v/
https://oscarliang.com/using-switch-to-control-piezo-speaker/