The original perceptron machine built by Rosenblatt used self-rotating potentiometer knobs to visualize and change weights for a perceptron that used a cadmium

The original perceptron machine built by Rosenblatt used self-rotating potentiometer knobs to visualize and change weights for a perceptron that used a cadmium sulfide "pixels" camera. I believe you could use LEDs and if you were really clever, you can use the fact that LEDs are also light sensitive to adjust the LED "weights".