The following is all from Daniël Mantione, who is doing great work on working out how to emulate a 1351 mouse using an Arduino, which we intend to use to make a solid-state joystick which will also work as a proportional 1351-compatible mouse:
The comparatorIt turns out that the analog comparator on the Arduino is located on digital pins 6 and 7. It does not share any functionality with the analog ADC pins.
Two pins are necessary: The comparator compares the voltage on both pins, one cannot compare with a voltage specified in software. The internal resistance of the Arduino is incredible, pins configured as inputs have a resistance of many megaohm, so a resistor from 5V to the pin doesn't really work, you need two resistors to create a voltage divider.
I took my box of electronic components and after a bit of searching and experimenting I came up with a 100 kOhm and a 15 kOhm resistor. 100 kOhm from 5V to the pin and 15 kOhm from the pin to ground: