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I have it together and have the motor going. So I have made some progress!

However, in your video it shows that it speeds up as you turn the potentiometer, for me though it is only on or off. I have to turn the potentiometer until 255 is written to pin3. Then it will turn on, but it is on full speed. (I have added a lot of debug statements to the firmware so I can see what it is doing).

It doesn't help that this motor driver doesn't have a datasheet, the only information we have for it is on the aliexpress site:

SET for the control side:
GND  control terminal ground
VR   the governor end 0-5V(corresponding to 10% of motor speed - 100%) <------pin2
ZF   reversing control(connected to 5V or ground can change the motor rotation direction)
M   tachometer pulse output(depending on the number of motor poles corresponding to the number of pulses per revolution of the output number)
EL  enable control terminal(connected to 5V or ground running then 5V, grounding the motor stops) <----- pin3

The firmware is doing a digitalWrite to pin2, but this doesn't make sense if that is the speed controller, shouldn't that be an analogWrite?

The firmware takes pin2 HIGH anytime RPM is greater than 0, which is all the time once you turn the potentiometer one notch.

I am kind of confused why I am not getting a gradual speed increase.

Motor is off when pin3 is 0-250:

iRPM (pin2): 980 RPM > 0? 1
pin3: 250

Turn it one more notch the motor is on full blast:

iRPM (pin2): 1000 RPM > 0? 1
pin3: 255

Turn it backwards on notch and the motor shuts off again:

iRPM (pin2): 980 RPM > 0? 1
pin3: 250