So I wired up this schematic today, and it works!!, but I need to change some values because the range of control is VERY narrow, like 60-80%. Any ideas on how to increase the range to 0-100%??

Also I found it strange that the large electrolytic cap C8 sits across the source voltage as opposed to the motor voltage, is this correct??

 

The 555 timer (dual 556) in the way it is generally implemented and implemented here as well is poor for wide frequency range.

C8 is OK where it is placed--the motor current is DC even though the voltage may look really nasty--this is due the effect of the motor inductance.