How do I run multiple vibration motors from an Arduino Mini with PWM?
You can control motor PWM by using AnalogWrite() feature. Arduino Mini is equipped with 6 PWM pin, so you can use 1 PWM pin for each of you motor. Check here about more information on AnalogWrite, and here about Arduino Mini    
This is the circuit if you want to use external power source. Normally, I use 1k ohm resistor, I'm sure there are someone out there that knows the exact formula.








simulate this circuit &ndash; Schematic created using CircuitLab