1. Resistor size between pwm output pin and base of transistor.
This resistor is call R b . YOU have to have this resistor between Arduino pin and a base of a transistor ( NPN or PNP )
The value I use are : 1K, 2.2 K , 3.3 K, 4.7 K are OK. The goal is to not overload the Arduino pin.
2. Load resistance.
I'm not sure, (I've been searching for a while now.)
When design a transistor switching circuit, You need : Supply voltage, load resistance and you simply calculate the current.
Like : I = V / R Example : 9 V / 1 K = 9 mA With that information ( 9 mA and 9 V ) , you can select the type of transistor to use by looking at the datasheet of transistor for Ic ( max ) , Vcb (max ) , Power value ( max ) . In this example, a transistor like 2N3904 is OK.
Positive of supply wires to one side of the load device. Negative side of supply wires to emitter of transistor AND to arduino ground pin.
Do you mean that I need to put both the Arduino and the external power supply ground/power into the same places on the circuit? (respectively)
Arduino power and power for the circuit is seperated. But the gnd goes together. Example : 5 V for the Arduino, 9 v for the transistor circuit is seperated and gnd of Arduino AND gnd of the transistor circuit connect together.
I hope you understand.