The electric screwdriver I got the motor out of had a resistor in series (on the negative I think). It was too damaged by over heatedness to see what value it was.
If the heat hasn't changed the value, you could measure it.
It doesn't matter what side it was connected to.
Was it there to protect the motor from the power adaptor? Or maybe the power adaptor from induction from the motor?!
What I was wondering was do I need to replace it? If so how do I work out the value and where would it go on the previously recomended 555 circuit?
The most likely reason for a resistor in series with the motor in a screwdriver would probably be to protect the motor and/or the driver against too high currents if stalled on a screw that won't budge.
Another possible reason could be to adapt a lower voltage motor to the battery.
With the 3.5V supply and the PWM, you may easily test, with increasing PWM output (say 1/4 the scale each step), if the motor gets hotter than you can hold it (around 50°C) a small value resistor may be needed (or the circuit could be rearranged to have a lower power max. output).
When you have the circuit ready and hooked up, just drop me a PM if you need more specific guidance.