James:

Here is a circuit of how I would suggest driving your motor. In this example the LM317 regulator would regulate the voltage to the motor to about 6 volts when the driving NPN transistor on the existing circuit board is in the ON state. When the existing transistor goes OFF the motor and the LM317 both go OFF. The regulator takes care of providing an even 6 volts to the motor during the time when it draws the in-rush start-up current and during times of varying load on the motor. These are attributes that just cannot be met with a simple series dropping resistor. This example circuit will be suitable for providing drive to a motor up to a couple hundred milliamperes. (I think you had suggested that the existing motor was driven by a 2N3904 and this supports the presumption of relatively low motor current. You can fine tune the voltage you require for the motor by adjusting the value of the 2.2K resistor up or down.



Michael Karas