I am not 100% on this so ...

If I am on the right track all you have to do is subtract 0.7 (the
base - emitter voltage) from the 5 volts.

This gives 4.3v.  Then if you know the base current required for
saturation say 5 mA

This gives R = 4.3/0.005  = 860.
So use a 1k resistor.  I use these for pull ups, pull downs and
preceding transistor bases.

Dont forget to use a diode for spike protection.

On 2/6/06, mrgizmo <mrgizmo@vianet.ca> wrote:
>  I am using a 2n3904 to drive a 12V relay off a 16F628 pic at 5V input, I'm
> not sure how to figure out the resistor for the base?
>
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