>   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? 

Neither am I unless you specift the relay current. And while you are
looking that up, check the 2N3904 datasheet for the minium Beta. Divide
the relais current by that Beta and you've got the base current. Divide
3.5V by that current and you've got the base resistor. But don't forget
the freewheel diode!

Wouter van Ooijen

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