Hi Les,

In response to your question (below), you're right in
that applying +5V to the base of your NPN transistor
will make it act like a switch; but the transistor is
basically a controlled _current_ source. So you design
the circuit to pass the desired current into the
collector and you get almost the same current out the
emitter (depending on your beta -- for your purposes,
you can probably assume it's the same) when the
transistor is on. BUT, the voltage difference between
the base and the emitter is 0.6-0.7 volts, like a
diode (there is a pn junction between the base and
emitter pins after all). So, if you apply +5 volts
from a PIC pin to the base then your emitter will be
at 4.4 volts.

Regards,
Todd.

> Thanks for the help..but I don't understand why I
> will only get 4.4v. I thought that if I supply 5v to

> the NPN base from a logic high PIC output pin, the
> transistor would act as a simple switch and pass the

> +9v across the C-E junction.  Why only 4.4v?
>
> Thanks
> Les


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