Ken Webster wrote:

>Vital characteristics of 2N3904:
>hFe 200 typ.
>BVCEO 40
>IC 600mA
>
>Also, I am assuming that the coils are 250mA or less .. you can
substitute
>1k for the 2.2k to drive up to about 500mA.

My data book says the ABLOLUTE MAXIMUM allowable collector current for a
2N3904 is 200 mA.
It also says that the hFe of 200 typical (100 min/300max) is at 10 mA.
At 100 mA, the hFe is more like 30 than 200.  The circuit Ken described
is fine if your relay will operate reliably with a current of 60-70 mA
with the 2.2K, 100 mA with the 1K resistor.

The 2N2222 is a little better, max about 200 mA. The 2N4401 is better
yet, but not quite as easy to find.

A better solution is one of the Zetex high-gain transistors available
from Digi-Key.  A ZTX689B will sink 2 amps with 10 mA of base drive (but
be careful - this is measured in very short pulses!).  I'd feel safe
planning on 500-600 mA with 10 mA of base drive.  Assuming you have a 5V
supply, you need about 390 ohms to get 10 mA into a transistor because
you lose 0.7V across the base-emitter junction and 0.3 to 0.4 on the PIC
output.  Check your databook and make sure the I/O pin can source 10 mA
or better at 4.5V output (they will specify it as Vcc-0.5V, I think).
This assumes you have a decent 5V supply.

Of course the easiest solution is to use an N-channel FET.  An IRLD014 or
IRLD110 would work great.  Just hook source to ground, gate to your PIC,
drain to the relay, relay to 9V.  At Digi-Key, the IRLD110 is even
cheapter than the Zetex part.   Don't forget the diode across the relay
coil, or you can destroy any of these transistors quick, especially the
Zetex ones.

Hope this helps.

Don