Re: Parallel port relay blues

>
> This doesn't address the issue of the port being expected to source

current,

> though, rather than sinking it...

Most likely the 'source' for current is a pullup to plus 5 of
something as high as 4.7K. If that is the case, the max current would
be about 2.5Ma even if he connected the base direct. Given that the
Beta of the transistor is something like 50, that means that it would
drive a realy coil of about 125ma or less. The problem with this
circuit is that it defaults to on, so when you fire up your PC during
bootup, there may be circumstances where the ouput of the pins is
undefined. Once the port initilizes during the Windows load it may
well let the pin go high. Nothing like having all your 'stuff' come
on during bootup!

Doing active low for triggering the relay is a good approach although
you may well be beset with some of the same bootup issues. One of
the simplist ways to deal with the transistion is to use a chip like
the ULN2003 series that has the drive to handle several relays.
There are other open collector chips as well. I grabbed a 74541 non-
inverting octal buffer and it worked to make the signal transitions
from a PP to optos nice and clean.

PNP transistors are easy to use. Ground the collector and connect
the emitter to the Coil -. Put a diode across the coil with the
cathode to + and the anode to -. Put a 1K resistor in the base to
the parallel port pin and use active low to turn things on. The port
will sink enough current to turn the pnp hard on. The 2N2907 is the
complemtary transistor to the 2N2222. Any PNP that will handle the
max current of the relay will work. You can also run loads that need
higher voltage. 12V relays are more common. If the relay needs to
drive a big load you may find that it's coil exceeds the current
capability of the TO92 style plastic transistors. YOur choice is to
use a small relay to turn on the big one or go to bigger transistors.