Re: Parallel port relay blues

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Andy Wander <awander@v...>
wrote:

> Off the top of my head, I will say the parallel port can source

10mA and

> sink 20mA.
>
> I could be wrong..

I think the specification for the parallel port is 2.5mA at 2.5 volts
when power external circuits. I'm pretty sure it can handle 20mA
when used in the NPN mode.

This means your 5 volt output can dip to 2.5 volts and supply 2.5mA
and still be in spec. this is why laptops are known for low output
power.


You are much better off connecting 5V from the PC Power supply to
your board and then pull the optos to ground to get the signal.

Don't worry about the PC side so much. use an opto and do all the
conversion/inversion on the power side, not the PC/Signal side.

Dave