Re: opto isolator and capacitors

Thank you.

I had not used a opto isolator before and it seemed that the field
switch (end switches in my case) just turned on an LED. The LED was
powered from some external power source. in that manner, only light
connected the 'protected' PC circuits and if one were to slip and
either ground or hit 220 to the LED, all you would loose is the
optocoupler. A friend bought a new circular saw some many years ago
and one if his first project was to saw the cord off. Always makes
think what if...

And I always say that one should learn the mistakes of othes, as you
will not live long enough to make them all yourself.

That being the case, The LED only needs minimal lumonsoity.
Enough for the receiver, but not so bright as to allow one to see it
from across the shop. ( I like double or triple powering LEDs and
pulsing the signal to get a remarkable bright display.)

Thanks, Now I need to play with inverting and non-inverting signals
from the parallel port to see what will work with what.

Dave

--- In CAD_CAM_EDM_DRO@y..., "Lee Studley" <indigo_red@q...> wrote:
> Hi Dave,
> The optocoupler led is just that, an led like you're thinking.
It'll
> need a current limiting resister. I'm making the assumption you mean
> the optos are for contact closure sensing going back into the port.
>
> I think your friend is trying to work up a debounce for the switch.
> reasons for placing a cap across the switch include:
> Eliminate arcing to contacts for higher voltages, and not really an
> issue at the <20Ma.
> -or- more likely, he probably means it will cause an RC time
> constant when the switch is opened and will act to keep the PC from
> reacting to contact release 'bounce', if the software could process
> it fast enough, and stretch the 'on' time enough so that the
> PC will see the 'contact' if it happens quickly. The cap's probably
> not necessary, but try/test it both ways. Too large a cap will
micro-
> weld the contacts upon closure during discharge unless you add an
> additional small R in series with the cap across the contacts. The
> led series resistor will be on the order of:
> (5v-2.1vledtypical)/0.015mA ~= 200ohms @ 5v. (330 ohms is a typical
> value for ~9mA @ 5v)
> -Lee
>
> --- In CAD_CAM_EDM_DRO@y..., "turbulatordude" <davemucha@j...>
wrote:
> > A friend said that I should use a small cap at the switch and a
> > resistor in the power line (5 volts).
> > I assumed that they worked just like a LED. figure current and
> > voltage then select a resistor.
> > Is he correct that a cap should be used too ? And if so, should
it
> > be at the switch or at the board ?
> >
> > Dave