Re: opto isolator and capacitors

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