Michael Algernon wrote:
> I tried building a do-it-yourself opto-isolator
> using a T13/4 red and
> green LED pointing at each other.

That doesn't sound like a good idea.  I think the reception wavelengths of
LEDs are usually close to their transmission wavelengths.

> I flattened the faces of the LEDs and polished
> them and glued them together with clear glue ( very thin coat ).  I
> drove the green LED with about 10ma of current.  I reversed biased the
> red LED with a 50M ohm pull-up resistor.  I fed the connection between
> the LED and the pull-up resistor into a 74C14 hex inverter ( with
> schmitt triggers ).

Sounds like a long way to go to build half of a 75 cent off the shelf part
yourself.

> The 74C14 is specified to have .005 ua of input
> current or 5*10**-9 amp.

Learn to say "nano".  It means 10**-9.

> The pull-up resistor was supplying about
> 10**-7 amp.   So I thought it might work.  No it did not.  Only when I
> shone a bright flash-lite on it, did it change state.

This means the leakage current wasn't getting you, but not enough of the
right wavelength light was getting into the receiver LED.

> Maybe I should have used a 500M ohm resistor.

Where are you going to get that from?  And even if you did, how are you
going to keep it and the board its on and all the other parts super clean to
maintain 500Mohm?

Personally I think
http://www.mouser.com/Search/ProductDetail.aspx?qs=bfRUmXT2lZgQAJSbX58%2fqw%3d%3d is rather less hassle.


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