I need an opto isolator which will switch on about 5mA. Just to drive an
ordinary LED off a feed for a low current one. I do need the electrical
isolation, though. I've got a few assorted ones here and none are
sensitive enough. I dunno if a darlington pair one would do?

--
*Marathon runners with bad footwear suffer the agony of defeat*

Dave Plowman London SW
To e-mail, change noise into sound.

It happens that Dave Plowman (News) formulated :
I need an opto isolator which will switch on about 5mA. Just to drive an
ordinary LED off a feed for a low current one. I do need the electrical
isolation, though. I've got a few assorted ones here and none are
sensitive enough. I dunno if a darlington pair one would do?

Have you looked at the opto in an old rubber ball type mouse?

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk

In article ,
"Dave Plowman (News)" writes:
I need an opto isolator which will switch on about 5mA. Just to drive an
ordinary LED off a feed for a low current one. I do need the electrical
isolation, though. I've got a few assorted ones here and none are
sensitive enough. I dunno if a darlington pair one would do?

I often use a quad opto isolator ILQ74 (CPC SC07729), and
I feed the output into darlington driver such as a ULN2804A.

This enables me to easily drive something like 50mA relay coils
from a 1mA input signal to the opto isolator. If you try driving
the relays directly from the opto isolator, you need to feed in
about the same current you switch, which requires more power (a
concern if battery powered) and the components run hotter.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

In message , "Dave Plowman (News)"
writes
I need an opto isolator which will switch on about 5mA. Just to drive an
ordinary LED off a feed for a low current one. I do need the electrical
isolation, though. I've got a few assorted ones here and none are
sensitive enough. I dunno if a darlington pair one would do?


Just look at the CTR figure for the opto in question. CTR = current
transfer ratio, i.e. the current gain of the opto. Most are 25% to 400%,
but split into narrower gain bands. Look for one from the higher end of
the range, or as you say a darlington one would do.
Default opto choice for me would be CNY17 series (mains isolation rated,
8mm creepage path options available). YMMV.


--
steve

In article ,
Steven Briggs wrote:
Just look at the CTR figure for the opto in question. CTR = current
transfer ratio, i.e. the current gain of the opto. Most are 25% to 400%,
but split into narrower gain bands. Look for one from the higher end of
the range, or as you say a darlington one would do.
Default opto choice for me would be CNY17 series (mains isolation rated,
8mm creepage path options available). YMMV.

Thanks for that. I wasn't clear if you needed a minimum drive current for
the internal LED to work - the data sheets just seem to give a max value.

The isolation needed is only between a 5 and 12 volt rail - so hardly
demanding.

--
*Learn from your parents' mistakes - use birth control.

Dave Plowman London SW
To e-mail, change noise into sound.