[sci.electronics] Opto-Isolator Advice Needed

deanr@sco.COM (Dean Reece) (03/01/90)

I'm building a circuit to control various functions on my audio/video
equipment by computer.  I realize that isolating my circuitry from the
existing A/V stuff is a really good idea, so I need some advice on the
use of opto-isolators.

The basic requirements are low power/low (electrical) noise switching.
The input side is to be driven be either 74LSxx or 74HCxx (I haven't
decided yet).  The output side is going to simulate a keypress on
the equipment, which is a standard momentary N.O. switch which completes
a path to ground when pressed.  Speed is not really a requirement, so
anything in the 1 ms (milli-sec) range is plenty.  As for logic levels,
the 74 series part I'm using to drive the opto-isolators is an 8 bit
addressable latch (7f[HC|LS]259) which can be cleared to all Lows; so
a low input to the isolator should result in a key-released condition
(not grounded).  The controls being tickled are themselves inputs to
CMOS controller chips, so 15 volts or so is all thats being dealt with,
though it would be nice to have a large margin (say 60 volts).

So... I'm asking for your favorite Opto-Isolator circuits to do the
above.  If you've got a known solution or a good reference, please
let me know (via email - I can summarize).  I would prefer to keep
the range of devices being used down to the easily available & low
cost ones available from Jameco, All electronics, Digi-Key ...

Such as: 4N25, 4N26, 4N27, 4N28, 4N32, 4N33, 4N35, TIL-116, TIL-154

Thanx in advance
 ______________________________________________________________________
| Dean Reece     Member Technical Staff |"The flames are all long gone |
| The Santa Cruz Operation 408/458-1422 | but the pain lingers on"     |
|___________deanr@sco.com_______________|___________________Pink_Floyd_|

eacj@batcomputer.tn.cornell.edu (Julian Vrieslander) (03/02/90)

In article <5020@scolex.sco.COM> deanr@sco.com (Dean Reece) writes:
>
>I'm building a circuit to control various functions on my audio/video
>equipment by computer.  I realize that isolating my circuitry from the
>existing A/V stuff is a really good idea, so I need some advice on the
>use of opto-isolators.
>
>So... I'm asking for your favorite Opto-Isolator circuits to do the
>above.  If you've got a known solution or a good reference, please

By coincidence, I am also starting a project that may need opto-isolators.
In my case, the goal is to send 20 digital lines (data, clock, control)
from a Macintosh II nubus card to a D/A convertor card outside the
computer.  Hopefully, the opto-isolation will reduce noise in the
analog output signal.  This is for a high performance sound synthesis
system: 25k to 100k sample rates, 16-bit sampling, 90+ dB S/N ratio.

I have seen products that have up to 4 opto-isolator channels in one
DIP package; this seems like a way to simplifiy life when 20 lines must
be transmitted.  Are there any "standard", or recommended models among
these?  Any tricks or pitfalls to know about when rates of up to 100 kHz
are being sent?  Any particularly useful Application Notes worth reading?

I would also appreciate hearing any tips for eliminating noise *without*
going to opto-isolators.  The main requirement is to reduce spurious
stuff at the low level end of the D/A convertor's dynamic range.  When we
wire up the external box containing the convertor, we will first try it
with a direct ribbon-cable connection.  Then we will look to see what sort
of clock and P.S. garbage is reaching the analog output.  If needed, we
will try resistors between grounds, taking the ribbon cable through a
ferrite common mode choke, etc.  Any other ideas?
-- 
Julian Vrieslander 
Neurobiology & Behavior, W250 Mudd Hall, Cornell University, Ithaca NY 14853    
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