From: Doug Sutherland (wearable_at_earthlink.net)
Date: 2001-12-13 02:42:08
Andreas wrote:
> A while ago (a few months?) someone (Yaniv?) proposed an idea
> that should satisfy those that don't feel that standard opto-
> couplers are safe enough: Build your own. Just mount a LED
> (on the PCB) facing a photo-diode (also on the PCB) or photo-
> transistor
I found some interesting info on opto-isolation at the
www.epanorama.net site (this is an excellent site). I
found these articles on the opto electronics page:
http://www.epanorama.net/links/opto.html
Here is an article about using LEDs and phototransistors for
opto-isolation of RS232 lines:
Circuit optimizes phototransistor bandwidth
http://archives.e-insite.net/archives/ednmag/reg/1998/040998/08di.htm#Circuit
optimizes
The images are no longer there but the article is in this PDF:
http://archives.e-insite.net/archives/ednmag/reg/1998/040998/08di.pdf
Here's a schematic for an analog opto-isolator
http://www.4qdtec.com/aoi.html
There was another article referencing opto-coupler temp drift
Circuit compensates optocoupler temperature coefficient
http://www.e-insite.net/ednmag/index.asp?layout=article&stt=000&articleid=CA181905&pubdate=11/22/2001&spacedesc=designideas
>> INTERESTING: The MAX232 (RS232 level converter) is powered
>> by a separate 9V battery from the main HAL circuitry to
>> maintain the isolation. Do we need this?
> Nope. Unless one needs a serial cable that is very long (as
> in more than 10 meters), a simple transistor amplifier can
> actually replace the MAX232.
Well, right now we have MAX232 on the PCB, and I assume that
it shares the same supply as the rest of the circuit. So is
this an issue? Assuming we stick with the 6N139 and MAX232
configuration, do those parts need separate battery power
to maintain isolation like the HAL does?
> If the group wants me to, I can design the
> DirtCheapSerialPortDriver if you want it added to
> the modular eeg. It ought to save a few bucks.
If it's actually safer than the 6N139 then I say go for it.
>> Is the irDa idea better or is it really overkill?
> I think IrDA is a cool feature, but the focus of this
> group is "cheap" not "cool". Right?
I agree. I would prefer RS232 done in a safe way.
-- Doug
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