From: sleeper75se (sleeper75se_at_yahoo.se)
Date: 2001-12-12 17:47:52
Hi Doug,
I'm all for optical isolation (it is vital), so my reply is really
about some technical solutions that might work.
--- In buildcheapeeg_at_yahoogroups.com, Doug Sutherland <wearable_at_earthlink.net> wrote:
> It seems that the opto-isolators with true optics-only
> paths, breaking all leads, are quite expensive (>$100).
A hobbyist could easily build a better isolator for almost no money
at all.
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 and cover them with
a piece of black plastic tube. The distance could be anything from,
say 1 cm to 10 cm depending on how unsafe you feel :-). Air is an
excellent insulator, so it will withstand thousands of volts and high-
amp currents. Probably not a direct lightning strike, though, but
everything else. Of course, one should be careful selecting plastic
material for the tube... something non-conductive. ;o)
Cost per optocoupler: perhaps $0.50 to $1.00.
> 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. The signal wires used for handshaking (RTS and DTR for
instance) can supply the power. If you doubt this, consider how many
serial-port mice there are out there... A MAX232 is useful when you
don't have -12V (or whatever) available, but since we do, it's not. :-
)
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.
> 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 hope my suggestion above shows that
you don't need it for safety.
/Andreas
This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:33 BST