From: sleeper75se (sleeper75se_at_yahoo.se)
Date: 2002-01-13 22:06:29
Hi Joerg and Jim,
--- In buildcheapeeg_at_yahoogroups.com, "Joerg Hansmann" <info_at_jhansmann.de> wrote:
> > Since the topic has been brought up, I'm attacing a
> > new serial-port-powered fiber-optic receiver I'm
> > considering for the TinyEEG. It is based on a
> > micropower (0.47 mA) comparator that is very fast
> > (60ns).
>
> Nice circuit.
Thanks. It took a few hours to find the right one...
> If the comparator could handle +-12V supply voltage,
> it could drive the RxD line of the PC without DS275.
Yeah, I wish it could. Unfortunately, it doesn't. It's designed for
interfacing logic.
> How much tolerance in current transfer ratio can the receiver
> handle ? So how precisely must the IR emitter and the
> receiver be aligned or how precisely must the fiber be mounted ...
> and how difficult is the mounting of the optical fiber ?
> > Option 1 relies on these parts (available at Digikey):
> >
> > FB134-ND - Plastic Fiber Optic IR-LED (7ns): $5.25
> > FB128-ND - Plastic Fiber Optic Red LED (100ns): $4.88
> > FB120-ND - Plastic Fiber Optic Photodiode (5ns):
> > $4.18
If you use the Digikey-parts above, all that is taken care of. You
just insert the fiber in the connector and tighten the nut and you're
done. The Red LED injects 50uW into the fiber and the IR-led 100uW,
at 10mA LED-current. The photo-diode has a .2uA/uW transfer ratio for
the red-led and .4uA/uW for the IR-led. Therefore, if the fiber is
short (say max 2m) and the comparator threshold is set to less than
33mV (10uA photo-current through a 3.3K resistor), both LED options
would work. You could use a bigger resistor (33K for instance), but
then the diode capacitance would start to distort the signal.
Digikey catalog page: http://info.digikey.com/T021/V5/089.pdf
Photodiode datasheet: http://www.i-fiberoptics.com/leds/IFD91.pdf
If both the LED and the photo-diode is mounted in a box, it gets
trickier, but it is just a matter of deciding we want *this* LED and
*that* photo-diode at a particular distance in a particular current
range (say 5-15mA LED current) and then stick to that.
> 1.) without DS275 (why a level converter if we have +-12V ? Or have
I missed
> something ?) it looks good to me.
Ok, I'll try the 4053 solution first.
The only practical difference between 1 and 2 is board layout, so
both can be acommodated.
> Have you yet tried to use a third party opamp model in SWCAD ?
>
> I have just downloaded a TLC272 macromodel from TI and now I
> am trying to include it in the SWCAD library...
> until now not very successfully.
Hmm, no. I've looked around a bit ... they probably did this on
purpose. It shouldn't be impossible to fix, but I don't have the
answer, sorry.
--------------------------------------------------------------------
--- In buildcheapeeg_at_yahoogroups.com, "Jim Meissner" <jpmeissner_at_mindspring.com> wrote:
> Dear Andreas:
>
> Nice post!
>
> As you know I was questioning your "simple" two transistor receiver
circuit.
> Sometimes "too" simple and "too" cheap can be "too" expensive in
the long run.
Actually, I was sort of questioning it myself. :-)
> Please post the rest of the circuit, the 5 volt regulator
> and where that power comes from.
Ok, but it'll take a little while (hands full at the moment).
Meanwhile I hope a written description is ok:
Power to the circuit is supplied from two status lines, RTS and DTR,
controlled by the PC.
One status line is programmed to a high level (+12V) and the other to
a low level (-12V) and together they form the power supply for the
circuit.
The +/-12V supply is connected to two LDO regulators, with the usual
filtering, bringing the voltage down to +/-5V. We need 5V because
neither the comparator nor the DS275/(74HC)4053 are designed for
more. I've spent quite some time looking for +/-12V comparators, but
all of them draw too much current or are too slow.
> Will the RS232 port have enough power?
I've looked at the data sheet for the industry-standard MAX232A.
http://pdfserv.maxim-ic.com/arpdf/MAX220-MAX249.pdf
On page four, there is a graph showing the available output current
at different data rates going up to 60kbps. I want 115kbps, so the
graph can only be used as a guide...at 60kbps the MAX232 can supply
6.5mA. Anyway, I'm pretty certain a MAX232 is capable of at least 3mA
at 115200bps.
I'm hoping that ordinary desktop-computers have the same drive
capability... I know some laptops have problems, but for those there
are alternate solutions, like RS232-power-boosters.
Now, turning to page 6 of the MAX232 datasheet:
RS232-input threshold low: 0.6V minimum (1.2V typical)
RS232-input threshold high: 2.4V maximum (1.7V typical)
So, at least the MAX232 accepts regular logic levels on the RS232
inputs. This is the ideal solution - the comparator in the circuit I
posted can drive the input of a MAX232 directly. That many desktop
PC's can handle this is proven by all the PIC-microcontroller-
programmers you can find om the net.
Of course, a main requirement is that the RS232 cable is kept *short*
(< 5 m perhaps?) or that the optical receiver is mounted in the
connector (no cable at all).
> What is the difference between the DS275 and the 4053?
DS275 is made for the purpose of driving an RS232 line, using very
little power (800uA).
Here's the datasheet: http://pdfserv.maxim-ic.com/arpdf/DS275.pdf
4053 is a voltage controlled switch. If it was used as a driver, it
would work by switching in either +5V or -5V to the RS232 line - the
host-pc would in effect drive its own input.
Datasheet:
http://www.semiconductors.philips.com/acrobat/datasheets/74HC_HCT4053_
CNV_2.pdf
> The data rate I used was 9600 Baud. Joerg keeps asking about 56 K
Baud? What do "you"
> need for the TinyEEG project?
115200kbps, but if I change setup to the same MCU as Joerg, or use a
separate ADC, I could lower it a bit, to perhaps as low as 19200bps
for two channels.
I Hope you guys can make something out of this post. I know I'm a bit
terse...
Regards,
Andreas
This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:36 BST