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'[PIC:] Optically-isolated analogue inputs'
2004\08\19@225321 by Jinx

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Any suggestions/recommendations for opto-coupling a DC or low frequency
signal (10s of Hz) to the AN pins for measurement ? Maybe V-F -> F-V for
example ? Pointers to a proven design would be absolutely spiffing

TIA

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2004\08\20@033055 by Jonathan Johnson

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Sorry about the bandwidth usage guys I realised what I did afterwards with
the attached emails.

JJ

{Original Message removed}

2004\08\20@033716 by Jan-Erik Soderholm

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Jinx wrote :

> Any suggestions/recommendations for opto-coupling a DC or low
> frequency signal (10s of Hz) to the AN pins for measurement ?
> Maybe V-F  -> F-V for example ?
> Pointers to a proven design would be absolutely spiffing

Hi !

Isn't that a bit depending on why you need the opto coupler
in the first place ?

Anyway, maybe an "Isolation Amplifier" ? Like :

http://focus.ti.com/docs/prod/folders/print/iso122.html

I have a couple of samples someware. My plan was to build an
isolated probe to the o-scope to be able to measure on
a mains (230V) curcuit. And that's still my plan... :-) :-)

Regards,
Jan-Erik.


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> TIA
>
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Jan-Erik Svderholm
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mob : +46 70 5241690

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2004\08\20@040535 by Jinx

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> Hi !
>
> Isn't that a bit depending on why you need the opto coupler
> in the first place ?

How about a customer who insists on it ? Isolated digital i/o is easy
but never had to contend with isolated analogue. Wondered if anyone
here was familiar with it in medical equipment for instance. Ideally a
linear relationship between i/p and o/p would be best (not sure what
to expect from an LED though - ISTR a distant thread along those lines),
but if it really came down to it I suppose the ADC reading could be
massaged, but I'd rather not. I think IR headphones use PWM ? Maybe
that's the way to go. Which perhaps implies a PIC or 555 driving the
opto

> Anyway, maybe an "Isolation Amplifier" ? Like :
>
> http://focus.ti.com/docs/prod/folders/print/iso122.html

Definitely worth consideration. I've just had a look in the RS
catalogue to what's locally convenient. A range of amps from
NZ$52.65 for the ISO124U to the ISO102 at, choke, cough,
splutter, pollute trousers, NZ$596.25

Hmmm, pass.........The ISO102 isn't exactly special either

http://focus.ti.com/docs/prod/folders/print/iso102.html

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2004\08\20@041625 by Wouter van Ooijen

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> Any suggestions/recommendations for opto-coupling a DC or low
> frequency signal (10s of Hz) to the AN pins for measurement ?

Maybe avoid it?

A/D on the source side (with a small PIC), send it digitally to the big
PIC over an optocoupler.

Wouter van Ooijen

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2004\08\20@042704 by Alan B. Pearce

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>Any suggestions/recommendations for opto-coupling a
>DC or low frequency signal (10s of Hz) to the AN pins
>for measurement ? Maybe V-F -> F-V for example ?
>Pointers to a proven design would be absolutely spiffing

Look at the Agilent HCPL-7800 (available through RS or farnell, cannot
remember which I got from). It is designed for isolated motor current
measurements, so the input side will handle only about 300mV, so you may
need to scale that, but it does have a small amount of gain across the
isolation. Requires 5v supply on both sides.

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2004\08\20@043534 by Jinx

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> A/D on the source side (with a small PIC), send it digitally to the big
> PIC over an optocoupler.

That's what occurred to me when I thought about PWM (it could be
serial of course). The only very slight downside is that you'd need another
opto to request - in this application at least, all the INT pins are taken -
but
both could be el cheapos. Actually that might not be such a bad idea. The
unit is expected to survive on a back-up battery, so asking for data would
be better than having it omnipresent, wanted or not

Would still be interested in analogue opto ideas though

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2004\08\20@044401 by Jinx

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> Look at the Agilent HCPL-7800 (available through RS or farnell

Thanks, I did see that one. Would have to get a better price for
it ( approx US$25) and any iso-amp before committing. The whole
of PortA on an 18F452 needs to be isolated so cost is a factor

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2004\08\20@052138 by Russell McMahon

face
flavicon
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> > Any suggestions/recommendations for opto-coupling a DC or low
> > frequency signal (10s of Hz) to the AN pins for measurement ?
> > Maybe V-F  -> F-V for example ?
> > Pointers to a proven design would be absolutely spiffing


Infineon IL300 dual opto.
Made for analog linear coupling.
Suggested application circuits range from 2 transistors and a few resistors
to a handful of opamps.

Infineon application note 50 has 17 pages on how to use it.

   <oase.uci.kun.nl/~mientki/ Lego_Knex/Lego_electronica/app50.pdf>



       RM

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2004\08\20@054252 by Jinx

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> Infineon IL300 dual opto.

>  <oase.uci.kun.nl/~mientki/ Lego_Knex/Lego_electronica/app50.pdf>

No luck with that link

I found it here. Will look into it, thanks

http://www.vishay.com/optocouplers/list/product-83622/

===================================

BTW, [OT], after the demise of a much-loved 98 HDD, I'm trying to set
this new XP box up. One thing that doesn't seem to work as it should is
that clicking on links doesn't always bring up a web page

For example, clicking on a link in OE doesn't open IE. I have to manually
start IE and copy/paste. And that ain't right. Many links in web pages open
a new window....and that's all that happens. Think I'm looking in the right
place to change something

Tools/Folder Options/File Types. Under HTML it says

Action - &Open

Application - "C:\Program Files\Internet Explorer\iexplore.exe" %1

DDE message - "%1",,-1,0,,,,

Application - IExplore

DDE application not running - *

Topic - WWW_OpenURLNewWindow

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2004\08\20@073017 by Russell McMahon

face
flavicon
face
> For example, clicking on a link in OE doesn't open IE. I have to manually
> start IE and copy/paste. And that ain't right. Many links in web pages
open
> a new window....and that's all that happens. Think I'm looking in the
right
> place to change something

Does shift and then click work ?

   RM

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2004\08\20@083451 by Bob Ammerman

picon face
> > A/D on the source side (with a small PIC), send it digitally to the big
> > PIC over an optocoupler.
>
> That's what occurred to me when I thought about PWM (it could be
> serial of course). The only very slight downside is that you'd need
another
> opto to request - in this application at least, all the INT pins are
taken -
> but
> both could be el cheapos. Actually that might not be such a bad idea. The
> unit is expected to survive on a back-up battery, so asking for data would
> be better than having it omnipresent, wanted or not
>
> Would still be interested in analogue opto ideas though

You would, of course, need an isolated power supply in addition to the opto
feeding the data in. So, just turn on the power supply to perform your
'request'.

Bob Ammerman
RAm Systems

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2004\08\20@083452 by Bob Ammerman

picon face
> Thanks, I did see that one. Would have to get a better price for
> it ( approx US$25) and any iso-amp before committing. The whole
> of PortA on an 18F452 needs to be isolated so cost is a factor

Be careful. Do you have to have each analog input isolated from the others,
or just all the analogs isolated from the rest of the world, but sharing
common circuitry among themselves?


Bob Ammerman
RAm Systems

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2004\08\20@084944 by Russell McMahon

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flavicon
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> Any suggestions/recommendations for opto-coupling a DC or low frequency
> signal (10s of Hz) to the AN pins for measurement ? Maybe V-F -> F-V for
> example ? Pointers to a proven design would be absolutely spiffing


Howzabout you give us the FULL spec of what you are trying to do.

How many channels
How many bits
Min update rate per channel (10's of Hz = eg 50 mS per channel or 50 mS per
8 channels or ...)
Isolation requirements.
... ?


       RM

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2004\08\20@085355 by Paul Lee

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PWM, or digital interface are only natural solution.

why still stick with analog?



----- Original Message -----
From: "Jinx" <spam_OUTjoecolquittTakeThisOuTspamCLEAR.NET.NZ>
To: <.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU>
Sent: Thursday, August 19, 2004 10:53 PM
Subject: [PIC:] Optically-isolated analogue inputs


{Quote hidden}

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2004\08\20@095415 by Jinx

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> Howzabout you give us the FULL spec of what you are trying to do.

Full spec is still in pencil. Basically the system can set and/or monitor/
report the status of various plug-in boards. I've made and have the host
board running, now looking at the interfacing

> How many channels

I've been thinking.......instead of using each AN as a separate input, by
using one bi-directional opto-link to a satellite PIC it would be possible
to
send commands to it to control a MUX that could select from many
analogue sources with eg a 4051. That satellite PIC then does the AD
and bit bangs the result to the host PIC via the other opto. Which may be
an answer to Bob's question ;

> Be careful. Do you have to have each analog input isolated from the
> others, or just all the analogs isolated from the rest of the world, but
> sharing common circuitry among themselves?

So it would be possible to use all the pins of the host as digital. AD can
be done on the plug-in

> How many bits

10 would be nice

> Min update rate per channel (10's of Hz = eg 50 mS per channel or
> 50 mS per 8 channels or ...)

The basic sampling rate of the sytem is once per minute and mostly that's
how
often I'd expect to normally check the analogues. One aim is to make smart
modules, so there is potential for each analogue source to, for example, be
compared against an alarm value in a module's PIC. Which may mean some
channels need ony be polled for alarm on/off condition. Forget the Hz for
the moment, DC levels will be more likely

> Isolation requirements.
> ... ?

Mains faults, electrical noise, etc. Not bothered about lightning

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2004\08\20@100419 by M. Adam Davis

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Ask an analog question in a digital group...!

Assuming there's power on both sides of the circuit, use an opamp on one
side to light the LED inside the optocoupler.  The brightness will
depend on the current flow, which will depend on the output of the
opamp, which will depend on the signal.

On the transistor side, measure the current flowing through the
transistor in the optocoupler.  Use an opamp to buffer it and convert it
back to a voltage.

Pick the correct values for the opamp gains and the voltage out will
equal the voltage in over the entire range.  Pick the correct opto and
opamps and you will get all the bandwidth you could ever use.

Maximum of one tiny opamp on each side, an opto, and maybe 5-10 passive
parts.  Bonus points if you use transistors instead of opamps, since
they'll be vastly cheaper and an opamp is overkill for this application

There are some great circuits that do exactly what you need in this data
sheet:
http://literature.agilent.com/litweb/pdf/5988-4104EN.pdf

Hope this helps!

-Adam

Jinx wrote:

{Quote hidden}

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2004\08\20@102043 by Spehro Pefhany

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At 10:04 AM 8/20/2004 -0400, you wrote:
{Quote hidden}

Note that for any kind of accuracy it is usually more complex than you
suggest since the transfer ratio varies with temperature and time. Thus
feedback is used to compensate for the horrible analog characteristics
of the optocoupler in general (and the LED in particular). That's why
the analog optocouplers have two sensors and just one LED. With enough
gain, trimming and luck you can just about get to the DC accuracy of the
ADC in a PIC. Maybe.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speffspamKILLspaminterlog.com             Info for manufacturers: http://www.trexon.com
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2004\08\20@104121 by hael Rigby-Jones

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{Quote hidden}

Or you could just use a multichannel ADC with I2C or SPI bus. Analog
devices,Linear technology and Maxim have numerous variants.  Definitely try
to keep any opto-isolation on the digital side, it will save you headaches.

Note also that cheap optoisolators tend to be very slow.  This will limit
your bit rate, but it sounds like this may not be a big problem in your
case.  You should be able to get a few kilohertz minimum however.

Mike

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2004\08\20@111249 by Bob Ammerman

picon face
> I've been thinking.......instead of using each AN as a separate input, by
> using one bi-directional opto-link to a satellite PIC it would be possible
> to
> send commands to it to control a MUX that could select from many
> analogue sources with eg a 4051. That satellite PIC then does the AD
> and bit bangs the result to the host PIC via the other opto. Which may be
> an answer to Bob's question ;
>
> > Be careful. Do you have to have each analog input isolated from the
> > others, or just all the analogs isolated from the rest of the world, but
> > sharing common circuitry among themselves?

Why use a 4051. Why not just use the built in analog mux on the satellite
PIC. ie:

   biggerpic < smallerpic + 4051

I guess your answer to my question is: "it is ok for the analog channels to
have common circuitry".

Bob Ammerman
RAm Systems

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2004\08\20@190551 by Mike Harrison

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On Fri, 20 Aug 2004 09:27:54 +0100, you wrote:

>>Any suggestions/recommendations for opto-coupling a
>>DC or low frequency signal (10s of Hz) to the AN pins
>>for measurement ? Maybe V-F -> F-V for example ?
>>Pointers to a proven design would be absolutely spiffing
>
>Look at the Agilent HCPL-7800 (available through RS or farnell, cannot
>remember which I got from). It is designed for isolated motor current
>measurements, so the input side will handle only about 300mV, so you may
>need to scale that, but it does have a small amount of gain across the
>isolation. Requires 5v supply on both sides.

I'd use a PIC12F675 on the remote side as an intelligent ADC, sampling the data and squirting the
data (e.g. as async data)  through an optoisolator. The fact that you can use a dirt-cheap optoisolator for the sort of data rates you need will
probably more than offset the cost of the 12F675 compared to using any sort of fancy analog
isolator.

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2004\08\20@191005 by Jinx

face picon face
> Why use a 4051. Why not just use the built in analog mux on the
> satellite PIC. ie:
>
>     biggerpic < smallerpic + 4051
>
> I guess your answer to my question is: "it is ok for the analog channels
> to have common circuitry".

Well, if I used daughter boards with something like a 12F675 on each,
then yes. The digital transfer option does seem at this stage to be more
attractive than individual analogue links to the 18F452 motherboard's
AN i/ps, which could be expensive and fiddly to set up

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2004\08\20@191212 by Spehro Pefhany

picon face
At 12:09 AM 8/21/2004 +0100, you wrote:
{Quote hidden}

Yup. Think of the 12F675 as an intelligent ADC. You get unadjusted
accuracy as good as the isolated power supply, pretty much, without the
issues associated with trimpots etc.

I also like the idea of cycling the power as the conversion-request signal.
Only one opto required, and a hardware serial port could be used on the
other side. Especially nice on chips that have a couple of those.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speffspamspam_OUTinterlog.com             Info for manufacturers: http://www.trexon.com
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2004\08\20@191629 by Jinx

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> I'd use a PIC12F675 on the remote side as an intelligent ADC

Thanks, I'd just this minute come to that conclusion. Maybe even
a 16F676, although in all probability the sources will be somewhat
scattered - looking at UHF data links to some daughter boards - so
minimal ADC channels per board (12F675 has 4) would be fine

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2004\08\20@193250 by Russell McMahon

face
flavicon
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> >Assuming there's power on both sides of the circuit, use an opamp on one
> >side to light the LED inside the optocoupler.  The brightness will
> >depend on the current flow, which will depend on the output of the
> >opamp, which will depend on the signal.

> >There are some great circuits that do exactly what you need in this data
> >sheet:
> > http://literature.agilent.com/litweb/pdf/5988-4104EN.pdf

> Note that for any kind of accuracy it is usually more complex than you
> suggest since the transfer ratio varies with temperature and time. Thus
> feedback is used to compensate for the horrible analog characteristics
> of the optocoupler in general (and the LED in particular).


Usually more complex, yes. BUT in this case he oversimplified what was
involved BUT referred to an Agilent app note that used a 1 x LED 2 x two
receiver opto that was specifically optimised for linear analog transfer.
They claim 0.01% linearity (not necessarily overall end accuracy). They also
give a range of cheap circuits to go with it. This is an identical solution
to the one I mentioned a day or so ago but he provided a different and
arguably better app note.


My device          IL300                                    Agilent   None
at Digikey
His devices        HCNR200. HCNR201        Infineon $US2.99 & 3.98 in 1s @
Digikey
($2/100 for HCNR200)


I think for what Jinx wants a standard solution and an isolated digital data
channel will do well BUT the Infineon parts look very nice for appropriate
applications.


       RM

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2004\08\20@195157 by Jinx

face picon face
> My device          IL300                            Agilent   None
> at Digikey
> His devices        HCNR200. HCNR201  Infineon $US2.99 &
> 3.98 in 1s @ Digikey  ($2/100 for HCNR200)
>
>
> I think for what Jinx wants a standard solution and an isolated digital
> data channel will do well BUT the Infineon parts look very nice for
> appropriate applications

And I'll get prices off Arrow when I call about the 12F675, just in
case an appropriate application does come up in this project

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2004\08\20@214733 by Rich

picon face
There is a circuit made from discrete components in an older HP OPTO
catalog.  It uses a technique of creating the same nonlinear current in each
side of the OP amp, if I remember correctly.  I do not have the circuit but
it may be available from HP.  The older catalog had excellent applications
notes.  I probably have the catalog, but during moving the lab and library,
it is packed among about 4000 books.  I will be unpacking them in November
but that may be too late for you.  Try contacting HP and Asking about is.
It is a very cost effective solution, utilizing low cost components, and the
linearity is improved by careful selection of transistor characteristics
which can be done without a curve tracer.  I have used the circuit in the
past and I will bet someone on the list may have the circuit.


{Original Message removed}

2004\08\29@221223 by Jinx

face picon face
part 1 249 bytes content-type:text/plain; (decoded 7bit)

This should work, but any thoughts on simplification ?

I intend to make a bank of 4 or 8 of these, interfacing with
completely isolated sensors, so using a 4066 or two is not
an issue, neither is cost (mostly)




part 2 3231 bytes content-type:image/gif; (decode)


part 3 194 bytes content-type:text/plain; charset="us-ascii"
(decoded 7bit)

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part 4 1 bytes

2004\08\30@000824 by Engineering Info

picon face
I don't see a pulldown on the far left transistor.  In fact, I don't
even understand it's purpose as drawn.

If you are wanting to have COMPLETE isolation between sensors, wouldn't
you need a seperate 4066 for each sensor to seperate the grounds?????
Could maybe use a single bilateral switch (74H1G66) instead of a quad.

Jinx wrote:

{Quote hidden}

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2004\08\30@003348 by Russell McMahon

face
flavicon
face
>  I intend to make a bank of 4 or 8 of these, interfacing with
> completely isolated sensors, so using a 4066 or two is not
> an issue, neither is cost (mostly)

Seems overly complex - depending on objectives.
Howzabout stating the objectives in approx order of priority

As shown

- It minimises pin count at each end, which may be desired.
- It minimises power used by remote by waking it up when required.

How important is power consumption of A2D end?
How often must it send?
How close to wakeup timer must it sample and send?

A "blurt out occasionally" version requires just an optocoupler and 2
resistors. It must send periodically and could power down between
conversions.




       RM

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2004\08\30@004020 by Jinx

face picon face
> I don't see a pulldown on the far left transistor.  In fact, I don't
> even understand it's purpose as drawn

Agreed, it could have a high value pulldown. That would also help
discharge the cap in a more defined manner. The transistor is there
to ground the 4066 switch control input (ie open the switch)

> If you are wanting to have COMPLETE isolation between sensors,
> wouldn't you need a seperate 4066 for each sensor to seperate the
> grounds?????

Grounds are not connected - I simply used the generic symbol. The
sensor would be on a completely separate mains or battery supply

> Could maybe use a single bilateral switch (74H1G66)

For one sensor, yes, although I did mention they would be in banks
of  4 or 8

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2004\08\30@005017 by Jinx

face picon face
> As shown
>
> - It minimises pin count at each end, which may be desired

One PIC pin / per A2D is desirable. Not overly fussy about 675 end,
it has i/o to spare, even if all 4 AD inputs used

> - It minimises power used by remote by waking it up when required.
>
> How important is power consumption of A2D end?

If on battery, important. If using minimum configuration a low quiescent
should be quite achievable

> How often must it send?

Max 1 once per minute. Could be as infrequent as once / day or
much longer intervals. It's user-set, with minimum interval = 60 sec

> How close to wakeup timer must it sample and send?

Not important. (Well, within a minute really)

> A "blurt out occasionally" version requires just an optocoupler and 2
> resistors. It must send periodically and could power down between
> conversions.

I'd prefer on-demand, but I guess unwanted transmissions could be
ignored. Might get a little messy s/w-wise because the 452 may have
to power-down too. It would be woken by events on its own /INT2

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2004\08\30@010638 by Engineering Info

picon face
Banks of 4 or 8?  Aww yes, you did say that.  My bad.

However, the transistor still doesn't look right. Power to the opto1 LED
as well as the transistor base would have to be applied THROUGH the 4066
switch.  I'm assuming that the 5V pullup on the opto2 transistor would
be pulled down within the 452 so as to not allow power to pass through
the opto1 LED.  As soon as you release the 452 I/O pin,  power is
applied to both the 4066 control transistor and the opto1 LED and the
4066 will open which I guess is your desire.  However, since power
through the 4066 is now shutoff, the base on the transistor will fall
back to ground, turning off the transistor and thus allow the 4066 to be
controlled on again.  My guess at this point is that the 452 is now in a
INPUT state to recieve data from the 12F675 which in turn will allow the
opto2 5V input to power the opto1 LED which in turn will short out the
12F675 interupt pin.

Jinx wrote:

{Quote hidden}

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2004\08\30@020207 by Jinx

face picon face
> However, the transistor still doesn't look right. Power to the
> opto1 LED as well as the transistor base would have to be
> applied THROUGH the 4066 switch.  I'm assuming that the
> 5V pullup on the opto2 transistor would be pulled down within
> the 452 so as to not allow power to pass through the opto1 LED

Yes. It would also compromise what's meant to really be happening
on the transistor's base. And pullups are high value, LED / base resistors
will be much lower

> As soon as you release the 452 I/O pin,  power is applied to both the
> 4066 control transistor and the opto1 LED and the 4066 will open
> which I guess is your desire.  However, since power through the 4066
> is now shutoff, the base on the transistor will fall back to ground,

But the switch will be held open for some time (> than the time it takes
to transmit the data) due to the capacitor. Once the transistor turns off
the whole thing would be back to where it started. I'll need to breadboard
it and work out both PICs timings, to try and characterise it. I'm sure
there's an optimised set-up in there somewhere

Also, the quality of the wake-up to the 675 isn't important. It just
needs to be "something/anything" on /INT

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2004\08\30@041829 by Russell McMahon

face
flavicon
face
You may not care, but I'm sure you could do something FAR simpler component
wise.
I don't know about the capabilities of the '675, but if it has a pin that
can wake on change and then be disabled, you could use that to wake the 675
and then disable it if necessary. You can use a separate 675 pin from the
675 data TX if desired to stop that pin seeing its own data via two optos
with potential for latching action

You can have the 452 line usually high and pulled low by receive data. It
doesn't matter that this low signal is also sent back to the separate pin on
the 675.

IF the 675 is NOT able to turn off its wakeup signal internally you could do
this with a separate pin of its own. ie when asleep it is able to be woken
up by an opto low but once running it disables this function so its own
returned data will not affect it.

Picture later maybe.
Imagine this. No resistors shown in series with the optos. Add them,
yourself.

OptoA Anode high
OptoA cathode to 672pin
OptoA emitter output to remote ground.
OptoA collector out to remote wakeup pin.
Pullup from wakeup pin to high if needed.

OptoB collector to 672pin
OptoB emitter to ground
OptoB cathode to remote ground.
OptoB anode to 675 TXD pin (resistor as required)

If necessary disable 675 wakeup / INT function with 1 extra transistor
driven from another 675 pin.

Whole hardware is two optos and 2 resistors (possibly an extra transistor
and resistor for wakeup turnoff).
The dual 8 pin (or quad 16 pin) NEC optos that I forgot to give you some of
the other day would probably do fine here.

You COULD send data 472 to 675 if desired for function control.


       RM

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2004\08\30@043842 by Engineering Info

picon face
part 1 2950 bytes content-type:text/html; charset=us-ascii (decoded 7bit)
Content-Type: text/html; charset=us-ascii Content-Transfer-Encoding: 7bit OK, makes perfect sense now.  I know your knowledgeable enough to have thought it out which is why I didn't say you were wrong but rather say it didn't look right (at least at the time).  The only thing I could add to the circuit would be the pulldown that I mentioned earlier and possibally changing the resistor between your cap and the opto1 LED to a diode instead.  Would allow you to charge the cap faster in the one direction and prevent the LED from discharging part of your caps capacity in the other direction (but you already knew that).

On the other hand, Russell is probally smarter then both of us put together so I would suggest you ignore everything I have said and listen to him.

Jinx wrote:
However, the transistor still doesn't look right. Power to the
opto1 LED as well as the transistor base would have to be
applied THROUGH the 4066 switch.  I'm assuming that the
5V pullup on the opto2 transistor would be pulled down within
the 452 so as to not allow power to pass through the opto1 LED
    

Yes. It would also compromise what's meant to really be happening
on the transistor's base. And pullups are high value, LED / base resistors
will be much lower

  
As soon as you release the 452 I/O pin,  power is applied to both the
4066 control transistor and the opto1 LED and the 4066 will open
which I guess is your desire.  However, since power through the 4066
is now shutoff, the base on the transistor will fall back to ground,
    

But the switch will be held open for some time (> than the time it takes
to transmit the data) due to the capacitor. Once the transistor turns off
the whole thing would be back to where it started. I'll need to breadboard
it and work out both PICs timings, to try and characterise it. I'm sure
there's an optimised set-up in there somewhere

Also, the quality of the wake-up to the 675 isn't important. It just
needs to be "something/anything" on /INT

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part 2 300 bytes content-type:text/plain; charset="us-ascii"
(decoded 7bit)
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part 3 5 bytes
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2004\08\30@071940 by Russell McMahon

face
flavicon
face
> On the other hand, Russell is probally smarter then both of us
> put together so I would suggest you ignore everything I have
> said and listen to him.

Flattery will get you somewhere :-)

Smarter than both - no. More experience with doing things really cheaply for
a long time - maybe :-). (I'm older than Jinx anyway).

There's the right way to do it and the cheap way. Sometimes they are the
same way. Truth be known an extra 4066 and a few transistors is not the end
of the world. But I'm sure that if the task is only what it sounds to be
then something like what I suggested will work. Has the advantage of (apart
from lower components count) no dependence on delays or analog parts playing
semi digital games. I'm a great fan of using such things when needed, but I
also like avoiding them when possible. (You can do almost anything with
enough CD40106 sections, 1N4148 diodes and requisite R's & C's :-) ).



       RM

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