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'[EE]: Foolproof input line'
2002\10\16@140252 by Mike Singer

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part 1 376 bytes content-type:text/plain; (decoded 7bit)

I need simple converting circuit to protect PIC input.
Input levels 0 - 5v - 12v, open or not collectors.
Could anybody comment attached .gif, please.
Isn't it an overkill?

Mike.
-------
If you think of something to be foolproof,
the fools are always greater then the proof!
       Eduard Teller
       American Nuclear Physicist


part 2 1476 bytes content-type:image/gif; (decode)


part 3 131 bytes
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2002\10\16@145831 by Bob Blick

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>  I need simple converting circuit to protect PIC input.
>  Input levels 0 - 5v - 12v, open or not collectors.
>  Could anybody comment attached .gif, please.
>  Isn't it an overkill?

It looks like it could hurt your PIC by raising the +5 Vdd.

What's wrong with a diode pointing from the PIC to your input and a pullup
resistor on the PIC pin? And if you need protection from negative
voltages, an input series resistor and also a diode from ground to the
PIC pin?

Cheers,

Bob

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2002\10\16@152504 by Byron A Jeff

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On Wed, Oct 16, 2002 at 11:56:43AM -0700, Bob Blick wrote:
> >  I need simple converting circuit to protect PIC input.
> >  Input levels 0 - 5v - 12v, open or not collectors.
> >  Could anybody comment attached .gif, please.
> >  Isn't it an overkill?
>
> It looks like it could hurt your PIC by raising the +5 Vdd.
>
> What's wrong with a diode pointing from the PIC to your input and a pullup
> resistor on the PIC pin? And if you need protection from negative
> voltages, an input series resistor and also a diode from ground to the
> PIC pin?

What about a series resistor and a 4.7 or 5.1V zener across the pin to ground?
Zener action when the voltage is too high, conduction to ground when it's too
low?

BAJ

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2002\10\16@171305 by Mike Singer

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Bob Blick wrote:
> >  I need simple converting circuit to protect PIC input.
> >  Input levels 0 - 5v - 12v, open or not collectors.
> >  Could anybody comment attached .gif, please.
> >  Isn't it an overkill?
>
> It looks like it could hurt your PIC by raising the +5 Vdd.
>
> What's wrong with a diode pointing from the PIC to your input and a
pullup
> resistor on the PIC pin? And if you need protection from negative
> voltages, an input series resistor and also a diode from ground to the
> PIC pin?

With few mA it couldn't raise the +5v power suply.
a diode pointing from the PIC to the input eats some
voltage, thus reducing bottom threshold to switch Schmitt
Trigger. External source might be unable to provide this
level. (5v might be 3v, then:  0.2v * 3= 0.6v minus
diode's 0.35-0.7v)
With the circuit the bottom threshold is much higher, as
well as top threshold. The cost is only one extra resistor,
compared to your 2 diodes + 2 resistors circuit.

Mike.
Correct me if I'm wrong, please.

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2002\10\16@172100 by Mike Singer

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Byron A Jeff wrote:
> > >  I need simple converting circuit to protect PIC input.
> > >  Input levels 0 - 5v - 12v, open or not collectors.
> > >  Could anybody comment attached .gif, please.
> > >  Isn't it an overkill?
.
> What about a series resistor and a 4.7 or 5.1V zener
> across the pin to ground? Zener action when the voltage
> is too high, conduction to ground when it's too low?

Vdd might be 3v and what about open collector source?

Mike.
Correct me if I'm wrong, please.

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2002\10\16@174206 by Russell McMahon

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> Vdd might be 3v and what about open collector source?

Diagram explicitly showed Vdd = 5V

       RM

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2002\10\16@175938 by Mike Singer

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Russell McMahon wrote:
> > Vdd might be 3v and what about open collector source?
> Diagram explicitly showed Vdd = 5V

Good morning, Russell.

O.K. I was slightly hasty. It should be read as 3-5v.

Mike, still thinking about "defining the criteria a little
more tightly and making this a design challenge"

1.00 am.

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2002\10\16@193119 by Bob Blick

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>  With few mA it couldn't raise the +5v power suply.
>  a diode pointing from the PIC to the input eats some
>  voltage, thus reducing bottom threshold to switch Schmitt
>  Trigger. External source might be unable to provide this

OK, so how about just one part - a series resistor, high in value. If you
need to drive with open collector, add a pullup resistor before the series
resistor.

There's no universal way to do this. There are too many PICs, too many
different pins on the PICs, etc...

Do what works best in each case.

-Bob

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2002\10\16@224528 by DFansler

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Bob Blick wrote:
> >  I need simple converting circuit to protect PIC input.
> >  Input levels 0 - 5v - 12v, open or not collectors.
> >  Could anybody comment attached .gif, please.
> >  Isn't it an overkill?


Another solution not yet mentioned is an HP3700 - this is a IC that allows
AC or DC input that can be limited with a single resistor to cover ranges
from a few volts to several hundred volts, has a Schmitt trigger and as I
recall an OC output.  Not sure if it is still available from HP, but I do
have the data sheet and about 100 of them available.

David V. Fansler
spam_OUTDFanslerTakeThisOuTspamMindSpring.com
http://www.DV-Fansler.com

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2002\10\17@135853 by Mike Singer

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Thank you Bob, Russell, Byron, David  for your responses.

Russell wrote:
.
{Quote hidden}

Yes, this is just what I thought of.


> I could draw a dozen (+) circuits that would meet this spec ...

Could you compose a little "official" FAQ on the theme?

Mike.

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2002\10\17@154843 by Peter L. Peres

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On Wed, 16 Oct 2002, Bob Blick wrote:

*>>  With few mA it couldn't raise the +5v power suply.
*>>  a diode pointing from the PIC to the input eats some
*>>  voltage, thus reducing bottom threshold to switch Schmitt
*>>  Trigger. External source might be unable to provide this
*>
*>OK, so how about just one part - a series resistor, high in value. If you
*>need to drive with open collector, add a pullup resistor before the series
*>resistor.
*>
*>There's no universal way to do this. There are too many PICs, too many
*>different pins on the PICs, etc...

Yes there is. The clamping circuit I posted on the piclist before will do
this, will not rise the Vcc line, will maintain the pic inputs within
manufacturer specs at all times.

Peter

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2002\10\17@171229 by Russell McMahon

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> *>>  With few mA it couldn't raise the +5v power suply.

> *>OK, so how about just one part - a series resistor, high in value. If
you
> *>need to drive with open collector, add a pullup resistor before the
series
> *>resistor.

> *>There's no universal way to do this. There are too many PICs, too many
> *>different pins on the PICs, etc...

> Yes there is. The clamping circuit I posted on the piclist before will do
> this, will not rise the Vcc line, will maintain the pic inputs within
> manufacturer specs at all times.

I assumed he meant that there is no single solution that would be cheapest
in every case (or close to cheapest) and meet spec and that it is better to
design to the current requirement than to try to propose a single solution
for all cases. .

One could conceive of a relatively complex circuit that met essentially
every need but it would be liable to be overkill in almost every case. I
assume you are referring to your clamp cct attached as a PDF to an August
email. This has 4 transistors which puts it outside the class of what was
desired and still needs extra components for the input resistor and default
O/C bias. Which is not to say it is a bad cct - just still not the universal
solution.

Maybe we need a FAQuette on input systems?


           Russell McMahon

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2002\10\18@042550 by Peter L. Peres

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On Fri, 18 Oct 2002, Russell McMahon wrote:

*>I assumed he meant that there is no single solution that would be cheapest
*>in every case (or close to cheapest) and meet spec and that it is better to
*>design to the current requirement than to try to propose a single solution
*>for all cases. .

Unless this is a one off or few off circuit and you do not want to have
trouble at the customer's site.

*>One could conceive of a relatively complex circuit that met essentially
*>every need but it would be liable to be overkill in almost every case. I
*>assume you are referring to your clamp cct attached as a PDF to an August
*>email. This has 4 transistors which puts it outside the class of what was
*>desired and still needs extra components for the input resistor and default
*>O/C bias. Which is not to say it is a bad cct - just still not the universal
*>solution.

That's true, the circuit as drawn requires 4 common transistors and two
resistors (one to set the bias and one in series with the input). I have
developed that circuit specifically because of trouble encountered with
'simple' external clamps on PICs with AD inputs in use. It works
exteremely well especially when set for low bias (10-20uA) using common
transistors (2N2222 BC547 etc). So I claim it is a universal solution that
is cheaper than the next best solution (which uses two OA65 or shottky
diodes - expensive - and doe have the potential to pull the supply up - my
circuit does not do that).

*>Maybe we need a FAQuette on input systems?

Definitely yes but imho without "semiexpert's" circuit examples, even if
they are tried. I have used and use the 'just a resistor' input limiting
but only for non-product devices, like RS232 during debugging and other
such applications. It would never occur to me to let such a device loose
into someone else's hands, with the PIC operating clearly outside the
manufacturer's specs. It would be an accident waiting to happen imho.

Peter

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2002\10\18@230000 by Jim

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Mike, have you considered simply using something
from the MC1489, MC14C89, SN75189, SN75C189,
DS1489, and DS14C89 family RS232 line receiver
family?

This chip series is basically billed as a:

 "ESD-Protected, Quad, Low-Power RS-232
  Line Receiver"

but will work to 'detect' TTL  and CMOS logic
levels *and* is protected for electro-static
discharge ...

Brief specs:

Input low:     approx. 1 V
Input high:    approx. 2 V
Working range: +- 25V
Approx Rin:    5 KOhms

http://www.maxim-ic.com.cn/Max038-157/MAX1489E.PDF

RF Jim


{Original Message removed}

2002\10\19@003407 by Mike Singer

picon face
Thank you, Jim.

I'll keep them in my mind. But from the first glance:
I need:
1. Wider input gap.
2. Higher bottom threshold.
3. Lower input impedance.
4. Ability to work from open collector source.
5. Some flexibility to cope with other requirements.

Mike.

Jim wrote:
{Quote hidden}

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2002\10\19@010733 by Jim

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Well, Mike, maybe we can adjust some
of those operational parameters.

>  1. Wider input gap.

How wide? 0 - 50V

How about using a simple resistive divider in
conjunction with the input interal resistance
of the line receiver?

Or - how about a resistive divider in front of
a CMOS 74C14 Schmitt Trigger hex inverter?

You could set your 'trigger' range AND have static
protection plus direct interface to 5V logic on
the output ...

>  2. Higher bottom threshold.

A divider network would give you this ...

>  3. Lower input impedance.

EASY to do - add external R's as req'd to lower
the Z (the 'impedance') ...

>  4. Ability to work from open collector source.

In any case - you would need a "pull-up" to some
voltage here ...

>  5. Some flexibility to cope with other requirements.

Like? (I didn't follow previous follow-ups to this
topic so I might be going over some questions that
have been asked already.)

Some of the airborne mil gear that I have worked on
in the past used actual "voltage comparators" as
the active interface device between the outside
world (where transients and RF can abound) and
the internal world of "protected" 5V TTL logic
around the bit-slice processor.

This was on the Panavia Tornado Nose-mounted RADAR
aboard the tri-country produced all-weather
bomb-delivery aircraft known as the Panavia
"Tornado" bomber designed originally for use
in the European theater as an "all weather
bomber RADAR-guided weapons delivery platform"
for use against the perceived 'Red Threat' (or
'Red Menace' <grin>) in previous decades ...

Switch inputs from the cockpit to the LRU 1 (Line
Replaceable Unit #1), the 'processor/computer'
were 'buffered' via these circuits using voltage
comparators (like the uA711, bascially op-amps
but with no particular emphasis on linearity and
logic-driving outputs) and protective capacitor
and resistor networks ...

RF Jim

{Original Message removed}

2002\10\20@012018 by Mike Singer

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part 1 4260 bytes content-type:text/plain; (decoded 7bit)

Hi, Jim.
The first message of this thread, I posted, included
the attached input circuit. Only two diodes and two
resistors. Rather wide input gap, bottom threshold
isn't too close to the ground level. The ability to
adjust parameters varying value of resistors and
type of diodes.
 What could IC add to this?
 I'm not faced to some strict specifications here. It's
me who writes them. (As the Russian sayings goes:
Chukchi man is not a reader, Chukchi man is a Writer,
odnako.)
Almost anybody on the List writes his own
specifications to small semi-hobby projects, I think.

 Regarding "Tornado" things, I'm not sure Dale gets
happy when weapon (even old) is discussed on the
List.

Thank you for your response.
Mike.

Jim wrote:
{Quote hidden}


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part 3 144 bytes
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2002\10\20@160048 by Dale Botkin

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On Sun, 20 Oct 2002, Mike Singer wrote:

>   Regarding "Tornado" things, I'm not sure Dale gets
>  happy when weapon (even old) is discussed on the
> List.

Weapons?  I'm all for 'em.  Especially old ones, and those that fly.  As
applied to the PIC-related, technical aspects of on-topic discussions, of
course.

Dale

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2002\10\22@003700 by Jim

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Mike:
  ">   What could IC add to this?"

Extra, un-needed cost?

Seriously, it depends on your market:

space-qualified, mil-spec, commercial,
automotive or consumer/mass-market where
'price-points' (what the marketing dweebs
call cost!) matter above all ...

Adding an extra 'buffer' may make your product
survive (and work!) in one environment and
not make any differnce in another ... if you
were to implement a logic (on-off) sensor on
a long wire in a building - it would pay to
'buffer' or even use opto-isolators to provide
some isolation between your remotely located
switch contacts (or sensors) and your 'electronic
brain' (processor) -

- which can get easily get 'spooked' (reset, garbled
data or even potentially destroyed) by transients
generated by motors (series wound vacuum motors!) on
the mains switching on and off or lightning strikes
(induced or secondary effects occurring as far as a
1/2 to a mile away!).

If you are building to your own specs - I can advise
that I always like to protect the CPU from the
'outside world', so, the cost of an extra buffer (I
love the 74C14 Schmitt triggers) with an extra series
1K through 10 K R (plus a .01 uF cap if I am worried
about RF suscpetability) is inconsequential ...

If intefacing to automotive applications is a
consideration I would be inclined to go with the
afore-mentioned MC14C89 series ICs (for my own
use) for the reasons previously cited:  bury the
CPU/processor at least one layer deep to gain some
ESD survivability of *program* operation and
this is aside from simple CPU survivability during
an ESD 'strike' ...

RF Jim




{Original Message removed}

2002\10\22@014252 by Russell McMahon

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> - which can get easily get 'spooked' (reset, garbled
> data or even potentially destroyed) by transients
> generated by motors (series wound vacuum motors!) on
> the mains switching on and off or lightning strikes
> (induced or secondary effects occurring as far as a
> 1/2 to a mile away!).

"Taser" extra high voltage shock devices applied to wiring/camera/PIR sensor
with aim of rendering your security system insensible :-) (Real security
industry event according to an alarm specialist on this list some while
ago). Has the beauty of travelling far and wide on your wiring and very
likely taking out not only interface it is applied to but also the
controller and other interfaced devices.


       RM

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2002\10\22@031934 by Jonathan Johnson

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Decent quality equipment should have protection built into the panel in the
form of gas arrestors and/or MOV's if it doesn't and there is a risk of this
happening then protective devices should be installed in addition to the
panel, also if the target asset is of high enough value then a direct line
monitoring system should be used. Any risk control system should be
proportional to the risk  you are trying to combat( or threat and likelihood
of occurrence).

Regards,

Jonathan

{Original Message removed}

2002\10\22@114110 by Jim

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  ''"Taser" extra high voltage shock devices applied
    to wiring/camera/PIR sensor ... (Real security
    industry event according to an alarm specialist
    on this list some while ago)''

A friend of mine, in the interest of science, has performed
experiments on these types of systems in the past using a
simple car-coil and a D cell battery or two in order to test
their vulnerability ... most of them do not fare well after
such 'test' sparks ...

  ''Has the beauty of travelling far and wide on
    your wiring ... ''

As a result, do you avoid even the simplest of protection
of your designs - or do you learn from such examples or
'tests' of your designs and using feedback learned from such
testing stumble upon the 'proper design'? (e.g. simple
spark-gaps to ground, in the same veign as color-tv
manufacturers use on their product in order to insure
it's survivial)

Unless your spec calls for survivability under these kinds
of conditions - I doubt most of us even consider the various
types of 'protection' that is necerssary in order to insure
a design can withstand this type of 'event'.

I have had experience with TTL-logic mounted on boards that
were attached to a TWT's cathode - boards which possessed a
negative potential WELL in excess of -20 KV in order to effect
electron emmission (literally, "thermionic emmission") from
said cathode for the purpose of setting up a rather healthy
electron beam down the center of said TWT amplifier tube.

Other circuitry on this board also incldued the TWT grid
drive circuitry as well as the filament regulator, assorted
BIT (Built In Test) circuitry and of course, the 5V regulator
circuitry ... arcs through the tube or in the grid pulse HV
isolation transformer or simply through the silicone oil were
catastrophic to the logic on the board - there was little
protection that could be designed-in given the limited
space available and the power involved in any particular arc
as sourced by the HV power supply ...

The lessons I *did* learn from all this was what *could be*
used to couple signals onto and off-of said board 'floating
deck' (the 'official' name of this board BTW) WELL in excess
of -20 KV ... so I don't think such design goals such as
imperviousness to 'taser attacks' are impossible - they JUST
need to be specified in the design docs, reflected on the
SOW (the statement of work) and mentioned in the final
'quote' ...

RF Jim



{Original Message removed}

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