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'[EE]: Using a pressure trans to turn power on to '
2002\07\03@114009 by A.J. Tufgar

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Hello all,
         Got an interesting problem.  Right now I have a battery
powered circuit that uses a pic and an adc to watch a pressure
transducer.  When a change in pressure is sensed it opens a valve.

The circuit is pretty simple however being battery powered (alkaline
1.5V) I'd like to have a standby mode so if there is no pressure change
in say 2 min. it shutsdown.  BUT, I'd like to have it able to wake-up
from a change in pressure.

My step-up reg (LTC3400) has a inverted SHDN pin so this may be one
method to control it.

Here are my two ideas so far:
Find a op-amp that will work on as little as 1.1V (if possible) and
power this off the pattery directly, then run the output to the inverted
SHDN line.  When a pressure change is sensed the logic board would turn
on and the pic would be able to hold shdn high until it wanted the
device to sleep.  I've looked for low voltage op-amp and comparators but
nothing comes close to 1.1V.  Any sources?

Second idea is to run the 3.3V reg continuously, but only run the op-amp
and sensor instead this time I'd have it control a transistor that
powered the rest of the board and again the pic would take over control
after it woke up.  But the trasistor might cause a signifigant drain
while on.

Well I'm looking for any ideas that might be better or comments on
these.  BTW my sensor is a typical wheatstone configuration.  So I need
an op-amp anyways for the ADC.

Thanks,
Aaron

P.S. Sorry the post was so wordy.  :P

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2002\07\03@120456 by Tal Dayan

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Regarding your first method, how will the pressure change be
detected by the op amp ?  You need to detect up/down deviation
from the last reading.

How about having a 1.5V timer that will turn the DC/DC and the pic on
every 2 minutes ?  You can do the reading, keep it in an EEPROM and turn it
off
again.

What is the idle current of your DC/DC (assuming the PIC is off) ?

Also, did you try to reduce the PIC current to minimum (e.g. reducing the
clock
speed to 32Khz) ?

Tal

> {Original Message removed}

2002\07\03@123855 by Brendan Moran

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> Find a op-amp that will work on as little as 1.1V (if possible) and
> power this off the pattery directly, then run the output to the inverted
> SHDN line.  When a pressure change is sensed the logic board would turn
> on and the pic would be able to hold shdn high until it wanted the
> device to sleep.  I've looked for low voltage op-amp and comparators but
> nothing comes close to 1.1V.  Any sources?

You'll need some form of wakeup interrupt when there's a pressure change.  I
can think of one way of doing this with one opamp, but it will only sense a
decrease in output voltage.  To trigger off of both rising and falling
voltages, you'd need a window comparator.

The single opamp method can esily be extended to a window comparator
technique, and as I'm drawing in ASCII, I'll leave that to you.


                               |\
Vin o--+------------------------|+\
      |                        |  \
      |   R1                   |   >---o Int
      +-/\/\/\/---+---+        |  /
                  |   |    +---|-/
                  |   >    |   |/
                  |  <  R2 |
                  |   >    |
                  |  <     |
                  |   >    |
                  |  <     |
                  |   |    |
               C ---  +----+
                 ---  |
                  |   >
                  |  <  R3
                  |   >
                  |  <
                  |   >
                  |  <
                  |   |
                  +-+-+
                    |
                  -----
                 / / /

Now, R1, with C controls how fast the opamp (in comparator mode) will adapt
to changes without triggering.  You will probably want a large value there
if you want to detect slow changes.  The input to this circuit should really
be buffered with either a unity gain follower, or a small gain stage.

R2 and R3 control how large of a change is required to trigger the opamp,
and provide some noise immunity and resistance to triggering on small
changes.  You will likely want R3 to be large with respect to R2.

As for the opamp, you will likely want one with FET inputs.

If you want to use a window comparator configuration, add a diode to the
ouput of each of the two opamps (with the anode towards the opamp), and a
load resistor off the output (after the diode) to ground.  The first opamp
will be connected the same way as the circuit I drew above, but the
additional opamp will will need its own input circuit, which will have to
have the divider on the buffered line, rather than off of the capacitor. so
that when the voltage rises sharply, the capacitor stays at the same
voltage, and the opamp triggers after the divider voltage gets above the
capacitor voltage.

Hope this helps you.

--Brendan

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2002\07\03@130620 by A.J. Tufgar

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Hmmmm, the pressure only changes for about two seconds, when there is a
change my op-amp currently outputs about 1.5V (3.3 volts is the
rail-to-rail supply).  So turning it off and on would probably not work
to well.

I run off the intrc so it should be a lower drain, but I dunno if going
to 32Khz is feasible.

Brendan, thanks for the circuit.  But my main problem is trying to find
an op-amp that works off a really low voltage i.e. 1.1V.

Thank you both for your suggestions,
Aaron

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2002\07\03@132659 by Brendan Moran
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> Brendan, thanks for the circuit.  But my main problem is trying to find
> an op-amp that works off a really low voltage i.e. 1.1V.
>
>

If size isn't too much of a problem, have you considered using a charge pump
for just the opamp?  It wouldn't burn too much power since charge pumps are
usually quite simple, but it would give you just a little power to run the
opamp on.

I'm sure linear has something for this.

--Brendan

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2002\07\03@134112 by A.J. Tufgar

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Hey I haven't really thought of that although might be too expensive.  I
use one already in the circuit (LTC3400) and I think they're 3 or 4
bucks a pop.  I guess this sort of goes back to the idea of using a
transistor to control the power to the board.  This could work.

Aaron

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2002\07\03@141842 by Peter L. Peres

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>Here are my two ideas so far:
>Find a op-amp that will work on as little as 1.1V (if possible) and
>power this off the pattery directly, then run the output to the inverted
>SHDN line.  When a pressure change is sensed the logic board would turn
>on and the pic would be able to hold shdn high until it wanted the
>device to sleep.  I've looked for low voltage op-amp and comparators but
>nothing comes close to 1.1V.  Any sources?

LM10 ?

Peter

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2002\07\03@151950 by A.J. Tufgar

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Hey thanks Peter I had tried national but I didn't see this opamp.

I think this should work out hopefully the bridge is sensitive enough
though after I decrease it's voltage supply.

One quick question about this:  How would I go about switching the 1.5V
to 3.3V on the op-amp supply? I can't just put the 3.3V across it or the
battery would probably be destroyed.  So how would I switch the 1.5V off
of the op-amp?

Thanks,
Aaron

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2002\07\04@152017 by Dwayne Reid

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At 03:19 PM 7/3/02 -0400, A.J. Tufgar wrote:
>Hey thanks Peter I had tried national but I didn't see this opamp.
>
>I think this should work out hopefully the bridge is sensitive enough
>though after I decrease it's voltage supply.
>
>One quick question about this:  How would I go about switching the 1.5V
>to 3.3V on the op-amp supply? I can't just put the 3.3V across it or the
>battery would probably be destroyed.  So how would I switch the 1.5V off
>of the op-amp?

The LM10 is a neat part - we use several thousand each year.

As far as your PSU switching is concerned - have a close look at the output
voltage of your boost convertor when the switcher is not active (in
shut-down mode).  I think that you will find that it is the input voltage
less the drop across the catch diode.  This may still be too low but it is
worth taking a look at.

dwayne

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2002\07\04@182356 by Peter L. Peres

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On Wed, 3 Jul 2002, A.J. Tufgar wrote:

>Hey thanks Peter I had tried national but I didn't see this opamp.

It is OLD. Try their cross reference or something like that. Or just type
LM10 into a web search.

>I think this should work out hopefully the bridge is sensitive enough
>though after I decrease it's voltage supply.
>
>One quick question about this:  How would I go about switching the 1.5V
>to 3.3V on the op-amp supply? I can't just put the 3.3V across it or the
>battery would probably be destroyed.  So how would I switch the 1.5V off
>of the op-amp?

I do not understand the question. I think that you want a free run diode
or fet switch.

Peter

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2002\07\04@233235 by A.J. Tufgar

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No luck Dwayne, 0V when SHDN is active.  That would be too easy.  :)

Also Peter (and anyone else that wouldn't mind lending a hand) here's
what I'm looking at now:

I'm running two LM10's off of the battery.  The first op-amp has two
purposes the first is to rereference the bridge output to ground and to
amplify the signal alittle bit.  The second LM10 is to amplify the
signal again.  The output of the second op-amp goes to the inverted
shutdown gate.  Simply put, when a pressure is sensed the inverted
shutdown gate on my boost reg goes high and the regulator switches on
while the pressure is sensed.

Here's my problem:
I want to keep this /shdn gate on so I get the pic to hold the gate
high.  The problem is at the same time I'd like to supply 3.3V now to
the transducer (to make it more sensitive).  But I can't apply the 3.3V
to the transducer directly as the batteries 1.5V is already powering it.
How can I remove the 1.5V from the transducer and reapply 3.3V?
I don't care if power is momentary shut-off.

I would like this to be lower power, as was the whole initial point.  :)

Thanks in Advance,
Aaron

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2002\07\06@034758 by Peter L. Peres

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You use a few switchover circuit, a regulator chip with standby power
capability (from Seiko/Epson f.ex.), or just two freerun diodes (shottky)
to supply 1.5V or 3.3V into your special power rail.

Peter

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