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'[PIC]: Low Battery Sensing'
2000\11\29@084506 by Brian Gregory

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I'm starting work on a project using a PIC powered by a battery.

The idea is that the PIC will spend most of the time in sleep mode so the
batteries will last a nice long time - at least a year would be our target.

The PIC will probably be something like a PIC16C711.

We need some way of providing an alarm when the battery is low. i.e. a
voltage sensor that doesn't itself use much power.

Are there any chips around to do this?

Anyone got any ideas about how I could do this?

Brian Gregory.
spam_OUTbriangTakeThisOuTspamcix.co.uk

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2000\11\29@090732 by Simon Nield

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if the pic you are using has an adc you could use that.

regards,
Simon

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2000\11\29@090736 by stouchton

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Try a low current voltage detector from Seiko or Telcom... should draw 2uA
at most!

-----Original Message-----
From: pic microcontroller discussion list
[.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU]On Behalf Of Brian Gregory
Sent: Wednesday, November 29, 2000 8:31 AM
To: PICLISTspamKILLspamMITVMA.MIT.EDU
Subject: [PIC]: Low Battery Sensing


I'm starting work on a project using a PIC powered by a battery.

The idea is that the PIC will spend most of the time in sleep mode so the
batteries will last a nice long time - at least a year would be our target.

The PIC will probably be something like a PIC16C711.

We need some way of providing an alarm when the battery is low. i.e. a
voltage sensor that doesn't itself use much power.

Are there any chips around to do this?

Anyone got any ideas about how I could do this?

Brian Gregory.
.....briangKILLspamspam.....cix.co.uk

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2000\11\29@093204 by Michael Rigby-Jones

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You'd still need a micropower reference though, you couldn't use Vcc as a
reference!

I mentioned on the list a while back that TI have a new range of ultra low
power uP supervisors, 220nA typical power consumption, maybe possible to use
one as a low battery detector?  Part number is TPS383x (x being the voltage
variants...)
Regards
Mike


> {Original Message removed}

2000\11\29@094523 by Eisermann, Phil [Ridg/CO]
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       I did this with discrete devices on a recent product. I wanted a
battery sensing circuit that could be turned off when not required. I used a
FET to turn a voltage divider on/off. The voltage of the divider was read
via an ADC. It was a 14.4V battery, so I used two transistors to control the
FET via the micro. Depending on your application, you could use just a
logic-level FET. The only current the circuit draws in the off state is
leakage current. In my application, this was in the nano-ampere range IIRC.


{Original Message removed}

2000\11\29@094935 by Simon Nield

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mike:
>You'd still need a micropower reference though, you couldn't use Vcc as a
>reference!

the reference need not be micropower... if you are not too bothered with the absolute accuracy then
a standard diode & resistor in series may do the trick - you do need another pin to switch power to
the cheapo reference, but it does have the advantage of not needing any fancy components.

connect a resistor to an output pin.
connect the other end of that resistor to one of the analog inputs.
connect a diode from the analog input to ground (anode to input)

pull the output pin high to enable the reference.
if the analog reading > some number then the power rail is getting low.

an led might make a good choice of diode, and you might just have one hooked up exactly how you
would like already, in which case it's only cost you the analog input pin...


Regards,
Simon

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2000\11\29@094939 by Brian Gregory

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Simon Nield <EraseMEsimon.nieldspam_OUTspamTakeThisOuTQUANTEL.COM> wrote:
> if the pic you are using has an adc you could use that.

I would need some kind of voltage reference or some resistors to divide the
battery voltage if I used the supply (from an LDO regulator I guess) as the
reference. That's run the battery down.

Brian Gregory.
briangspamspam_OUTcix.co.uk

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2000\11\29@101306 by David Kott

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>         I did this with discrete devices on a recent product. I wanted a
> battery sensing circuit that could be turned off when not required. I used
a
> FET to turn a voltage divider on/off. The voltage of the divider was read
> via an ADC. It was a 14.4V battery, so I used two transistors to control
the
> FET via the micro. Depending on your application, you could use just a
> logic-level FET. The only current the circuit draws in the off state is
> leakage current. In my application, this was in the nano-ampere range
IIRC.
>

Dammit, this is one of those really good, simple ideas.  As I look at a
recent design, I see that I just decided to run a 900Kish voltage divider
across a battery's terminals to do low battery detection via a comparator
(LT1304 et. al.).  I could have saved the 35 uA or so by placing a $.35 FET
in series with it, and selectively biased it when the system was running.

Wish I had thought of this.

-d

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2000\11\29@101317 by Michael Rigby-Jones

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{Quote hidden}

Good idea.  Maybe you could even use the classic single slope software ADC.
An R-C network from Vcc to 0v, with the midpoint going to a pin.  Ground the
pin for a couple of ms, tristate the pin and count how long it takes for the
pin to toggle.  Although, the switching point of the port pin is related to
supply voltage, it's definately not linear* so this may be workable.  Once
the cap is fully charged the only current will be cap leakage and the input
current of the port pin.

Regards

Mike

* just been playing with an AN512 circuit, and tried it without a reference
resistor.  The resulting circuit was horribly sensitive to supply changes.

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2000\11\29@110102 by Andy Jancura

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What about LTC184x, total 3.5uA, reference and 2x open collector on chip?

A.

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2000\11\30@012742 by Dan Michaels

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Brian Gregory wrote:
>Simon Nield <RemoveMEsimon.nieldTakeThisOuTspamQUANTEL.COM> wrote:
>> if the pic you are using has an adc you could use that.
>
>I would need some kind of voltage reference or some resistors to divide the
>battery voltage if I used the supply (from an LDO regulator I guess) as the
>reference. That's run the battery down.
>


Yes, it would seem that just about anything you connect to the
battery itself is likely to pull it down - except possibly a zener
with high enough voltage and hopefully low leakage [??????]

Here is a variation on some of the other - diode+resistor schemes.
Assuming you aren't running straight off the battery - but are using
an LDO to power the bus - eg, LM2936-5.0 [15 uA]:


battery---+-----LM2936--+----> +5v
         |        |    |
         |       gnd   |
       zener           |
         |       +-----+---+
         |       |    Vdd  |
         +--10K--|RB7      |
         |       |         |
         +-------|RA0      |
        Vsense   |         |
                 +---------+

Assume battery = 9v, then use 5.1v zener. RB7 = 1 while in sleep
mode. Wake up --> pull RB7 = 0 --> measure voltage on RA0 -->
Vsense = Vbattery-5.1v --> accurate reading, since the A/D Vref=5v
from the LM2936.

At full charge, Vsense ~ 9-5.1v = 3.9v. At low-charge, when
Vsense ~ 0.1v, then Vbatt has dropped to 5.2v, and the LDO is about
to fall out.

If the zener has too much leakage, then assign RB7 as input
during sleep, instead of holding it as output = high.

Who knows, this thing might even work,
- Dan Michaels
Oricom Technologies
http://www.oricomtech.com
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2000\11\30@043237 by staff

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Hi Dan,

       I have a battery voltage sensor using one PIC pin
that doesn't need the ADC feature, so it can be used on
any cheaper PIC that only has digital inputs. It does
require that the PIC is run from a regulator and the battery
voltage changes but the PIC Vdd does not. It uses about 30uA
so is useful for some circuits.
It uses two resistors and one small ceramic cap.
-Roman



Dan Michaels wrote:
{Quote hidden}

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2000\11\30@120343 by Dan Michaels

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Roman Black wrote:
>Hi Dan,
>
>        I have a battery voltage sensor using one PIC pin
>that doesn't need the ADC feature, so it can be used on
>any cheaper PIC that only has digital inputs. It does
>require that the PIC is run from a regulator and the battery
>voltage changes but the PIC Vdd does not. It uses about 30uA
>so is useful for some circuits.
>It uses two resistors and one small ceramic cap.
>-Roman
>

Of course, mine is soooo much slicker, cause I do use an
A/D channel <-- choice of control freaks everywhere.

So when you gonna show your ckt, anyways, Roman? [use large
Rs, pull down pin, switch pin to input, count seconds for pin
to exceed Vin(threshold)].

- danM
============================

{Quote hidden}

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2000\11\30@124259 by Bill Westfield

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I wanted to sense battery voltage, in a circuit run directly from the
battery, by ...
1) use IO pin(s) to run voltage doubler
2) voltage doubler goes to zener on Vref

This would let a pic running from (say) a Li-Ion batter (3 to 4.1V useful
range, I'd guess) sense the battery voltage on an absolute basis, at the
equivilent of very low dropout voltages...

Does that sound reasonable?

BillW

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2000\11\30@132008 by Dan Michaels

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BillW wrote:
>I wanted to sense battery voltage, in a circuit run directly from the
>battery, by ...
>1) use IO pin(s) to run voltage doubler
>2) voltage doubler goes to zener on Vref
>
>This would let a pic running from (say) a Li-Ion batter (3 to 4.1V useful
>range, I'd guess) sense the battery voltage on an absolute basis, at the
>equivilent of very low dropout voltages...
>
>Does that sound reasonable?
>

Hi BillW, forget the voltage doubler, just use a 2.5v ref diode
run straight off the I/O pin.

- danM

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2000\11\30@142238 by Don Hyde

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I would suggest hanging the whole battery checker off a PIC I/O pin,
assuming that the PIC Vcc is directly connected to the battery.

When the PIC wakes up, make the I/O pin high, which powers the
comparator/voltage reference or whatever.  It measures its own supply
voltage, which is pretty close to PIC Vcc, which is the battery.  Once the
circuit has made its determination, switch the PIC pin either low or
tri-state.

This way you don't need to take any particular precautions about the battery
comparator being low-power, it is not on long enough to make any differenct
to battery life.

If you're really clever, maybe you can figure out a way to get the results
back via the same I/O pin (maybe the battery checker leaves its answer in a
cap hooked to the same pin or something?)

> {Original Message removed}


'[PIC]: Low Battery Sensing'
2000\12\01@013600 by staff
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Dan Michaels wrote:
{Quote hidden}

Actually Dan, your circuit is better than mine! I used a
resistor divider from battery to PIC pin to GND, with
the cap on the ground side. I pulled the pin high and
count cycles for it to get down to low level. Bottom
R is much less ohms of course.

Your idea just needs one R from battery + to the pin,
with the cap to gnd. Then pull the pin low and count for
it to get high again. With a 560k resistor and the pin
held high by software, your circuit would use much less
power too. Good way to measure a battery voltage with
just a digital pin (not ADC). Pretty cool!
-Roman

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2000\12\01@051610 by James Hillman

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>Your idea just needs one R from battery + to the pin,
>with the cap to gnd. Then pull the pin low and count for
>it to get high again. With a 560k resistor and the pin
>held high by software, your circuit would use much less
>power too. Good way to measure a battery voltage with
>just a digital pin (not ADC). Pretty cool!
>-Roman

You would probably need a small R from cap to the pin to limit the current
discharge when the I/O pin is pulled low. Nice idea.

James

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