Post by thread:[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. spam_OUTbriangTakeThisOuTcix.co.uk -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

Try a low current voltage detector from Seiko or Telcom... should draw 2uA at most! -----Original Message----- From: pic microcontroller discussion list [.....PICLISTKILLspam@spam@MITVMA.MIT.EDU]On Behalf Of Brian Gregory Sent: Wednesday, November 29, 2000 8:31 AM To: PICLISTKILLspamMITVMA.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. .....briangKILLspam.....cix.co.uk -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

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 -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

> 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 -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

{Quote hidden}> -----Original Message----- > From: Simon Nield [SMTP:@spam@simon.nieldKILLspamQUANTEL.COM] > Sent: Wednesday, November 29, 2000 2:49 PM > To: KILLspamPICLISTKILLspamMITVMA.MIT.EDU > Subject: Re: [PIC]: Low Battery Sensing > > 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 > 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. -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

Brian Gregory wrote: >Simon Nield <RemoveMEsimon.nieldTakeThisOuTQUANTEL.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 ========================= -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

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}> > Brian Gregory wrote: > >Simon Nield <spamBeGonesimon.nieldspamBeGoneQUANTEL.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 > ========================= > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

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}> > >Dan Michaels wrote: >> >> Brian Gregory wrote: >> >Simon Nield <TakeThisOuTsimon.nieldEraseMEspam_OUTQUANTEL.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 >> ========================= >> >> -- >> http://www.piclist.com hint: The PICList is archived three different >> ways. See http://www.piclist.com/#archives for details. > >-- >http://www.piclist.com hint: The PICList is archived three different >ways. See http://www.piclist.com/#archives for details. > > > > -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

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 -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

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}

Dan Michaels wrote: {Quote hidden}> > 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 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 -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email RemoveMElistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

>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 -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservEraseME.....mitvma.mit.edu with SET PICList DIGEST in the body