Post by thread:[EE:] Voltage measurement

Hi All, I need to measure the voltage present at a car battery - but this is simpler (I think/hope) than a full-blown voltmeter type thing. Basically I care about only three states: A total lack of power (e.g., V-no_battery = 0V), the engine is not running (V-battery = about 12V) and the engine is running (V-alternator = about 13-14V). Any way I can do this by using just two inputs on my PIC? I'd like to avoid a chip that has an a/d converter if possible. I'm not necessarily looking for someone to hand me a complete solution - a point in the right direction would be most useful. I'll learn alot if I just know what such a circuit is called - I can Google for the rest. Thanks! -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spam_OUTlistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

Herbert, I'm trying to use t5he smallest PIC possible for my application and also don't want to burn up ports needlessly. Of course, if the cost of external components required to support this makes a bigger PIC the more attractive option then that's what I'll do. -matt On Mon, 1 Dec 2003 19:22:03 -0500 Herbert Graf <mailinglistKILLspamFARCITE.NET> wrote: {Quote hidden}>> I need to measure the voltage present at a car battery - >> but this is simpler (I think/hope) than a full-blown >> voltmeter type thing. >> >> Basically I care about only three states: A total lack >>of >> power (e.g., V-no_battery = 0V), the engine is not >>running >> (V-battery = about 12V) and the engine is running >> (V-alternator = about 13-14V). >> >> Any way I can do this by using just two inputs on my >>PIC? >> I'd like to avoid a chip that has an a/d converter if >> possible. >> >> I'm not necessarily looking for someone to hand me a >> complete solution - a point in the right direction would >> be most useful. I'll learn alot if I just know what >>such >> a circuit is called - I can Google for the rest. > > Just wondering why you'd like to avoid one with >A/D? Baring that, any PIC >with a comparator on board can do that. Baring that? You >can use an external >comparator or op amp to do it. TTYL > >---------------------------------- >Herbert's PIC Stuff: >http://repatch.dyndns.org:8383/pic_stuff/ > >-- >http://www.piclist.com#nomail Going offline? Don't >AutoReply us! >email .....listservKILLspam.....mitvma.mit.edu with SET PICList DIGEST in >the body -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email EraseMElistservspam_OUTTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

> Herbert, > > I'm trying to use t5he smallest PIC possible for my > application and also don't want to burn up ports > needlessly. Of course, if the cost of external components > required to support this makes a bigger PIC the more > attractive option then that's what I'll do. Hmm, ok, fair enough, how small? The 12F629 has comparators on board and it's 8 pins? There is of course the 14pin version (can't recall the part number at the moment... 16F630? If you need more the 18pin 16F628 would be my choice. TTYL -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservspam_OUTmitvma.mit.edu with SET PICList DIGEST in the body

> I need to measure the voltage present at a car battery - > but this is simpler (I think/hope) than a full-blown > voltmeter type thing. > Any way I can do this by using just two inputs on my PIC? You could use a zener diode that pulled a port pin high when 12v was present and a slightly higher value one that pulled the 2nd pin high when say 13v was present. Distinguishing between the two with non-precision thresholds (either zener or Schmitt) could be slightly tricky. If pins used were more important than components you COULD do this with 1 PIC pin. Have a cct that clamped the pin low when at say 12v, high when at say 13+ v and float when << 12v. The PIC can then detect hi/low/float easily enough. Another approach to consider is that there will be voltages that change when the motor runs - eg alternator output, detecting their presence would give you a separate motor run indication. Also, direction of current flow will swap in some locations when alternator runs and be zero when power is off. Using a VCO (voltage to frequency converter) would allow the PIC to determine voltage from frequency. An eg 555 or similar would do for this. No voltage = no signal. This has the advantage of giving you somewhat more information than just the simple go/no go system that you propose. Odds are that this will prove useful in due course. RM -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email @spam@listservKILLspammitvma.mit.edu with SET PICList DIGEST in the body

> was present. Distinguishing between the two with non-precision > thresholds (either zener or Schmitt) could be slightly tricky And don't forget that an auto environment is not always a happy place for a PIC. There are spikes and voltage fluctuations that could give false readings if too simple an interface is used. You should consider an RC filter at least, with external Schottky diodes to protect the pins. The PIC's PSU should be well-buffered and smoothed too, to avoid resets and other unwanted glitches coming through Vcc or ground. Depending where the measurement is to be done, it might even pay to metal-shield the circuit to stop eg alternator / ignition noise or, as I've come across lately and had to fix, interference from RT's that severely and adversely affected s/w A comparator input will do what's needed -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email KILLspamlistservKILLspammitvma.mit.edu with SET PICList DIGEST in the body

> Basically I care about only three states: A total lack of > power (e.g., V-no_battery = 0V), the engine is not running > (V-battery = about 12V) and the engine is running > (V-alternator = about 13-14V). > I'm not necessarily looking for someone to hand me a > complete solution - a point in the right direction would > be most useful. I'll learn alot if I just know what such > a circuit is called - I can Google for the rest. The magic term you are looking for is "window comparator". However, are you sure this is what you want to be looking for ? Do you want to know if the engine is running or do you want to know what state the battery is in ? If you put your fingers near the fanbelt, you will learn that they are different things. You can expect the battery voltage to be high just after you turn the engine off and you can also expect the battery voltage to be low just after if first starts, especially if it was hard to start on a cold morning and has a slow idle. Just something to think about. If you power your circuit from the battery, then the battery not present is solved because your circuit won't run. If the light is of ...... Steve. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email RemoveMElistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

You could measure the battery voltage using a RC circuit. The battery voltage charges the capacitor through a resistor. Connect the capacitor positive terminal to an input of the pic, so that, based on how long it takes for the pin to go to HIGH you can approximate the battery level. To discharge the capacitor, make the pin an output in LOW logic level. You will need a diode to prevent the capacitor from charging to 12V and damaging the input pin. (CAP+) ----|>|--- (+5v) If using PIC12F629 you could use the T1G pin, so Timer1 count will be proportional to the voltage. Lucas Hartmann -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spamBeGonelistservspamBeGonemitvma.mit.edu with SET PICList DIGEST in the body

How about 1 pin and a 555 timer? Can you measure a pulsewidth? http://www.bobblick.com/techref/projects/a2d555/a2d555.html Cheers, Bob > I need to measure the voltage present at a car battery - > but this is simpler (I think/hope) than a full-blown > voltmeter type thing. > > Basically I care about only three states: A total lack of > power (e.g., V-no_battery = 0V), the engine is not running > (V-battery = about 12V) and the engine is running > (V-alternator = about 13-14V). -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email TakeThisOuTlistservEraseMEspam_OUTmitvma.mit.edu with SET PICList DIGEST in the body

Matt Redmond wrote: > Basically I care about only three states: A total lack of > power (e.g., V-no_battery = 0V), the engine is not running > (V-battery = about 12V) and the engine is running > (V-alternator = about 13-14V). > > Any way I can do this by using just two inputs on my PIC? > I'd like to avoid a chip that has an a/d converter if > possible. Why? Two resistors and a capacitor tied to an A/D input seems like the obvious and simple answer. Many PICs have a built in A/D, and there is usually at least one in a subfamily. You could do this with a couple of opamps or comparators. Since you don't need high accuracy, you could use bipolar transistors instead as comparators. Another option is use a 12F675 to look at the voltage with its A/D, then output the two digital signals you want. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestTakeThisOuTmitvma.mit.edu

Matt Redmond wrote: > Jake Anderson <grooveeeEraseME.....OPTUSHOME.COM.AU> wrote: >> use a schmit trigger buffer combined with risistor >> dividor networks IE set it up so that one IO line is >> high when voltage is say 10, one when 13 >> if both are off then there is no power ;-> > > Awesome Not really. Check the data sheet for the schmitt triggers and note that very large range where the output could flip. It will be tough to guarantee you can detect the different between 12 and 14 volts. Also, the hysterisis will be working against you, not for you as in normal schmitt trigger useage. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com -- http://www.piclist.com hint: To leave the PICList EraseMEpiclist-unsubscribe-requestmitvma.mit.edu

One more thing to watch out for. A dead Lead Acid battery is not 0V. It's 12.00V or so. Each cell is about 2.0 to 2.2 volts....6 cells and it's 13.2 for fully charged and 12.0 for empty. And I'm with the group on this one....a resister division network and a PIC with A/D is the way to go.... Tim >>> RemoveMEolin_piclistEraseMEEraseMEEMBEDINC.COM 12/02/03 06:39AM >>> Matt Redmond wrote: > Basically I care about only three states: A total lack of > power (e.g., V-no_battery = 0V), the engine is not running > (V-battery = about 12V) and the engine is running > (V-alternator = about 13-14V). > > Any way I can do this by using just two inputs on my PIC? > I'd like to avoid a chip that has an a/d converter if > possible. Why? Two resistors and a capacitor tied to an A/D input seems like the obvious and simple answer. Many PICs have a built in A/D, and there is usually at least one in a subfamily. You could do this with a couple of opamps or comparators. Since you don't need high accuracy, you could use bipolar transistors instead as comparators. Another option is use a 12F675 to look at the voltage with its A/D, then output the two digital signals you want. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestspam_OUTKILLspammitvma.mit.edu -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestTakeThisOuTspammitvma.mit.edu

Okay, thanks Tim & everyone else. I'll digest all of these responses and figure out how to do it. Tim - gotcha on the 'dead' battery. It's actually a missing battery I'm worried about - fairly certain that will be 0v :-). It sounds like the most difficult thing is going to be differentiating between engine on and engine off. Perhaps I can set up a calibration routine where the chip measures and stores the voltage readings during known engine-on and engine-off states & thus is calibrated for that particular alternator/battery. Thanks! -matt On Tue, 2 Dec 2003 11:21:29 -0500 Tim Hart <EraseMETim.HartspamspamBeGoneHAWORTH.COM> wrote: >One more thing to watch out for. A dead Lead Acid >battery is not 0V. It's 12.00V or so. Each cell is >about 2.0 to 2.2 volts....6 cells and it's 13.2 for >fully charged and 12.0 for empty. > >And I'm with the group on this one....a resister division >network and a PIC with A/D is the way to go.... > >Tim -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestKILLspammitvma.mit.edu

Detecting the engine being on should be really easy....there are lots of ways... Hall effect sensor (or even reed relay) and a Magnet on a shaft. Lights that come on. Many things come on with the Key...in the case of car. Noise is easy to detect. Temperature sensor? Air flow sensor in the intake? The exhaust could spin a fan. I'd say noise would the easiest....that's the preferred method my brain uses.... Missing Battery? You might lose it? Someone stealing them? Tell us a bit more about your project....we all love finding solutions :) Tim >>> mdredmondSTOPspamspam_OUTCHARTER.NET 12/02/03 11:18AM >>> Okay, thanks Tim & everyone else. I'll digest all of these responses and figure out how to do it. Tim - gotcha on the 'dead' battery. It's actually a missing battery I'm worried about - fairly certain that will be 0v :-). It sounds like the most difficult thing is going to be differentiating between engine on and engine off. Perhaps I can set up a calibration routine where the chip measures and stores the voltage readings during known engine-on and engine-off states & thus is calibrated for that particular alternator/battery. Thanks! -matt On Tue, 2 Dec 2003 11:21:29 -0500 Tim Hart <spamBeGoneTim.HartSTOPspamEraseMEHAWORTH.COM> wrote: >One more thing to watch out for. A dead Lead Acid >battery is not 0V. It's 12.00V or so. Each cell is >about 2.0 to 2.2 volts....6 cells and it's 13.2 for >fully charged and 12.0 for empty. > >And I'm with the group on this one....a resister division >network and a PIC with A/D is the way to go.... > >Tim -- http://www.piclist.com hint: To leave the PICList KILLspampiclist-unsubscribe-requestspamBeGonemitvma.mit.edu -- http://www.piclist.com hint: To leave the PICList EraseMEpiclist-unsubscribe-requestEraseMEmitvma.mit.edu

> It sounds like the most difficult thing is going to be > differentiating between engine on and engine off. Perhaps > I can set up a calibration routine where the chip measures > and stores the voltage readings during known engine-on and > engine-off states & thus is calibrated for that particular > alternator/battery. Highest "engine off" voltage is almost certainly going to be higher than lowest "engine on" voltage. The former at some period of running with a light electrical load and the latter soon after starting. Also state of battery over time will make the ranges vary, especially if the battery is not well maintained. There will be other points in the system other than the battery itself where the conditions will be much better defined. RM -- http://www.piclist.com hint: To leave the PICList @spam@piclist-unsubscribe-request@spam@spam_OUTmitvma.mit.edu

Engine on: a possible idea.. wrap a small coil around one of the spark cables (ignition). When the engine is on, probably (?) you can detect high voltage spikes that can sensed as a frequency by the PIC (before this, is necessary to convert these pulses to a lower voltage and to a clean wave with a simple circuit). But if the engine is "Diesel" type.. regards Marco ---------- Initial Header ----------- From : "pic microcontroller discussion list" spamBeGonePICLISTKILLspamMITVMA.MIT.EDU To : .....PICLISTspam_OUTMITVMA.MIT.EDU Cc : Date : Tue, 2 Dec 2003 12:18:18 -0500 Subject : Re: [EE:] Voltage measurement {Quote hidden}> Okay, thanks Tim & everyone else. I'll digest all of > these responses and figure out how to do it. > > Tim - gotcha on the 'dead' battery. It's actually a > missing battery I'm worried about - fairly certain that > will be 0v :-). > > It sounds like the most difficult thing is going to be > differentiating between engine on and engine off. Perhaps > I can set up a calibration routine where the chip measures > and stores the voltage readings during known engine-on and > engine-off states & thus is calibrated for that particular > alternator/battery. > > Thanks! > > -matt > > > On Tue, 2 Dec 2003 11:21:29 -0500 > Tim Hart <TakeThisOuTTim.Hart.....TakeThisOuTHAWORTH.COM> wrote: > >One more thing to watch out for. A dead Lead Acid > >battery is not 0V. It's 12.00V or so. Each cell is > >about 2.0 to 2.2 volts....6 cells and it's 13.2 for > >fully charged and 12.0 for empty. > > > >And I'm with the group on this one....a resister division > >network and a PIC with A/D is the way to go.... > > > >Tim > > -- > http://www.piclist.com hint: To leave the PICList > TakeThisOuTpiclist-unsubscribe-requestKILLspamspammitvma.mit.edu > -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

> > > > Tim - gotcha on the 'dead' battery. It's actually a > > missing battery I'm worried about - fairly certain that > > will be 0v :-). > > > > It sounds like the most difficult thing is going to be > differentiating > > between engine on and engine off. The alternator "no charge" warning lamp signal should be good enough. you'll need to condition it a bit because it'll be a mucky 12 volts. Jon -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

I got the assumption that the OP was trying to determine the three states: 1: No battery 2: Engine off 3: Engine running By only looking at a +12v line in the car, without any other sensors. I would think that 2 vs. 3 could be distinguished easily by the amount/type of noise on the line over a short period of time. Bob Ammerman RAm Systems -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads