Help me design a simple voltage meter for my car. - Page 1
Forum section: electronics / projects

author: Rodville
author_rank: Contributor
timestamp: November 05, 2013, 10:45:14 pm
content: I am not so good at the design aspect but I can solder and assemble on a semi-pro level. My car just recently had the alternator die on a trip and I decided I want a volt meter so I can see it coming next time. I don't want to add some big clunking gauge to the a-pillar or anything as I live in a so-so neighborhood and I don't want my car stolen because they think it's souped up. I want to just put a display in a blank switch panel and hide the electronics behind the dash. If anyone knows an easy way to do this I would be grateful.Some basics:1. Just a display visible2. Green LED's please (dim-able would be awesome. )3. as inexpensive as possible as I am poor at the moment.If someone can help awesome. If not I understand completely.

author: dr.diesel
author_rank: Super Contributor
timestamp: November 05, 2013, 10:48:43 pm
content: I don't recall a vehicle that didn't have either an analog gauge or an idiot light for the alternator?What make/model is it?

author: AG6QR
author_rank: Frequent Contributor
timestamp: November 05, 2013, 11:09:25 pm
content: http://www.eevblog.com/2011/09/29/eevblog-204-designing-a-li-ion-battery-gauge-with-the-lm3914/There's Dave's tutorial on how to make a battery gauge. He was doing it for lithium ion batteries, at a different voltage level, but the same general idea would be applicable to a car's battery. Display is just one ten-segment LED. You might use red, yellow, and green LEDs for extra style points.

author: retiredcaps
author_rank: Super Contributor
timestamp: November 05, 2013, 11:18:24 pm
content: I bought one of these off ebay.http://www.ebay.com/itm/310786797061So far it works fine through summer and Canadian winter. It only one digit of resolution.I leave it plugged into the cigarette lighter all the time, but if you live in a so-so neighborhood, someone might think it is something valuable?

author: dannyf
author_rank: Super Contributor
timestamp: November 05, 2013, 11:32:19 pm
content: 1: Use a serial resistor + LEDs - simple but not as "sharp" in turning on / off;2: Use a serial resistor, a voltage reference + leds: slightly more complex but sharp on/off.

author: Rodville
author_rank: Contributor
timestamp: November 06, 2013, 12:21:55 am
content: It's a 1996 Honda Accord. It has the light in the dash that comes on AFTER the car dies. I want to know what it reads on a continuing basis.

author: w2aew
author_rank: Super Contributor
timestamp: November 06, 2013, 02:42:00 am
content: Quote from: Rodville on November 06, 2013, 12:21:55 amIn my experience, alternators (and the regulators embedded in them) often don't give you any kind of early warning before they die - they just up and quit when their time is up. So, the voltage gauge probably isn't going to help you much.

author: Unknown
author_rank: Unknown
timestamp: November 06, 2013, 03:08:43 am
content: "expanded scale car voltmeter" is the magic phrase to search for.At one stage designs appeared semi-regularly in EA/ETI/SC & presumably similar magazines, and a quick google shows a few places selling kits. They're all essentially the same design (LM391*-based) and much the same as the linked Li-ion monitor of Dave's, so shouldn't be hard to come up with a circuit.Oh look,here's one now...

author: ddavidebor
author_rank: Super Contributor
timestamp: November 06, 2013, 06:14:22 am
content: There are chinese ones with just 2 wire for 6$ in ebay

author: rr100
author_rank: Frequent Contributor
timestamp: November 06, 2013, 07:26:09 am
content: Quote from: w2aew on November 06, 2013, 02:42:00 amCorrect, there are just too many things that can go wrong, circuitry, belts, etc. You can learn some things by watching a voltmeter connected to your battery (even better if it has enough counts and maybe some logging) but they are usually related to how good your battery is and how long you can leave your car before having the battery flat. But for the operation of the alternator itself I don't think you'll be able to see a failure coming.

author: PA4TIM
author_rank: Super Contributor
timestamp: November 06, 2013, 05:59:33 pm
content: dave has a funamental friday about an out of window comparator. 2 leds, a quad comparator and some resitors. Cheaper and better then this would become difficult.

author: dannyf
author_rank: Super Contributor
timestamp: November 06, 2013, 06:34:29 pm
content: QuoteA voltage reference is effectively a comparator.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: November 06, 2013, 07:08:03 pm
content: Quote from: ddavidebor on November 06, 2013, 06:14:22 amthere are 3 wire, 4 digit, for only $4.503 digit doesn't have much advantage in a car, unless you want to measure the voltage at the battery, while turning the gauge on using the accessories circuit in the car.In house, it has the advantage that you can measure down to 0 volts.

author: deth502
author_rank: Frequent Contributor
timestamp: November 06, 2013, 09:47:06 pm
content: while its true that you cannot really tell the future to know if an alternator is going to crap out or not, having the gauge can be helpful in telling you that it HAS given up the ghost. once an alternator dies, the car will still run for as long as the battery can supply enough juice to run the plugs and injectors, so knowing that your alternator died can give you a few minutes (possibly even up to an hour) of run time to get to a shop or parts store. better than finding out once youve stalled on the side of a road in the middle of nowhere.

author: rr100
author_rank: Frequent Contributor
timestamp: November 06, 2013, 10:20:29 pm
content: Quote from: deth502 on November 06, 2013, 09:47:06 pmThe dash light would do just the same.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: November 06, 2013, 10:29:06 pm
content: Quote from: dannyf on November 06, 2013, 06:34:29 pmWhat?

author: smashedProton
author_rank: Frequent Contributor
timestamp: November 06, 2013, 10:31:57 pm
content: You can buy a voltmeter to jam into the cigarette jack. If you smoke, and don't want to have a voltmeter in tge way... stop it

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: November 06, 2013, 10:51:26 pm
content: QuoteBut I use that to power my bar fridge!

author: Rodville
author_rank: Contributor
timestamp: November 06, 2013, 11:24:56 pm
content: Actually I use it to power an old android phone running Torque because the speedo died when the alt did. I haven't had a chance to pull it back out and look at it.

author: smashedProton
author_rank: Frequent Contributor
timestamp: November 06, 2013, 11:35:57 pm
content: Quote from: Rodville on November 06, 2013, 11:24:56 pmNice fix!

author: dannyf
author_rank: Super Contributor
timestamp: November 07, 2013, 01:04:37 am
content: QuoteA voltage reference is effectively a comparator.

author: deth502
author_rank: Frequent Contributor
timestamp: November 07, 2013, 01:20:12 am
content: Quote from: rr100 on November 06, 2013, 10:20:29 pmnot if its faulty.he has already replied that the dash light will not come on as long as the car is running.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: November 07, 2013, 04:13:12 am
content: QuoteI dont agree.

author: ddavidebor
author_rank: Super Contributor
timestamp: November 07, 2013, 01:22:03 pm
content: Quote from: dannyf on November 07, 2013, 01:04:37 amYeeeah sureAnd i'm a horse

author: dannyf
author_rank: Super Contributor
timestamp: November 07, 2013, 01:31:52 pm
content: QuoteThat's OK.QuoteYou are the authority on that, obviously.

author: Zero999
author_rank: Super Contributor
timestamp: November 07, 2013, 01:35:43 pm
content: How about adding a hall effect current sensor to the alternator? You could then connect the output to a volt meter to measure the charge current.

author: Excavatoree
author_rank: Frequent Contributor
timestamp: November 07, 2013, 01:37:32 pm
content: Quote from: deth502 on November 07, 2013, 01:20:12 amI don't know about the OP's car in particular, but many won't light the light when the alternator fails, if the battery is in good condition. The light will only show after the battery dies. For those cases, it's better to have a voltmeter or a simple low voltage lights - some cars have this function built in, however.Also, it's good to know when the alternator is overcharging the battery due to regulator failure. Often that condition won't light the warning light either, if you don't have a voltmeter, you'll miss that.

author: Rodville
author_rank: Contributor
timestamp: November 08, 2013, 03:18:01 am
content: The light will only tell you that the batt doesn't have enough charge in it. By that time it's too late to really do anything but pull over. With a gauge you can keep your eye on the value and if it goes high then you know the regulator has gone bad (have had that happen) if you know where the normal is for your car and it is a volt or two under it could mean that the alt is not long for this world. Every car that I have had an alt failure that had a gauge and not a light I was able to see that it was on it's way out and take the necessary actions to not leave me stranded on the side of the road like this car just did to me.

author: jwm
author_rank: Regular Contributor
timestamp: November 08, 2013, 04:04:18 am
content: Quote from: rr100 on November 06, 2013, 10:20:29 pmHah. I have had two different cars alternators die with absolutely not a peep on the panel. That's the problem with indicators that only go on when something is wrong, you have no way to know if they are working or not until after something bad has happened. A voltmeter would be much better, seeing it fluctuate as you drive turn on the A/C etc is proof positive that it is actively working.Stick a current sensor on the battery line too (https://www.sparkfun.com/products/11005) while you are at it, to justify not just getting a cheap ebay one. Plus it may be more useful, if your alternator dies your battery may still keep up the voltage for a while, but if you suddenly start seeing a huge current draw from the battery while driving you know the alternator is not doing so well and you should head for somewhere safe to break down.Modified:Doh. just realized those clamp current sensors don't work on DC. A hall effect sensor would, not sure how easy that is to do non-invasively. But what should work is one of those clamp meters on the output of the alternator before DC rectification? that would tell you at the source whether it is putting out juice.John

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: November 08, 2013, 04:23:36 am
content: Quote from: jwm on November 08, 2013, 04:04:18 amhttp://www.ebay.com.au/itm/DC-4-5-30V-100A-Red-Blue-Led-Dual-Panel-Digital-Voltmeter-Amp-Ammeter-with-Shut-/281189191513?pt=AU_B_I_Electrical_Test_Equipment&hash=item41782e8f59

author: Six_Shooter
author_rank: Regular Contributor
timestamp: November 08, 2013, 04:35:56 am
content: Quote from: AmmoJammo on November 06, 2013, 07:08:03 pmI disagree. Having the tenth of a volt displayed can be great in keeping an eye on the electrical system in a vehicle., especially for people that have aftermarket accessories, namely audio systems that puts a large demand on the charge and electrical system. Knowing that the system is closer to 14V and not 13V can really be effective in an automobile, since the system runs on a limited voltage range, effectively 12.6V to 14.4V, and knowing that the charge voltage is actually 13.6 and not 13.5, means that you know you're getting a charge and not just keeping the car on. 13.6V is the industry standard threshold that there is a charge, below that the vehicle is considered to not be charging.The only real issue is where to reference from. When I'm adding a voltmeter to a vehicle that didn't originally have one, I use 2 points or techniques for connecting it. I either tap into the main ignition lead from the ignition switch, so that the voltmeter is switched and it's measuring the voltage feeding the ignition system, especially on older vehicles where relays to run the vehcile were less common.The other way I like to connect a voltmeter is to the battery through a relay, to measure the battery (and charge) voltage, but the relay is connected to only turn the voltmeter on while the vehicle is on.So basically what I'm saying is that on a voltage range system the tenth of a volt display is as important or more important than on many other systems.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: November 08, 2013, 04:44:52 am
content: Quote from: Six_Shooter on November 08, 2013, 04:35:56 amI meant to say 3 wireedit: hence my comment:"unless you want to measure the voltage at the battery, while turning the gauge on using the accessories circuit in the car."Which is pretty much what you're describing using a relay, but without the need for a relay!Just have a look at the ebay special gauge you're buying, a lot are 3 wire, meaning you can measure the voltage at the battery, alternator, or amplifier(s) while switching the gauge on only when the accessories circuit in the car is on

author: Six_Shooter
author_rank: Regular Contributor
timestamp: November 08, 2013, 04:48:22 am
content: Quote from: AmmoJammo on November 08, 2013, 04:44:52 amWell, that changes everything, and depending on what the 3rd wire is for, I might still disagree with you. LOLIf it's 3 wire I would suspect there is a power (battery) connection, ground and a switched input, which would eliminate the need for an additional relay.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: November 08, 2013, 04:51:09 am
content: Quote from: Six_Shooter on November 08, 2013, 04:48:22 amI edited my previous postAnd you're right, it can be very helpful! but not really if all you want to do is make sure your alternator is working

author: dannyf
author_rank: Super Contributor
timestamp: November 08, 2013, 11:53:07 am
content: QuoteIt would be power connection (Vcc and ground), and the measurement point (switched or otherwise).Technically, you could get by with just two wires.

author: Jon86
author_rank: Frequent Contributor
timestamp: November 08, 2013, 12:01:37 pm
content: Quote from: dannyf on November 07, 2013, 01:04:37 amDo you know what a voltage reference is?

author: dannyf
author_rank: Super Contributor
timestamp: November 08, 2013, 12:20:25 pm
content: QuoteDo you know what a comparator is?

author: ddavidebor
author_rank: Super Contributor
timestamp: November 08, 2013, 01:25:33 pm
content: A thing with a couple analog input and a digital output that tell you which input is higher

author: dannyf
author_rank: Super Contributor
timestamp: November 08, 2013, 01:40:28 pm
content: QuoteSo an opto coupler is a comparator;A transistor is a comparator;A mcu is a comparator;and diodes are comparators too....I encourage you think out of the box.

author: ddavidebor
author_rank: Super Contributor
timestamp: November 08, 2013, 02:01:12 pm
content: CAN BE USED AS!!!!!!!I suggest you to think more normally!

author: dannyf
author_rank: Super Contributor
timestamp: November 08, 2013, 02:04:45 pm
content: If something can be used as a comparator, is it a comparator?Everything I mentioned earlier fits your definition of "comparator" but you still insist that they are not comparators.Then what really is a comparator to you?

author: G7PSK
author_rank: Super Contributor
timestamp: November 08, 2013, 04:29:59 pm
content: In my experience an ammeter is more useful than a voltmeter in a car it tells you which way the current is flowing and how much, and if you keep an eye on how much for how long so you will see if the battery is taking too much current or not enough so if I had the choice I would always opt for an ammeter rather than a voltmeter both of course is even better but very few cars have either as to most people they are meaningless but judging by some of the driving I have seen recently so is the speedo.

author: deth502
author_rank: Frequent Contributor
timestamp: November 08, 2013, 09:50:59 pm
content: Quote from: G7PSK on November 08, 2013, 04:29:59 pmam i the only one who read this long run on sentence in their head like a character from speed racer??

author: ddavidebor
author_rank: Super Contributor
timestamp: November 08, 2013, 10:03:09 pm
content: No, i had to take a deep breath too

author: chasxmd
author_rank: Contributor
timestamp: November 09, 2013, 02:42:04 am
content: I recently purchased one of these off eBay:http://www.ebay.com/itm/390568398159?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649$17 and it does current (up to 100A).I don't know why you'd want to design something yourself because its so cheap to get something that does what you want.. but I get sometimes I build stuff I could have bought for much less money. There are plenty of other recommendations as I read through some of the other comments.Good luck!

author: Six_Shooter
author_rank: Regular Contributor
timestamp: November 09, 2013, 03:16:55 am
content: Quote from: G7PSK on November 08, 2013, 04:29:59 pmAn ammeter by itself can be very misleading, especially in a car.I know far too many people that have arrived some place to have an undercharged battery, even though the ammeter showed a charge the entire time it was running. A voltmeter is much more indicative of what is happening in a charging system, if only one had to be chosen.

author: Six_Shooter
author_rank: Regular Contributor
timestamp: November 09, 2013, 03:37:48 am
content: Quote from: dannyf on November 08, 2013, 11:53:07 amThat's perfect, ignition to turn it on and off and then being able to measure at the point you want to. Voltages can vary a lot from one point in a vehicle to another.

author: dannyf
author_rank: Super Contributor
timestamp: November 09, 2013, 12:17:00 pm
content: QuoteIf the writer doesn't bother to make his/her statements readable, why bother reading it at all?

author: rr100
author_rank: Frequent Contributor
timestamp: November 10, 2013, 05:35:20 pm
content: Certainly there's no such things as overkill for us. I jump-started cars with a 60A power supply (best option yet, I still have the extension cord and nice thick wires prepared:https://www.eevblog.com/forum/chat/jump-cables-mix-up/msg209886/#msg209886) and from an electric bike battery ( I did not get to build or buy what was thinking about in this thread:https://www.eevblog.com/forum/projects/switched-current-limiter-about-10a/but I used a simple 12V 55W bulb). I also used an sd logger based on ATmega connected to a cheap serial logging multimeter (which isn't that straightforward as it seems because you need to feed power to the serial for the opt-coupler to work) and I also used a 5 digit logging multimeter worth probably more than the car for such investigations.However, I do think that more than a voltmeter is overkill unless you need something for a particular investigation. Even the voltmeter is too much if you know already that the idiot light is working as it should. Of course the voltmeter is useful to know if you don't overcharge or if there's some extreme drain over the night or weekend and so on. But normally not much can go wrong. We tend to concentrate on the simple part of the electric part but there are many more things that can go wrong with a car and let you stranded (or even crashing) immediately.

author: IdahoMan
author_rank: Regular Contributor
timestamp: March 24, 2016, 06:25:38 pm
content: Quote from: chasxmd on November 09, 2013, 02:42:04 amHello. Mind if I bump this thread?I'm putting in an auxiliary, isolated deep-cycle battery in my truck for accessory use and have to monitor itsstate-of-charge. Knowing how many amps the accessories are drawing, as well as any parasitic draw that might be present, is important too.I see a lot of cheap DPMs like the one you linked too on eBay and Amazon. I guess the question is: Are they quality enough to be accurate, and to not break?

author: TimFox
author_rank: Super Contributor
timestamp: March 24, 2016, 08:21:17 pm
content: Quote from: deth502 on November 07, 2013, 01:20:12 amI had two alternators die in a 1981 Dodge Omni. The dashboard included an ammeter, but it only indicated charging current, not discharge (despite having a zero-center movement). The indication of alternator failure (while the engine kept running on the battery) was that the radio faded out. Apparently, when the dealer replaced the first alternator, he didn't check the voltage regulator, so the second one burned out as well.

author: SeanB
author_rank: Super Contributor
timestamp: March 25, 2016, 11:54:19 am
content: Cheap cigarette lighter plug in LED voltmeter on Ebay, and mount it in the instrument panel somewhere neatly. They use a 2 wire supply, and are really cheap and reasonably accurate., plus come in multiple colours. I mounted it on my car in the centre console in an otherwise unused slot ( think it was there for keeping cassette tapes, but have not used one in years in the car, as well the deck is dead sometime over the last 5 years, did not notice when) and power it off an ignition switched feed. Works from around 4V to 24V, and easily works on 12V supplies. Was under $2 including shipping, you could not build it for that price in the small case with smoked display.

author: IdahoMan
author_rank: Regular Contributor
timestamp: April 10, 2016, 08:53:50 pm
content: *Frustrated*The empty spot on the instrument cluster where I want to put the power monitor gauge is around 1.75" in diameter, which means the digitial display needs to be around 1-1.25" in length and around .5-.75" in height. Now, I found out about these:Really neat little OLED USB volt/amp meters. Perfect display size, color, accuracy, and crispness, but the volt and amp ranges won't work. Don't know if it could easily be modded, and I think they (YZXstudio) may be discontinuing the OLED model.BUMP. Anybody know where a little, accurate OLED volt/amp meter could be found??One that can be modded/adjusted for 5-15VDC/0-500A?