Getting every detail perfectly right is often the goal in automotive restorations, and some people will go to amazing lengths to make sure the car looks and acts just like it did when it rolled off the dealer’s lot all those decades ago. That ethos can be pushed a little too far, though, especially with practical matters like knowing how much gas is left in the tank. Get that wrong and you’ll be walking.
Unwilling to risk that cruel fate with his restoration of 1978 Volkswagen Bus, [Pegork] came up with a replacement fuel gauge that looks identical to the original meter, but actually works. The gas gauges on ’60s and ’70s VWs were notoriously finicky, and when they bothered to work at all they were often wildly inaccurate. The problem was usually not with the sender unit in the tank, but the gauge in the dash, which used a bimetallic strip heated by a small coil of wire to deflect a needle. [Pegor]’s “SmoothBus” modification replaces the mechanical movement with a micro servo to move the needle. The variable voltage coming back from the fuel sender is scaled through a voltage divider and read by an analog input on an ATtiny85, which does a little algorithmic smoothing to make sure the needle doesn’t jump around too much. A really nice addition is an LED low fuel indicator, a feature that would have saved us many walks to the gas station back in our VW days. Except for the extra light, the restored gauge looks completely stock, and it works far better than the original.
Hats off to [Pregor] for this fantastic restomod. As we’ve noted before, classic VWs are perhaps the most hackable of cars, and we applaud any effort to keep these quirky cars going.
” These things don’t normally break, it’s the voltage regulator that they are connected to that often goes bad”
It would be unusual for a bimetallic strip gauge to break – although being inaccurate is a good description of how it works, since the reading literally depends on the weather.
And speaking of the voltage regulator:
” It contains a voltage regulator that can handle up to 35V”
That’s the problem. Car power is dirty with transients up to 50-90 Volts, You can get intermittent ground, induction, negative voltages, whatever – which is the reason why the original regulator breaks. The linear regulator in their circuit diagram has no smoothing capacitors, input or output, and no transient voltage suppression, no negative voltage shunting, so it will break sooner or later just as the original did. The whole power input filtering section is missing.
There is a minimum requirement of 0.33 µF for the input capacitance, which is missing in the circuit. This helps the stability of the circuit and absorbs some of the transients. There’s also the NCV-prefixed version of the same part, which is specifically made for automotive applications, although it’s not clear from the datasheet what the advantages are.
https://www.diodes.com/design/support/technical-articles/transient-voltage-suppression-in-automotive/
What you need is a suitably sized TVS diode at the input. This acts like a fast zener that dumps the excess voltage to ground and clamps reverse voltages to prevent damage. After that, a low value resistor and a reasonably large capacitor to limit the maximum current and filter out high frequency noise that might bleed past the regulator and glitch the MCU.
Thanks for the feedback Dude! I cranked this thing out over a weekend so definitely room for improvement.
I usually put the unipolar TVS after the limiting resistor, limits maximum thermal power dissipated on the TVS and doubles as reverse polarity shunt.
That reminds me, a linear regulator should have a reverse current diode from the output to the input unless otherwise specified, because any reverse voltage applied on the regulator will push current through the body diode of the regulating transistor and that might cause damage.
This looks so awesome. Too bad you would have to go through huge expenses and probably have to sacrifice your firstborn before you can legally use something like this here in europe.
Also i often thought about how to rebuild analog gauges. But it never occurred to me to just use a tiny servo. Its so obvious.
Now i feel stupid.
A lot of cars that have dials use stepper motors. There exist some pretty cool ones and there relatively cheap when bought from a vendor far far away. Automotive dial stepper like the BKA30D which is a dual motor stepper, in case you want a dual dial display or a clock or whatever.
You mean the TüV is going to find out ?… I do not believe it.
“In Europe”
Do people forget that Europe is bigger than Germany?
This would be perfectly acceptable in several places. E.g. when building your own car from scratch in Sweden you are allowed to use a bicycle speedometer if you wish…
“Do people forget that Europe is bigger than Germany?”
No. But as a German, I do often speak very vaguely about my country, too, if possible.
And that’s for a reason. If we would say “in Germany ” all time then people would think we’re having a superior complex or something.
Or worse, think the Nazis are on the loose again or something
(we have to assume that history classes in schools in foreign countries are all negative about Germany and do cover WW2 only).
We also often describe ourself as “European” in forums and so on, I think.
Or we mention “Europe” as our current location in our profile.
Especially to prevent of being hated or to receive hostile treatment.
That’s why I think we were barely outing ourselves as Germans in the first two decades of the world wide web, at all.
Except in our own, small German speaking part of the web, of course.
You know, we aren’t very proud of our country, after all.
There’s no national pride besides when there are sports events.
What we are socially attached with are rather our clubs, our home town, our federal state maybe – and Europe as a whole.
But not because we think that we’re representing Europe or any special whatsoever.
It’s rather that we want to make a meaningful contribution to Europe, want to improve it were we can.
We’re rather trying to feel as a “Weltbürger” (world citizen) rather than as Germans.
We’re trying to look forward, without forgetting the past.
Speaking under correction, of course. This is just my opinion or view on the matter.
Some of my fellow citizens may disagree. Especially those from former GDR, I can imagine.
They had different political views and maybe weren’t raised with same humility when it comes to German history. Not sure, I can’t really speak for them. 🤷♂️
That’s funny. I’m Dutch so you are my neighbor. I will never say I’m “european” as that means nothing to me. No, I’m Dutch. I don’t feel european as it’s just some continent my country happens to be on. I don’t feel connected to Hungary, England, Russia or Sweden. Those are different european countries I don’t really care or think about. It’s like Djibouti, it exists but other than remembering that it exists, I don’t really care about it. Different cultures, languages, foods, etc.
Besides the offhand jokes about WW2 people make when they meet a German, I don’t think most people associate modern Germans with WW2. Most people barely know what happened back then anyway. When I think about Germany I think about boring beer, great food, very social behavior on the roads, Saalburg, Xanten and Wuppertal.
You are of course mostly correct on your summation of world history in other countries as far as my experience goes but here is my anecdote which does not invalid it but i hope makes you feel better: My German teacher in Junior High School was obsessed with Ludwig the 2nd and conspired with our history teacher to not only make most of that years classes about him but had a field trip to Munich and environs where we did nothing but visit the castles he built. Not a single thing past 1900.
If the government didn’t care that the gauge was broken the government won’t care how it’s fixed. The UK MOT doesn’t check for a functioning fuel gauge.
You might technically be right if the broken dial had been the speed dial, but even there the only checks that are done are a basic accuracy check.
If it was the speed dial, you’d have to mind that the odometer is in the same box. It might be technically illegal to open it and there’s probably going to be some sort of anti-tamper seal on it.
Even if the MOT doesn’t care – because they just care that the number keeps going up between checks and write it down – selling the car and then the next guy finding out that it’s been opened, they can raise trouble.
If anybody cares about the odometer of a car of this vintage they’re fools. No matter what it’s either had its engine rebuilt (or wheel bearings replaced, etc.) or will need it soon enough, and that’ll be down to statistics and actually inspecting the car’s condition, but a number on the dashboard.
In this case, yes, but in general, if your speedo is broken you’d better have it replaced professionally so you get the paperwork that says you did. It saves a lot of trouble later.
The MOT don’t care if the number goes down or if the speedo is sat in a box on the passenger seat – both things that my cars have passed MOT’s with.
They just note the mileage, it’s up to future buyers of the car to question why it went down.
Likewise nothing illegal about opening a speedo. It’s illegal to reduce the mileage and sell the car pretending to be lower mileage but the MOT records make that harder now.
Here as well, the MOT doesn’t care what the meter says for mileage – but having the meter missing from the dash or not working is an automatic fail.
Honest question- if something breaks on your car, are you required to use OEM replacement parts? What happens if, like in this case, it is an older car and the parts simply aren’t available?
.
In CA there are a lot of rules like that concerning anything to do with the induction/ air intake/ fuel system. It is rolled into emissions laws so messing with anything “in front of the catalytic converter” counts as tampering with emissions equipment and you car will fail smog check. Some plastic thing that was part of the air intake system broke on my car, and there are aftermarket metal ones that won’t break again, but I could not buy or have them shipped to CA. I could argue all day about how dumb it is that making your car more efficient and less polluting is illegal for some reason, but the laws are what they are.
For the vast majority of parts, no. As you said, engine part involved in the combustion process might fall under the rule, but replacing the belts or spark plugs, (e.g. consumable parts) air mass sensor, water pump, alternator, oil cap… those generally make no difference.
The older cars are usually “grandfathered” in the law. That’s why people who build and tune up cars often use older donor vehicles. The rules apply according to the manufacturing year, not the latest regulations – otherwise it’d be impossible to pass the smog checks.
Where in Europe would you be limited to factory gauges? Never heard of that one. Not all of Europe is like that.
Alright, gather around children. It’s story time:
I got a RPM gauge for my 70’s car from AE, which uses a stepper motor. It did not work. It would just jump around like crazy and flash red, even when i used a clean 12 volt signal generator. So after i got my refund, i took it apart and it was a pretty simple circuit, and usable as some kind of other gauge, based on the unpopulated parts.
It does the gauge startup run and changes background color, but otherwise does not work correctly. I even thought it might have a wrong firmware (for an analog gauge), but feeding it different voltages to the input (which is analog signal capable) did not change anything. At certain voltage (2,5 or somewhere around there) it just went crazy. The signal circuit up until the MCU worked fine.
I did find the microcontroller datasheet, which was in chinese, but i auto-translated it. Unfortunately it’s an OTP chip, so i need to replace it with something else.
I also have thought about doing the same thing to some analog gauges, you can get the steppers from different online vendors.
Here[tm] such a car would get a “historic” number plate (with a trailing “H”), until they find out you built in parts that were not available for that car at that time; then you are back to the normal procedures and requirements. The historic-car-guys look very closely if the car is really original before granting a “H” number plate, so this and the replacement fuel gauge is mutually exclusive.
You’re massively generalising – plenty of Europe you can do stuff like this no problem, and here in the UK you can nail a car together from random parts and road-register it with a fairly reasonable safety check.
I would have seen if a Switec X27.168 could be stuffed in there, but need 4 pins to drive it.
Also, I use a long 30-60 second average for fuel level, tank slosh can be that much. On power up, capture the tank level reading and display that until the average catches up.
A low fuel light? Hmmm.
I’m working on a project like this, using 2pins (i2c) to speak with a stepper driver, driving an x27.
My second car didn’t have a fuel gauge. I just carried a can of diesel and used the odometer as a crude proxy. On the rare occasion when I got it wrong, I had a spanner to bleed the injectors.
Eventually I got around to fitting a second hand gauge. The scale was in Roentgens/hour, which made it look like the car was nuclear powered
My first car was a 1959 Beetle. It should have been junked, and my efforts at repair were dodgy, to say the least (brakes were not my strong point), but I learned a lot, survived, and sold it for what I paid for it.
No gas gauge…it had a lever on the “firewall” (actually the rear of the gas tank), above the “transmission hump” that you flipped 180 degrees to use the couple of gallons in the bottom of the tank.
I did once repair the Smiths tachometer in my friend’s MG Midget. It stopped working, a new one was mucho bucks from the dealer, so we took it out, disassembled it, and found a PCB with nothing but a TI chip. I tried to order the chip from TI, but it turned out to be a custom for Smiths, so no luck. I persisted, called the TI sales rep’s office (my company bought a lot of parts from TI) and the person who answered the phone checked his desk drawer, found a chip and mailed it to me! This would have been 1980 or so. I saw him a few weeks ago, he still has the car, still drives it, and the tach still works!
I was pretty sure I’ve been in a beetle that had that, thanks for reminding me. You just drive it till it starts to quit, then you flip the switch and have a while longer to go before you are hitch hiking. When we were all starting to drive, beetles were still pretty common as the somewhat-junk cars that your family had kicking around and that a lot of kids got them as hand me downs or whatever. Well, junk cars that your average joe could actually repair themselves and keep them going- kind of! Now they are legit collectors vehicles. ha. Oh and there was a very practical book called something like How To Keep Your Beetle Alive and Happy.. it has been decades but it was as amusing as it was practical.
That era of cars had a lot of things to repair even if nothing was broken per se. Valve gap adjustment every 5,000 miles…
Every 1000 on my old motorcycle 😓
https://www.amazon.com/Keep-Volkswagen-Alive-Step-Step/dp/1566913101
Still published! Mine had a b&w cover, and was my go-to reference. I think I gave it to the new owner when I sold the car.
The original “…for complete idiots” book.
Oh, and there were a bunch of hacks…and warnings about design deficiencies (of which there were many). The old Beetle also served as a platform for a number of kit cars, notably the dune buggy. Leaving the engine out in the open was probably a benefit cooling-wise. Another hack was CDI and fuel injection. CDI systems were popular, because nobody liked adjusting the points.
I never got to that stage…I was just happy to get mine running (third engine’s the charm!) and (barely) legally on the road.
So what happened to just putting small rocks in the fuel tank?
If the rock noise started getting noticeable as the fuel sloshed about, it was time for fuel.
I think every farm truck that I ever rode in (tank inside cab, behind the seat) had a couple of stones or hex nuts in them. Almost any maneuver that sloshed the fuel (when low in the tank) made a racket.
At an angular crossing of a ditch or turning a corner, driving in or out of parking lots with sloped curbs etc, gave a slosh to the fuel and the rock made a noise as it tumbled about in the shallow fuel.
Definitely a hack! never heard of doing that but it makes a lot of sense.
thanks
And makes a lot of the senses.
P.S. I should mention the the stones need to be basically rounded, like a tumbled rock from a creek or river. Flat side pointy (crushed gravel) or thin pieces can tend to get in a seam and might not tumble very well. Been many years, but seems like the stones were something in the size of large marbles to around the size of tree nuts. Needs to be heavy enough to be heard, but not so big that it tends to stay put.
And the fuel tank needs to be in a location that you can hear the noise. A rear mount tank under the trunk is a bit isolated acoustically, so not ideal.
That is ingenious.
Personally it’s against my preference, I would have my car be perfectly silent if I could manage that.
Ingenious for sure. To answer the question:
On modern cars: they’re too well insulated for you to hear the rock.
On my old Land Rover: You wouldn’t hear the rock over all the other noises
Also, if you have an in-tank pump the rocks might damage it.
I don’t think I’m alone in asking this but…
Where’s the log on the restoration. I would love to see even a 1978 Bus getting restored!
Now if they could just do something about VW’s “engineers” having run both huge mechanical stresses and high currents through terminals soldered to circuit boards such that they violently burned out at random…in six lanes of Bay Area traffic…in the rain…
Excellent job, really professional!
As someone who was a automotive tech for 3 years, I have to stress that “Dude”s comment above is really important in that the automobile electrical system will kill your design; it’s only a matter of when, not if.
It’s the 90volt transients from the alternator that do it.
Input power conditioning must be used. The common method is a 100 ohm 5 watt resistor in series with the input and a fuse. This connects to a 24V Zener diode of 1W rating.
The main failure mode: the Zener shorts out and blows the fuse so only these need to be replaced to repair the instrument. The fuse is usually just a thin (by design) copper track segment about 5mm long on the PCB. It should blow when shorted across 12V without burning the rest of the pcb, and is repaired by soldering a equivalent piece of thin copper wire across the burn out segment.
Without this kind of protection, when the regulator shorts out it will blow the vehicle fuse, wreck the pcb, maybe even start a fire.
I like polyfuses, they self reset. Agreed re: the rest, but aim to make future you happy with less need to tinker.
The TVS is faster than a zener, and designed to handle a larger load dump without burning out and shorting.