help creating analogue temp and fuel guage

Thread Starter

Bear_2759

Joined May 23, 2008
120
Hi Gibson,

good to see you're keen. as long as you're willing to put in the time and effort you'll get this working fine. FYI I haven't actually finished mine yet, I had to put this project on hold shortly after the last post. I've had a few other things come up but am hoping to get back to it within the next couple of months

I am already part way into completely redoing the gauge cluster in my car as well, and have found some great resources here and here and here, among other places.. i'm fairly confident that i can build the circuit just fine. the fabrication will be the hardest part - making it look good.
that vwvortex link is actually where I got the diagrams and some good help from the original poster there for my Tach guage. FYI, that tach schematic is designed for the VW golf, there's a few typo's on that diagram that meant I spent allot of time getting very frustrated trying to understand why it wasn't working. ;). it can be allot simpler than what's there. also one problem I found with that circuit is heat. the 3914's get very hot, where they really shouldn't be producing much heat if working efficiently.

With making it look good, that's not quite as hard as you might think. check out the project forum on this site. it's for custom built PC's. it might not be what you're interested in, but allot of the fabrication work is very clever, and I'm getting allot of ideas from it.

what kind of car is it for? FYI the temp ckt works great, so does the PWM, haven't tested the fuel ckt yet and I'm using the tach ckt for the speedo too.

i found a guy who used a single lm3914 to make a VERY basic tach for his car, and it looks like dimming the LEDs at night will be a pretty simple affair based on his schematic located about 1/2-way down the page. my question is - will i need a copy of his "section for headlight auto-dimmer" for each individual 3914, or can i just feed all of them from just one circuit? I plan to have a total of 10 lm3914's in my new gauge cluster: 3 for tach, 3 for speedo, 2 for vacuum/boost, and 1 each for fuel level and coolant temp.
with the dimmer, I too had the same questions, but Wook's suggestion for using PWM ckt is much better, you can have all the seperate guage ckts and the + side of all LED's goes through the PWM, so the brightness of all LED's is controlled by the one PWM ckt.

One other question - when using the 3914 in bar mode, apparently you can wire it up so that the whole display will flash when a specific led comes on. This would be ideal as a shift light in the tach, or as posted earlier, a warning light for over-temp. or maybe even a warning for when you're speeding. ;) my question is, do you simply reduce the size of the capacitor in the flash circuit to make it flash faster, and if so, how much should you reduce it by to get something like a 4-6hz flash?
it is possible to make it flash, and switch from dot to bar mode. I've attached the datasheet for the lm3914. check out the example ckts. second pic page 12 and first pic page 13. the frequency at which the LED's flash could probably just be change with the cap/res combo you see in that example (pg 13.)

one last comment... my factory fuel gauge is fluid-damped, so it simply can't move quickly - most cars seem to be this way, causing it to take 2-3 minutes to move from E to F when you refuel. is there any way to "damp" or average the signal coming from the float in the tank so that when fuel is sloshing around in there, the fuel gauge isn't wildly oscillating? for anyone else who's already done this, is this even an issue?
I haven't tested the fuel guage ckt yet, and can't remember if that issue was discussed but I do remember thinking about it.:p

feel free to either keep posting here or PM me if you want to know more (and I know you will :D)
 

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Gibson99

Joined Jul 12, 2009
9
Hi Gibson,

good to see you're keen. as long as you're willing to put in the time and effort you'll get this working fine. FYI I haven't actually finished mine yet, I had to put this project on hold shortly after the last post. I've had a few other things come up but am hoping to get back to it within the next couple of months
My wife and I just had our first child about a month ago (hard to believe it's already been an entire month!) so i feel your pain.

that vwvortex link is actually where I got the diagrams and some good help from the original poster there for my Tach guage. FYI, that tach schematic is designed for the VW golf, there's a few typo's on that diagram that meant I spent allot of time getting very frustrated trying to understand why it wasn't working. ;). it can be allot simpler than what's there. also one problem I found with that circuit is heat. the 3914's get very hot, where they really shouldn't be producing much heat if working efficiently. [\quote]

i do remember thinking that something looked a bit odd in the published "simplified" diagram, both on the vortex and on his website. can't remember what made me question it at the moment, but it was something small. seemed simple enough though - pulse counter that converts freq into a voltage, and the 3914s use that. oh - do you think one single voltage regulator will be sufficient for running 10x 3914s and associated bits? i'm worried about pulling too much current through it, but again i'm not used to working with such small values.

With making it look good, that's not quite as hard as you might think. check out the project forum on this site. it's for custom built PC's. it might not be what you're interested in, but allot of the fabrication work is very clever, and I'm getting allot of ideas from it.
the trick is a steady hand, and persistence. i'm thinking that rather than gluing leds to the back of the stock gauge face, i will make my own new gauge face from tinted lexan/plexiglass. then the only issue will be trying to keep spacing consistent around the edge of the face, and maybe new lettering/labels for each section. except for the speedo - i don't really feel the need to modify my speedo/odometer, so i will put leds on the back of it just like the VWs.

what kind of car is it for? FYI the temp ckt works great, so does the PWM, haven't tested the fuel ckt yet and I'm using the tach ckt for the speedo too.
it's a 1998 Toyota Camry CE V6 m/t. I'm not adding all this instrumentation just for giggles... it's supercharged (among other numerous mods), and i tend to drive it like a supercharged car ought to be driven. ;) The best part is, it's a boring beige 4-door sedan. it looks almost completely stock on the outside except for a very conservative TRD body kit and a large muffler... and 15" hubcaps. total sleeper. :D

with the dimmer, I too had the same questions, but Wook's suggestion for using PWM ckt is much better, you can have all the seperate guage ckts and the + side of all LED's goes through the PWM, so the brightness of all LED's is controlled by the one PWM ckt.
the factory dash lighting on my MX5 also uses a pwm from what i remember reading in the factory service manual. maybe that's why that markdepot link is able to keep the dimmer circuit so simple. well, besides the fact that he's only adding one 3914, not 10+

it is possible to make it flash, and switch from dot to bar mode. I've attached the datasheet for the lm3914. check out the example ckts. second pic page 12 and first pic page 13. the frequency at which the LED's flash could probably just be change with the cap/res combo you see in that example (pg 13.)
yeah, i'd already looked at the datasheet (as well as your prior posts) and learned that it can flash. that's how i knew about the cap i mentioned - their examples.

I haven't tested the fuel guage ckt yet, and can't remember if that issue was discussed but I do remember thinking about it.:p
only one way to find out, eh?

feel free to either keep posting here or PM me if you want to know more (and I know you will :D)
Oh, I"m sure i'll be around. btw - here's what i have to work with. i'm currently hacking up a cluster from a wreck so that i still have something to drive around with - this car is my daily driver! i'm taking the gauges down off my A-pillar and mounting 2 of them into the stock cluster. I've thought about replacing all 3 of my additional gauges with 3914's, but i don't really feel like trying to recreate fuel pressure or exhaust gas temp - these are too critical for me to experiment with, so i'll just continue using the existing instrumentation, just in a new location. vac/boost isn't nearly so critical, so i can make that from LEDs. the right side is actually already put together for real - the left (as you can probably tell) is a digital manipulation. What do you think so far?

 

Thread Starter

Bear_2759

Joined May 23, 2008
120
ha, well my wife and I had our first in july last year, and now we've got the second due late august. On top of that we're moving some time over the next month or two...

i do remember thinking that something looked a bit odd in the published "simplified" diagram, both on the vortex and on his website. can't remember what made me question it at the moment, but it was something small. seemed simple enough though - pulse counter that converts freq into a voltage, and the 3914s use that. oh - do you think one single voltage regulator will be sufficient for running 10x 3914s and associated bits? i'm worried about pulling too much current through it, but again i'm not used to working with such small values.
if I remember correctly the problem with the diagram was that the + and - labels were on the wrong sides... can't be too sure without looking at it though.

with my ckt I'm going to have 1x regulator per ckt, and the PWM for all the LED's. the LED's are running on a seperate power source to the ckt that controls the led's. with using PWM there is only 1 component that you need to be concerned about when it comes to how much current is going through it and that's the MOSFET. the MOSFET I'm using from memory can handle up to 5.3A and the LED's that I bought, are 20mA each (max). which adds up to just over 250 LED's from the PWM I've got. all you need to do for more current is swap that MOSFET for one that has a higher current rating.

best bet would be to try and choose the LED's you're going to be using first, get a copy of the datasheet, then use that to calculate what kind of current they're going to be using all up to then work out what MOSFET you need.

sound's like a fun car to drive... I was thinking about putting a blower on my charger but decided to keep it N/A mainly because of cost.

I think some time I need to sit down and breadboard a simple ckt and experiment with the dot\bar switch and the flashing... if you don't have one already, get a breadboard... it save's wasting allot of time if you make simple mistakes and really easy tot test stuff.

not a bad idea having the smaller guages inside the larger ones should look pretty good when it's done...

ok, so where do you want to start. I think I'm going to use this as an opportunity to refine some of my ckts;).

FYI, when I'm completely done with mine, it'll be a lasercut brushed alluminium panel with tinted perspex behind so I can do the guages in almost any shape/design I want, then led behind that. LED's for all guages and 1xsmall LCD for speed and odometer. I'd leave out the LCD but here in Aus it's Illegal to have a car on the road without an odometer.

Edit: Just thought, your fuel tank should have baffles to stop the fuel sloshing around, otherwise if you go round a corner fast enough in the right direction the fuel would move away from the pump and your engine would cut out. the baffles in the tank should hopefully be enough to stop wild fluctuations in the guage.
 
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Gibson99

Joined Jul 12, 2009
9
my main concern with the fuel gauge was the fact that when i refuel from completely empty, my factory gauge takes about 2-3 minutes to move from E to F. That and when i physically move the needle with my finger, I feel a lot more physical resistance compared to the other needles - temp, speedo and tach. Those need the ability to move quickly, so fluid damping isn't desirable there.
 

Thread Starter

Bear_2759

Joined May 23, 2008
120
totally understand, let me do some tests on the design I've got and I'll let you know. if it is a problem, I'd like to work out how to "dampen" the display so it's not wildly fluctuating as you hit some corners... I'll let you know how I go and if some changes to the ckt need to be made, then SgtWookie would be the one to help us... I think...;)
 

Gibson99

Joined Jul 12, 2009
9
gwidno:

glad you found us here! I haven't had time to work on this project at all since August. It's amazing how much having a new baby will consume all your spare time!

That said, I know lots of companies make LED fuel gauges for motorcycles so you don't have to fabricate anything. In fact, when I was using google to search for threads like this one, i found loads of them for sale, anywhere from $50 on up. not all of them were for harley type bikes either - many were for sport bikes. i suggest letting them do the hard work for you and just buy one premade if you can afford it.

edit: i actually did another google search looking for those motorcycle gauges again and found this:
http://www.mez.co.uk/ms13.html - go about 3/4 of the way down and look for "2/1/07 Self Calibrated Digital LED fuel gauge"

the title is a little misleading, since he's the one who calibrated it, but it seems like it'd do the job and seems simple enough. he also expressed concern with "needle jumping" but says it hasn't been a problem.

since his fuel tank isn't linear when being emptied (most aren't), he uses individually adjustable comparators to accomplish it. looks like it wouldn't be too hard to add 2 more to get slightly better resolution, so that the number of segments would match our 3914 bits.
 
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