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Old 07-08-2007, 09:16 AM   #1
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diy mpg gauge

Update, skip ahead to a working model:
http://www.gassavers.org/showthread....32#post6473232


It seems a simple enough task, take a vehicle speed sensor "reading" and divide it by the injector duty cycle and put it on a meter. In fact my el-cheapo tach/dwell meter has both circuits in it (and a meter) and could be made to reflect vehicle speed (perhaps with an op amp on the vss) or injector duty cycle (which probably isn't much more than an inductor). Throw in some pots for calibration and an op amp to combine the signals and...

So has anyone come up with a simple mpg gauge circuit for the masses? I suppose digital would be better (i.e. keep track of every duty cycle for every vss pulse and add a "trip" type function to it), but I kind of like the "gomi"-ness of dissecting my old tach/dwell meter and making an mpg gauge out of it

Thoughts?

P.S. Scangauge is in my wifes hands and I don't have another $160, that's why I'm considering this , though something for the DIY sector would be useful.
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Old 07-08-2007, 10:06 AM   #2
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Came across a link on pg 4 & 5 has the circuit diagrams(originally posted by retrorocket) . It appears to use a sunpro tach . Add a vaccum gauge along with some fabbing skill you *might* save a few dollars .
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Old 07-08-2007, 10:32 AM   #3
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Yah, that would be an obd approach, I was thinking analog stuff (and about $5 in parts added to my extremely simplistic sunpro dwell/tach). Mebbe those days are gone?
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Old 07-08-2007, 10:43 AM   #4
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Didn't SVOboy(I think it was anyway) do a fuel gauge with a dwell tach ? I'd be willing to bet that the same thing *can* be done the way you described . You just need to take the time to get the calibrations down .
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Old 07-08-2007, 12:22 PM   #5
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The cheapest way to do it would be to...

(1) Get an Atmel processor, and programming kit from ebay. Get the flash-reprogrammable processor, so you can experiment.
(2) Find some code + circuit diagram for wiring it to a calculator screen (8-digit LCD)
(3) Connect it to the Auto power supply (using the info in that PDF). Use code to simply read the injector pulse duration, and the vehicle speed sensor reading.
(4) Have one or two buttons on it. You could make a simple user interface, so you can 'reset' the 'TANK' when you refill, and reset the 'journey' each time you make a journey. Display instant MPG on the left of the calc display, and tank/journey MPG on the right hand side!

This is the basic idea behind the SuperMID, except that a lot more work has gone into it, in particular in interfacing it to a much more complicated display.
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Old 07-08-2007, 06:43 PM   #6
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Quote:
Originally Posted by landspeed View Post
The cheapest way to do it would be to...

(1) Get an Atmel processor, and programming kit from ebay. Get the flash-reprogrammable processor, so you can experiment.
(2) Find some code + circuit diagram for wiring it to a calculator screen (8-digit LCD)
(3) Connect it to the Auto power supply (using the info in that PDF). Use code to simply read the injector pulse duration, and the vehicle speed sensor reading.
(4) Have one or two buttons on it. You could make a simple user interface, so you can 'reset' the 'TANK' when you refill, and reset the 'journey' each time you make a journey. Display instant MPG on the left of the calc display, and tank/journey MPG on the right hand side!

This is the basic idea behind the SuperMID, except that a lot more work has gone into it, in particular in interfacing it to a much more complicated display.
so how much do you think that will run??? if its less than $50 i might just have to try it...
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Old 07-08-2007, 10:17 PM   #7
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Looking at the Mnfocus linky, I'd say the design could be simplified for a laptop:

There's 1 signal line from the ODB-2 picked up. Just pass it on to a laptop's serial port and have a program scan that serial port and do the computation, logging, and display.

That way it's ALL SOFTWARE, with $2 worth of parts. May need a pull down 12V->5V resistor bridge, or at worst an op-amp to decouple it all.

Just need a guy who knows how to code with visual basic or visual C a serial port reader...

If you wanna do it digital, replace their analog display with a cheap LCD bar display.
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Old 07-08-2007, 11:24 PM   #8
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Yah, the laptop->obd route is a well traveled path. Problem is obdii is a hodgepodge of protocols, and laptops aren't exactly cheap.

Also, I'm not ready to sort out PICs and deal with programmer compatabilities,specs,etc.


However, there may still be a dirt cheap option for iso, an old blackberry 957 perhaps (or mebbe a serial based palm) and something to convert 5v<->12v. In that way it is relatively easy to program/reprogram and comes with a display,input system, and a serial port based on the windows iso stuff:
http://www.andywhittaker.com/ECU/OBD...9/Default.aspx
BUT, it ony works with iso vehicles.



Here is another laptop idea, tap into the vss and injector pulses via the line-in right and left channels on the sound card and sort it out from there. They are basicaly A/D converters.
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Old 07-09-2007, 01:13 AM   #9
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Most people have laptops nowadays, and all have serial ports.

If there's an explanation of what to monitor on the ODB-2, then a simple program can be made, which then will be ported etc.

I'll ask a friend when I have a chance if he knows free PC software to monitor ODB2... Maybe we can find a free one that does the trick or can be modded for FE analysis.

Google Examples:

http://www.er-forum.de/obd-diag-dl
But it needs:
http://www.obdpros.com

http://jdash.ludicrous-speed.com/wik....php/Main_Page
http://www.tactrix.com/

http://www.scantool.net/?mode=displa...rd&#37;20Party

A good explanation:

http://www.thinkythings.org/obdii/
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Old 07-09-2007, 04:21 AM   #10
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Originally Posted by thisisntjared View Post
so how much do you think that will run??? if its less than $50 i might just have to try it...
No idea, but, I know I could get the raw materials for $50 if I look around on ebay + other places. LCD screens seem quite expensive, but, if you find somewhere that sells simple calculator ones, it should be pretty cheap.
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