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Discussion Starter #1
Well to make a long story short, I've got a microcontroller class that I'm taking at night. I have a final project that I'm working on and I need some help with selecting a Analog to Digital Convertor (ADC) for this.

Here is the highlights of the project:<UL TYPE=SQUARE>
<LI>Using this LCD screen
<LI>An O2 sensor.
<LI>My BS2
<LI>Analog to Digital Convertor
[/list]

I'm hoping to make a portable Air/Fuel ratio gauge that I can plug into my cigarette lighter and hook up the O2 sensor wire to and read the ratio on my LCD screen.

I know there are cheaper ways to do this and there are some plans online over at www.diyefi.org, but I want/have to use my BS2 for this project.

Any thoughts or ideas from the electrical experts here? I need a ADC that has a range from 1000mV to 0mV. Any thoughts, ideas or suggestions there?

Thanks for your time and help! This should be built by mid Dec.

Joe


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Why not select a microcontroller that has an ADC in it, like a PIC12F675?

[This message has been edited by Gene McGill (edited 11-01-2002).]
 

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Discussion Starter #3
Gene,
I wish I could do that, but unfortunately the rules of the project from the professor is to use only the BS2. So I have to use an external ADC.

I wish I knew more about this stuff, but I'm going to be an ME and not a EE or CS


Joe

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Not an EE either, but I can check with some guys at work next week to see if there's anything they know of that would be easy to use.
Take a look at the datasheet for the National Semiconductor ADC8031x chips to see if one of them may work for you - they're pretty easy to find.
Datasheet: http://www.high-techgarage.com/pdf/ADC0831.pdf

Another option would be to use circuitry out of a voltmeter. Here's a link to a datasheet for an A-D converter that has built in LCD drivers
http://www.elementy.pl/katalog/element.pdf/908817.pdf

Good luck, and I'd be interested in hearing of your final solution.
--
Ed

[This message has been edited by 68Elkie (edited 11-01-2002).]
 

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Discussion Starter #5
Ed,
Thanks for the links. Looks like the ADC0831 will do the job, but I really only need something that will read from 0 to +1V and the ADC0831 goes from 0 to +5V.

Is the resolution pretty good with the ADC0831?

Sorry for all the questions, as I'm new to this stuff and I'm more at home solving 4-bar linkages using Grubler's criteria than "specing" out components for this project.
But its interesting and something different at least


Joe

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You didn't specify if you wanted a serial or parallel interface on the device. An ADC0801 is a fairly simple parallel one for a single channel (addresses like memory). It should be able to operate from 0-1V by applying the correct Vref/2. The datasheet claims that 0.5 volts as the reference would do it.

If it isn't enough for you, you can also make a op-amp amplifier that has a gain of 5 (or whatever value you choose).

In all cases, you should use some fairly low tollerance components (1%) in the analog circuit and you should also use a precision voltage reference for the ADC converter since running off the power supply can add extra error. I guess this depends on how accurate you want it to be. You can divide the reference from the pwoer supply but that's usually not a really stable voltage.

Peter
 

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Actually, you did say a Basic Stamp (BS2), right? I guess that would mean a serial one would be the easiest since I don't believe they can use data and address busses in the traditional sense.

The ADC0831 can have a reference voltage of 1VDC so it reads from 0 to 1V.

In case you hadn't thought of it, by careful choice of the reference voltage you can make the math much simpler. For example, using 1.28VDC would give 5mV per step. Using 1VDC would give 3.9mV per step. As you can see, using a reference of 1.28V could make the math much simpler than using 1V. You would divide the counts by 2 or right shift (remember that 256 counts = 1.28 volts) and then display the number directly with 2 digits after the decimal point. Well, there's likely a BCD conversion in there to get the data to the display but you get the idea.

Why not go for some big LED's so that it's easier to read? They're a little harder to drive but would make it more useful if you actually install it. The stamp probably can drive them directly. Hint, use a look-up table on the BCD number to turn on the segments.

Peter
 

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Discussion Starter #8
Peter,
Thanks again!

I want to go with an LCD as I've seen plenty of plans on the 'net to run the LED's and I want something different.

I want to be able to use this as a tuning tool and not install it permanently in the car. Just something I can plug into the cigarette lighter and install the O2 wire.

How hard is it to get a PCB made up? Are they expensive for one off PCB's?

Thanks again. I'm pretty excited about this!

Joe

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You should be able to find a place that can make a PCB for no more than $60 for 2. That usually seems to be the pricing without real quick turnaround.

As for the layout, That is more difficult but not impossible. ExpressPCB has their own layout tool and you use it for free then get the boards from them. No experience here so no recommendation. http://www.expresspcb.com/

Here is a site with a list of PCB layout tools if you're so inclined to find one somewhere
http://www.free-pcb-software.com/html_uk/uk_service_1.htm

I think the hard part is visualizing how the traces connect the parts and als getting the parts in the right places with the right clearances. If you buy all the parts first you should be able to do it by printing out the layout 1:1 and sticking the pieces into it to make sure they fit. Use a piece of foam behind the to put it together.

If you try it and have questions I can try to help you out further. I haven't done any complicated boards but I have done 5 different small boards at work.

Peter
 

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<BLOCKQUOTE>quote:</font><HR>Originally posted by 72SSAbody:
How hard is it to get a PCB made up? Are they expensive for one off PCB's?

<HR></BLOCKQUOTE>

If you would like, email the finished shematic to me and I'll draw up the PCB. I would also need each component for the caps and resistors. You can hang on to the larger modules, I'll just need their part numbers. I will then generate the "gerber" files a manuf. will need to process the boards.

-The local companies I deal with have a $750 lot charge. Setup fees alone are $100.

They do sell "Home made PCB" kits. envolves some harsh chemicals
, but get's the job done. If the ciruit isn't that complex, and I doubt it should be, just leave it on a breadboard. OR, solder it up on some perf board.

I love projects that re-invent something that you can buy for $5.99
education at it's best. One class tried to convince me to program a calculator processor (assembly) for a damn pocket calculator. "whoopty freakin doo!"


I'll try and check back, if I don't feel free to email me. A->D and vise versa is my life.



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Discussion Starter #11
<BLOCKQUOTE>quote:</font><HR>If the ciruit isn't that complex, and I doubt it should be, just leave it on a breadboard. OR, solder it up on some perf board.<HR></BLOCKQUOTE>

I'll fab it up on the breadboard first, but eventually, if everything works (I hope it does), I'll see about getting a PCB made.

<BLOCKQUOTE>quote:</font><HR>I love projects that re-invent something that you can buy for $5.99 education at it's best. One class tried to convince me to program a calculator processor (assembly) for a damn pocket calculator. "whoopty freakin doo!" <HR></BLOCKQUOTE>

Yeah, gotta love what the professor's come up with. This is at least interesting to me though, unlike my C++ professor that made us do some wacked out programs. Why the hell does a "mechanical engineer to be" need C++ anyways? I would rather have a class in MatLab than C....oh what rants I could go on


Thanks again guys for the help!

Joe

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[This message has been edited by 72SSAbody (edited 11-06-2002).]
 

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Discussion Starter #13
<BLOCKQUOTE>quote:</font><HR>Originally posted by airrj:
Some days it is nice to be just a drafter
<HR></BLOCKQUOTE>

Yep, somedays I wish I could just lay on the couch and watch tv all day long like my roommates


Joe

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Discussion Starter #15
Well just to give an update on the project...

I got it working! And it works beautifully! Just got done today helping my brother with tuning on his car (dialing in the adjustable fuel pressure regulator) and last week helped another friend out with his '30 Model A with a 302/Edelbrock RPM packaged.

Thanks again for the help!

Joe

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