Car tachometer

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Hi,

I want to build a tachometer for my car using a AVR. I think I have no problem with the software, but I'm having trouble finding out how to get a signal to measure the revolutions.

So far I've found out that it is possible to read the ignition pulses from the coil by either connecting directly to the -ve terminal of the coil and the chassis of the car or by inductive coupling of the power wire of the coil. Is this right?

What i haven't found is detailed descriptions of what voltages and currents to expect from the approaches described.

My car is an old 3 cylinder Chevrolet sprint.

Thanks for any help you can give me.

D.A.G.A

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Maybe you can use the AC ripple from the alternator? 14.5V... need to divide it down to 2.56V and use the comparator. 3 bumps per alternator rev, and 3 alternator revs per crank rev?

Imagecraft compiler user

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How old is old? If the car is less then 20 years old, I assume it has a computer controlled engine. That computer gets a tach reading from a crank sensor, which you should be able to borrow for your tach.

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But think how cool my way is.... just plug into the cigarette lighter and it works!

Imagecraft compiler user

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dksmall wrote:
How old is old? If the car is less then 20 years old, I assume it has a computer controlled engine. That computer gets a tach reading from a crank sensor, which you should be able to borrow for your tach.

No it doesn't have a computer.

@Bob:I like your idea will look in to it, do you have any info on alternators? I don't know that much about car electrics.

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Dont need to know anything if you have a scope.... set it to ac coupling, hi gain, about 1ms per cm and stick the probe on the + terminal of the battery with the engine running. See little humps? Rev the engine. Humps occur faster? All you have to do is voltage divide it down to 2.5V so it will trigger the comparator and you write a program to count them. A cleverly chosen timer interrupt period will let you read out RPM directly.

Imagecraft compiler user

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dguerre wrote:
No it doesn't have a computer.

Maybe nothing visible to you - but it'd be a pretty unusual modern car to not have some kind of "black box" under the bonnet if for no other reason than keeping the CO2 limits in stoichiometric limits using sensors in the engine and adjusting the ignition timing.

See: http://en.wikipedia.org/wiki/Air...

Cliff

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bobgardner wrote:
Dont need to know anything if you have a scope.... set it to ac coupling, hi gain, about 1ms per cm and stick the probe on the + terminal of the battery with the engine running. See little humps? Rev the engine. Humps occur faster? All you have to do is voltage divide it down to 2.5V so it will trigger the comparator and you write a program to count them. A cleverly chosen timer interrupt period will let you read out RPM directly.

The problem with that is that I don't have a scope.

clawson wrote:
Maybe nothing visible to you - but it'd be a pretty unusual modern car to not have some kind of "black box" under the bonnet if for no other reason than keeping the CO2 limits in stoichiometric limits using sensors in the engine and adjusting the ignition timing.

See: http://en.wikipedia.org/wiki/Air...

Cliff

I'm sure there is no computer on that car

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Well! Then one of your first AVR projects should be building a scope using a graphics LCD! That's sort of a joke, BUT there are several scope programs that use the line in on the soundcard... should work if you use a cap in series from the batt + terminal! You want to see the AC ripple from the alternator.

Imagecraft compiler user

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dguerre,

Either of your approaches will work. The negative side of the coil will deliver a 0-12v nominal pulse, and an inductive pickup will be, um, whatever voltage comes out :)

Both of these will be bouncy, probably repetitive, but you can generally get away with triggering a monostable multivibrator or smoothing with C/R filtering.

You'll get three ignition pulses every two revs for a three cylinder engine.

Neil

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dguerre wrote:
I'm sure there is no computer on that car

I guess the big question is: "does it have a carburretor?"

Old cars did and this controlled the fuel-air mixture mechanically

Modern cars have fuel injectors and they are computer controlled.

Cliff

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clawson wrote:
dguerre wrote:
I'm sure there is no computer on that car

I guess the big question is: "does it have a carburretor?"

Old cars did and this controlled the fuel-air mixture mechanically

Modern cars have fuel injectors and they are computer controlled.

Cliff

Yes it does have a carburetor.

barnacle wrote:
dguerre,

Either of your approaches will work. The negative side of the coil will deliver a 0-12v nominal pulse, and an inductive pickup will be, um, whatever voltage comes out Smile

Both of these will be bouncy, probably repetitive, but you can generally get away with triggering a monostable multivibrator or smoothing with C/R filtering.

You'll get three ignition pulses every two revs for a three cylinder engine.

Neil

Ok, thank you. Should I worry about high voltage peaks in the signal?

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If you had a scope, you could see how hi the voltage was going. Catch 22

Imagecraft compiler user

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Quote:
Should I worry about high voltage peaks in the signal?
Yes, you need to take precautions. Peaks can be as high as 300V :!:

What brand/type of car are we talking about ? How old is it?

A GIF is worth a thousend words   They are called Rosa, Sylvia, Tricia, and Ulyana. You can find them https://www.linuxmint.com/

Dragon broken ? http://aplomb.nl/TechStuff/Dragon/Dragon.html for how-to-fix tips

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He mentions the car in the 1st msg... 3 cyl Chevy Sprint in Columbia... carbureted

Imagecraft compiler user

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I am getting old, Bob.

No, I am not ashamed ;)

A GIF is worth a thousend words   They are called Rosa, Sylvia, Tricia, and Ulyana. You can find them https://www.linuxmint.com/

Dragon broken ? http://aplomb.nl/TechStuff/Dragon/Dragon.html for how-to-fix tips

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Attached a schematic for safe and proper tacho-signal for AVR.

Enjoy !

Attachment(s): 

A GIF is worth a thousend words   They are called Rosa, Sylvia, Tricia, and Ulyana. You can find them https://www.linuxmint.com/

Dragon broken ? http://aplomb.nl/TechStuff/Dragon/Dragon.html for how-to-fix tips

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Thank you very much.
Now is kind of late, but will take a closer look at it tomorrow.

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Plons wrote:
Attached a schematic for safe and proper tacho-signal for AVR.

Enjoy !

Nice one, but did I miss something?
Where is KL1 in the car?

/Johan

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"KL1 is the switched side of the ignition coil"

Comment block in the lower left corner ;)

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jayjay1974 wrote:

Comment block in the lower left corner ;)

:lol: