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ATMega168 Tachometer help

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StealthRT

Member
ATMega168 Tachometer help using LM2907 chip

Hey all i am trying to make a tach that i can read the cars RPM's. I am only looking for the chip to see what the RPM's are and not display it out to the user.I've draw up how i think it should be hooked up but I'm still now 100% sure.

I am using the LM2907 chip and tapping into the tach wire from the ECU of the car. I'm looking at it as if the RPM's are 200+ then the car is started. Anything lower then the car must be off. I'm checking because i will be remote starting the car and need to know when the stop cranking it.


http://www.electro-tech-online.com/custompdfs/2009/03/LM2907.pdf (Page 8)

Any help would be great! :)

David
 
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Diver300

Well-Known Member
Most Helpful Member
I agree with Blueroomelectronics.

To measure frequency, count the number of ignition pulses in a fixed time. It's all done in the programme on the microcontroller.

Warning no.1:- At low frequencies like 200 rpm, with say 2 ignition pulses per revolution, you are only looking at 6.666 Hz. To measure to 5% accuracy you need to count 20 pulses which takes 3 seconds, so the response time will be too slow.

The alternative is to measure the time between pulses and take the inverse of that. That gives you the frequencies much faster.

Warning no. 2:- You need some way of debouncing the signals or you could get noise being detected as ignition pulses.
 

StealthRT

Member
I'd feel more comfortable in using the LM2907 chip though. Diver300, isn't that what you used in one of your projects? How did it turn out?

David
 

StealthRT

Member
I'm just going after the nicest way to get what the rpm's are. It doesn't have to be 200. I just wanted to tell if the car was started... be that the cars idle at 200,300,400,...1000. I would guess that any number above 50 would be considered started as 0 would indicate that the engine is not running.

David
 

StealthRT

Member
I saw a schema that you made up for someone that looked like what i was trying to do. I wish i saved that post... Ring any bells?

David
 

hansdals

New Member
I'm just going after the nicest way to get what the rpm's are. It doesn't have to be 200. I just wanted to tell if the car was started... be that the cars idle at 200,300,400,...1000. I would guess that any number above 50 would be considered started as 0 would indicate that the engine is not running.
David
You could count time between pulses or count pulses per given time. If you only want a trip-point, you can connect the timers reset to the pulses. Let the timer run (if it can run slow enough). If the timer overflows, your motor is revolving too slow. This method is actually the same as a watchdog circuit as i mentioned in another post.
 
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StealthRT

Member
I'm bumping this back up in hopes i can find some help in dealing with the tach using the lm2917n-8 chip.



Current status with running 203k:
Engine off: 5.01v
Cranking : 4.xxv to 0.3xv
Idle : 0.4x and less

David
 
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crutschow

Well-Known Member
Most Helpful Member
So what help do you need?
What's 203K?
Are the voltages what you expect?
Can you post a schematic?
 

Mr RB

Well-Known Member
Cos he's probably not making a tacho, maybe just a car bomb. If he was making a tacho with a microcontroller he would not need a silly "engine is running" detector. :)

Why not use an old tried and true method to see if the car is running? Like battery voltage has risen above 13.5v or test if the exhaust manifold is hot. ;)
 

StealthRT

Member
Sorry for the delay.. it never sent me a notice that someone posted something on the forum...

blueroomelectronics: I do not think the ATMEGA168 chip can convert frequency to voltage?

crutschow:
So what help do you need? I need to be able to bring the "RUN" down to 0v when the car has started. Just not its about .40 to .12 and it bounces up and down here and there.

What's 203K? That is the resistors in series. 100k + 100k + 3k. It took the place of the R5 on the schema.

Are the voltages what you expect? Sure. It's taking the constant 5v on R5 and once the engine starts running then it should go down. But like i stated, it needs to be 0v instead of hovering above it (or less than 1v)

Can you post a schematic? yes.


David
 
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Chippie

Member
Pin 5 of that chip is an Open Collector output....
Can you explain the reason why the output has to be exactly 0volts please?
 
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StealthRT

Member
Chippie: It doesn't *exactly* have to be 0v and i dont think it ever could. Just want to get as close to it as possible. In other words, lower than 1v constant.

David
 
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