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tach frequency to voltage


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KPierson 
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Posted: January 01, 2009 at 11:04 PM / IP Logged  

As Jim has alluded to there is more then one way to do this, but that is what I would do.

I would run a fuel injector signal through a 30K ohm resistor to the base of a 2n3906.  I would ground the collector and I would hold the emitter to 5vdc through a 10K ohm resistor.  Your signal out would then be between the 10K ohm resistor and the emitter of the 2n3906.  This is NOT a standard way to operate the PNP 3906 and will result in a little bit of stray voltage on the output pin when the transistor isn't on, but that stray voltage will be plenty below the digital switching threshold of that Atmel Mega uC (~1.7vdc from memory).  If you want to get fancy you can hook the 3906 up how they are typically hooked up (to +voltage instead of gound) and then drive a 2n3904 from the 3906.  I have found in my experiance that the added transistor adds no real benefit.

if you get a second give us some background info on this - I know you are using it to control the starter on the car, but what tells it to start cranking?

Kevin Pierson
redstealthrt 
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Posted: March 12, 2009 at 6:10 PM / IP Logged  
OK, revisiting this again.
I'm still thinking about using the tech wire in the ECU to get if the engine is running or not.
I have the following:
NPN 30v 100mA BC548
2n4401
2n3904
2n2222
How can i hook it up from the tach wire to the 2n then to a analog pin on the arduino?
Thanks,
David
KPierson 
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Posted: March 12, 2009 at 8:22 PM / IP Logged  

A transistor alone won't work with an analog input - as mentioned before you would need to connect it to an external interupt and write code to measure the time in between pulses.

You may be able to hook a transistor up to the tach signal and then put an RC network on the output of the transistor to convert the pulse to a voltage, but I'm not 100% sure that would work because your frequency is extremely low and your duty cycle, at least at idle, would be so low it probably wouldn't be measurable.

Kevin Pierson
redstealthrt 
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Posted: March 12, 2009 at 9:21 PM / IP Logged  
Well how about with the frequency to voltage chip?
David
KPierson 
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Posted: March 13, 2009 at 6:27 AM / IP Logged  

That's an option - you'll have to download the datasheet and calculate the values of all the support components.

Kevin Pierson
redstealthrt 
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Posted: March 13, 2009 at 3:24 PM / IP Logged  
Well that's the problem. I do not know how to hook the NTE995M chip to the tach and to the ATMEGA168 chip.
Here is the pin out of it:
tach frequency to voltage - Page 3 -- posted image.
I also found this:
http://www.xdesignlabs.com/main/images/stories/xtal_images/opto.png
But sadly, i do not know what parts were used for that..
David
KPierson 
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Posted: March 13, 2009 at 4:37 PM / IP Logged  
the data sheet has diagrams that show how to hook it up
Kevin Pierson
redstealthrt 
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Posted: March 14, 2009 at 1:27 PM / IP Logged  
I do not see anything on the data sheet on how to hook one up other than telling me the VCC = 12v...
David
KPierson 
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Joined: April 14, 2005
Location: Ohio, United States
Posted: March 14, 2009 at 2:34 PM / IP Logged  

Check out page 8 here:

http://www.national.com/ds/LM/LM2907.pdf

Kevin Pierson
redstealthrt 
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Joined: October 15, 2008
Location: Tennessee, United States
Posted: March 14, 2009 at 9:00 PM / IP Logged  
OK i took some time and drew up how i think it would be connected. Give me your feedback please tach frequency to voltage - Page 3 -- posted image.
tach frequency to voltage - Page 3 -- posted image.
David
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