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Topic: Arduino and tachometer from 12V car battery gen (Read 12859 times) previous topic - next topic


wich is the safest method to read the data

sry but i am not an engineer so my thoughts might be wrong

i also have some pcs of the above

tip35n NPN 25A
s8050 NPN 1.5A
irfz44n MOSFET 25A

thanks a lot


i made them yesterday

the fisrt

i thinh i should decrease the amps with the 3 parallel resistors

and the three resistors in series should decrease the voltage

the second one gives an HIGH 5V output when the input i at 12V
but i dont know if the LM7805 is working so fast

in the 3rd sircuit the though is to drive the transistor with the pulse

the output is on 5V from the voltage regulator and the amps are decreaced with the 2kOhm


the code is ready already


the rev burner gives an output the same as the cars singal
because it drives real tachos
except it is running on 5V

i am not sure which pin on the car is the ouput
i will test them all


For a petrol engine it is common to use the negative terminal on the coil as a tach input signal. This signal (decaying sinus) peeks at +/- 200V for every ignition pulse (2 times per RPM for a 4 cylinder 4 stroke engine, 3 for a 6 cylinder etc.). For this type of input, a single rectifier diode plus a peek detector (could be a simple voltage divider) wired to a digital input pin works well.

For a diesel engine it is common to use the W output from the alternator as a tach sensor. This is a pulse train at approximate alternator output voltage (e.g. in the 13.7V to 14.4V range against Gnd).  The pulse rate is proportinal to engine RPM, but typically not in a 1:1 ratio so calibration is needed. I would wire this input directly to an Arduino digital pin through a large series resistor (e.g. 100k) and then count pulses using interrupts. You would need another tach to figure out the calibration constant needed for your RPM calculation unless you can get documentatioin on the alternator RPM to engine RPM ratio from the car manufacturer.


i have an audi 80 model of 1980 for testing

does the alternator has W wire

all the alternators have the W output?


Jun 05, 2010, 09:01 am Last Edit: Jun 05, 2010, 09:02 am by mrmeval Reason: 1
An opto-isolator will allow you to do safe level conversion and will protect your Arduino from the nastiness of an automotive environment.

You can make one using an IR LED and a photo-transitor to do this for you. Figuring +15V a 2.1v 20ma  LED you'd need a 680ohm resistor. Use the calculator here http://led.linear1.org/1led.wiz

or use R = (VS - VL) / I  I.E. R=voltage source-voltage led divided by current  (15-2.1)/0.02 = 645 and pick the next higher standard value.

Then hook up a the photo-transistor, this gives a digital output with a high of +5 volts

Once you get it working you can put them together with heat shrink or tape. This provides pretty good isolation.

You can buy these but that's no fun:)

This is a tad expensive but it can drive a signal farther due to the external transistors.

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