Tachometer Signal readout

Hi all,

I realise that this topic has been discussed a number of times already on this form. However i cant find a good one that explains to me how i most build my circuit and what elements i need.

The idea is simple. I have a Tachometer signal wire that runs my revcounter in my car (1999 miata) by rapid frequent pulse signals. Now i want to read out this signal true my arduino and let the arduino calculate my engines rpm. Now i know you have to convert the signals to a voltage so the arduino can read it by using a analog input to create a programmable signal, but thats all i know so far.

Now i have read some things about using a LM2907 chip from TI. But i also read some things that say you dont need it.

Can someone point me in the right direction or give my an explanation of how to do this?

I thank you all in advance.

Ps: I am pretty new to arduino, so try to write in beginners language, thx!

basanova:
Now i know you have to convert the signals to a voltage so the arduino can read it by using a analog input to create a programmable signal, but thats all i know so far.

Unfortunately that one thing you know is wrong. The Arduino can measure the pulses directly which will generally be more accurate than converting them to analog and back to digital.

Ah well that is my mistake then. Good you let me know that. i will dive in to that. I guess il have to split the cables of the Tach pulse signal and the ground cable?

You will need a signal and a Ground. If the signal is 0V to 5V (or at least 3V-ish) then you can hook it to an input pin directly. If the voltage is higher or goes negative you will need to condition it to make it compatible with the Ardiuino input.

It would really help to have an oscilloscope to look at the pulse waveform. Do you have one or know someone who has one you can borrow?

I just implemented this in a 1984 Land Rover. It was pretty straight forward. You will have a 12 V pulse signal coming out of every pole of the alternator (i.e., 4 pulses per RPM in my case). I used a voltage divider (http://en.wikipedia.org/wiki/Voltage_divider) to drop the signal from 12 V to around 3.5 V. In my case it did not require any filtering of the signal (as John said, an oscilloscope really helps!). Signal fed right into one of the interrupt pins (see: http://arduino.cc/en/Reference/attachInterrupt). Then you simply count the number of pulses over a given time period, divide by 4 (in my case) and voila!

To John:

Yeah i think my school has one i can borrow, but i am not sure. I will look in to that after the weekend.

To Azarur:

Yeah i dont know if it works for my car that way, because the signal is comming from the ECU and not directly from the Alternator or Coil. I can maybe look into splitting the signal before it hits the ECU or atleast try to read out the signal that is comming from the ECU.

Its hard to find any documentation of how it works on my car, so its gonna be a lot trial and error to find out.