Reading 12v Automotive engine RPM's

I did a simple read on my motorcycle, but that used 5v to the ecu; now I want to do the same on my truck; which has a "HEI" coil with a tach lead directly off the coil -- I would think this tach is 12v; I don't think I can use a simple voltage divider considering a car's "12v" system fluctuates a lot.

I would like to use the tach lead; but I have seen a tach (tinytach) that wraps a wire around the #1 spark plug wire; while not as elegant, that's a solution I can go with too.

I have done a lot of searching, but almost everything I have found, with a schematic, ends up with "It doesn't work".

Surely someone must have succeeded in this that can give me some guidance/advice?

you need an opto-coupler. That separates the voltages of the Arduino and the truck.

Simple as that? Awesome. thank you.

I think I suffer from "information overload"; there are a lot of optocouplers out there! I need something fast (? 316 reads per second) I have read a lot of data sheets; and unsure which ones would work. Is there any chance you could make a suggestion on which optocoupler would work?

I thought I would want a 'forward voltage' of 12; but then I'm not sure...


I guess it's another rpm dead-end...

I picked up several different types of optocouplers, including some that claim "high speed" (& at high prices); just gotta wait for the package & try each one until I find one that works....

Hi, can you post a copy f the circuit you are trying to use to get the signal.

A copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png or pdf?

Thanks Tom… :slight_smile: