Jeff Cesnik wrote:

> Once again I'm drawing upon the infinite wisdom of the PICLIST...
>
>   I'm building a simple pic-based automotive tachometer and would like
> to count pulses from the primary lead of the ignition coil and display
> the RPM on an LCD.  For peace-of-mind (and other reasons), I'd like to
> isolate the input pin of the pic with a 4N35 optoisolator.  I can handle
> everything but the connection to the coil...
>
> I have 2 questions -
>
> 1.  What is the maximum frequency that the 4N35 will reliably switch at
> (will it keep up with the pulse train from the coil)?
>
> 2.  What do I need to do at the optoisolator's input to handle the
> ignition pulses?
>
> -Thanks

 Hello,

I intend to build such a device too,
for my small yacht, that has a Sabb 8 hp Diesel engine.. no ignition by
electric means, and mechanical fuel injection.
Has anyone EXPERIENCE  with getting RPM info out of a diesel engine? Please no
untested ideas, I have plenty myself, just proven technology! Accuracy is not
the primary goal.
One of my better ideas is using the jerk of the fuel injection pipe.. anyone
tried that?
Must be verry reliable, and sea water resistant, in a dirty environment...
The engine has a flywheel with a diameter of about 50 cm (20"), that is
difficult to access from the front. The clutch is in the engine block, so the
outgoing shaft may be disconnected, so using that is not absolutely reliable.
The engine drives an AC generator through a V-belt. The start motor is a Bosch
Dynastart, attached by V-belt also.
No more V-belts can be driven.
Getting the RPM info is in my opinion the hardest part, interfacing it to the
PIC is a challenge, the rest is FUN though a challenge to me, as a novice in
PIC world!