FAQ? Correct way to sense RPM from coil (logic levels)

[Stephen Webb]

I figured this might be a FAQ, but I couldn't find much of a FAQ
listing.  I'd be glad to look there first, if someone can point me (or
archives).

Basically I want to read the pulses from the ignition coil so I can sense
RPM.  My understanding is that the coil gets +12 from the ignition switch,
and then switched ground from the ignition control module.  I'd love to
end up with a nice TTL level square wave.  So, basically I'm inquiring as
to what the best approach is (or any viable approach, I should say).

I have seen a circuit that uses a transistor and a 555, and a variation
that uses an optocoupler instead of the transistor.  I'm guessing the 555
is being used as a low pass filter, or is somehow cleaning up the signal,
but I must confess that I don't have muc experience with them.  

Help would be appreciated.

Thanks
-Steve



----------------------------------------------------------------------------
To unsubscribe from diy_efi, send "unsubscribe diy_efi" (without the quotes)
in the body of a message (not the subject) to majordomo at lists.diy-efi.org