EFI ignition pickup

[Chris Morriss]

In message <331D389A.4D96 at gil.com.au>, David Chambers
<dchamber at gil.com.au> writes
>Hi,
>I am currently building a fuel injection computer based on a 68HC11 and
>am having trouble getting a reliable pickup for RPM. I would like to
>design the circuit so that it can be triggered from any sort of ignition
>pulse. ie hall effect, optical, reluctor or good old points. I can
>handle the first three inputs with a comparator input and a little bit
>of filtering this works, but when i try to pickup off the points side of
>an ignition coil i get lots of spurious pulses. Anyone have any ideas
>for a general circuit that will work on any ignition system.
>Thanks David
Because a normal points ignition system looks like a flyback switch-mode
PSU, the voltage across them rises to about 300v when they open.  There
can also be a lot of spurious HF oscillations as various parasitic
components form lots of resonant circuits.  You need to low-pass filter
(with 400v working capacitors btw) and also clip the filtered signal
with a 15V zener diode or similar.  Do you have access to a 'scope so
that you can see the effect of various filters?  To start with I would
suggest a series resistor of about 10k and then a 15V zener (anode to
the ground side) with a 220nF capacitor across it.


The following attempt at a quick ASCII diagram will probably come out
all wrong :-(

    _______/\/\/\/\_______________________________
                         |        |      
                         |        |
                         |        |
                       __|__    -----
                       | /\     -----
                        /  \      |
                        ----      |
                         |        |
                         |        |
---------------------------------------------------- 

I doubt if you'll find an input that will cope with all transducers.
-- 
Chris Morriss