Tach from spark wire...

[Grant Beattie]

Well I'm finally able to get back to my karting data acquitision system 
and so far the status is:

- Remote front panel (under construction) contains 2-line x 16 char LCD 
and 4 panel switches (as well as three jacks for switch inputs like a 
SPLIT switch).  Quite small and uses serial (SPI) interface.

- Main CPU which at this time is recording MPH @ 5/sec and SPLITs on
demand (using the above steering wheel mounted switch).  The unit supports
multiple sessions, review data on the LCD and upload the data to a PC. 

Next I want to add tach and CHT so that I can throw away my present gauge. 
Does anyone know how I can interface my little HC11 micro to count the
sparks?  Many of the kart gauges appear to have a little coil placed next
to or around the spark wire.  HAs anyone done this?  Remember this is a
go-kart and I can't get it from the existing tach ('cause I'm throwing it
away and it kindo sucks anyhow ... Horstman brand). 

I can't really get it by using a hall sensor on the engine shaft 'cause 
it's too messy and the if the chain falls off it will trash the 
electronics.  So I *think* I'm limited to getting it from the spark wire,  
but I'm open to other suggestions too.

Thanks,
GB