[Diy_efi] coil (-) tach signal conditioning

[Tomas Sokorai Sch.]

Hi!

I'm trying to get a clean 5V square signal from the (-) terminal of an old 
Ford Duraspark electronic ignition for a display/datalogging circuit I'm 
building (PIC16F877A w/ hd44780 LCD, GPL'ed; PC datalogging & prog. tools 
software for Linux. I'll release the first ver. it when it's doing something 
useful :)
I tried with the tach input stage from the Megasquirt circuit, and tried the 
tips for tuning this part of the circuit, but I'm still having problems. 
If the capacitor values on the coil side are normal (like the one on the MS 
circuit), the tach signal is sometimes "doubled" or it's timing jumps too 
much. But with a big enough capacitor values to get a steady and real value 
for idle, it doesn't want to "go up" with RPM.
In the software the time between 4 pulses is 50% "lag filtered" (as described 
in the GM_ECM paper) to do the RPM calc. I'm sure the software is working OK, 
tested with a clock generator circuit (a megaexpensive one... a 555 ;), and 
always get a steady and get a very near to the theoretical RPM value for the 
calculated RPM.

Ideas for another (-) coil input? or does the Duraspark 1 have a particularily 
brain-damaged waveform?

Thanks in advance


-- 
Tomas J. Sokorai Sch.


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