Conditioning 12v spark signal

[Bryang]

Hi all
I have 2x Historic bikes we are racing in a week’s time, neither of which have tacho’s that are worth watching! All I want are 4-5 LED’s for approaching gear change points. I’m comfortable doing the programming side, using “count”, etc is plenty accurate enough, and I'm OK with the output side of the electronics too, and having searched through the forums there is plenty to re-check my coding anyway, but where I would like some help is if someone can throw a quick schematic up somewhere so I can condition/filter the pulses.

Hippy mentioned in one of the threads to use a flop-flop circuit to capture the rise in the voltage on the 12v side of the coil – sounds like this is the way I probably have to go, and I assumed I would be using an optocoupler to make sure I don’t blast my PIC out of the water, but just how/what does such an input signal conditioning circuit look like?

FYI, if interested, the bikes are…1972 Suzuki GT750 – 2-stroke 3-cyl, with a coil per cylinder run off of points, and a 1980 Suzuki GSX1100 4-stroke 4-cyl, with hall effect ignition feeding via an amplifier to 2x coils, so I thought the easiest way of getting the timing signal was from the +ve feed to the coils, so the hardware would be the same, just need to change the counts

Any help would be appreciated.
Bryan.

 

[RexLan]

Bryan I don't think the Count function will work because it is not accurate for what you want to do. There are not enough counts to give any sort of decent resolution.

I used the Pulsin function and, as Hippy said, if you want it right you should use the Flip-flop to get the proper rising edges. Otherwise you result will have a factor of two error which is an easy fix with math.

Here is a good circuit and the code for a 4 cylinder. The math is quite difficult I might add and not a snap with the Picaxe limitations. Your $2 caculator is much more powereful in this application. I have another circuit just using diode clamps and it too works fine for the input. I would look for the signal that fires the coil. Try the circuit but not with the Picaxe connected and look at it on the scope. 5v is MAX input to the Picaxe.

Can you put up a scope picture of the pulse you want to use for your signal to the Picaxe?

http://www.tach.rex-deb.net