hey thanks this looks awsome! and i recon i can build it no problem, will it run on 12v?
Yes, it will run on voltages up to 15V, but I would not go higher than the resting voltage of a lead acid battery, say 12.6V.
thinking further about it, the whole thing needs to be slowed down to about half or even less 160 - 190Hz is perfect, the camshaft turns at half engine speed so the syncronising puls is only read at every second engine rev
The left 555 determines the running rate. Making C2 47nF will slow it down by a factor of 22/47= 0.46, so you will get about 180 Hz, while still preserving the 30% duty cycle. If you vary only R1 or R3, the frequency will change, but so will the duty cycle. If you want to preserve the 30% duty cycle at each new frequency, either change the capacitor, or you will have to change R1 and R3 at the same time.
what variations will this require and can we make it adjustable with a variable resistor without upsetting anything?
Unfortunately, to get a 30% duty cycle out of a 555, you need the funky second resistor/diode in the 555 timing network, which means varying two resistors simultaneously. A ganged dual-pot would work, or adding a new divide by 3 counter (just to get a digitally derived 33% duty cycle) and running the 555 at 3X the frequency would work.
lastly can i get different divider intervals by simply using different pins from the 4017 counters? again without upsetting anything, this would be handy for engines with different numbers of trigger teeth.
Yes, the first divider can be strapped to divide by 1, 2, 3, 4, ..., 10, and so can the second divider, just by moving the feed back tap that goes to the R (reset) input. This means you can get any total divide ratio which can be expressed as the product of two integers from 1 to 10. For example it the first divided by 7 and the second by 5, you would get a total divide of 35...