Gee, thanks for the patronising response - that's really useful.
I have already learned a load thus far from taking 'reference circuits', building them, and then understanding how they work.
By tried and tested I don't mean a commercial product; just something that's worked for someone else that will save me hours of tail chasing.
For anyone looking to do similar - I am going to try using an LM555 with wire coiled around the spark plug lead. In case I forget to post back, this is the most useful reference I've found is the 555 circuit in this article: http://fly.srk.fer.hr/~wgottwe/e98a034.pdf
Many years ago I built a rev counter using a miniature micro switch to drive the measurement circuit. You could perhaps try one of those strapped to an HT lead or one of the fuel injector feed wires. The current flow in the lead creates a magnetic flux that might be strong enough to operate the switch. Then use the reed switch to feed a digin pin.
And to those who say a reed switch won't stand up to that sort of switching abuse, work out how many miles you get at 2500 RPM, one pulse per 2 engine revs, 60MPH at that RPM. Belive me it's a lot and the switches are usually rated at >10million operations.
Way back in the 60sI used a similar method to directly light a small neon lamp to see if each plug was firing.
There is no need to use a 555 for this. You could use a small neon lamp and then attach a photo transistor so you had a sort of opto isolated sensor. That would be the safest option, but whether this has been documented I don't know.
Part of the fun of making stuff is to step outside the "tried and trusted" to develop another way.
Hi Mike,
Off topic, but as you mentioned using miniature neons, here's a little story.
Around 50 years ago the cheaper models of vessel echo sounder used a rotating neon light to indicate water depth. One flash at 12o'clock as the zero reference then as the lamp rotated it would be re-struck to indicate depth.
One of the problems was that the neons were fired with a sharp pulse and would eventually blacken on the side of the glass such that they couldn't be seen. One of the jobs I used to do, as a youngster, for local fishermen was to replace these neons.
Some varieties wouldn't stike successfully but I found that shining a torch light onto them, made them fire OK. Grey cells into gear and doing some research, I found that any form of photon "exciting" would do the trick - so I ended up painting a little blob of luminous paint onto the back side of the neons. In those days it was mildly radio-active !!
These types of neons were also widely used as high voltage satbilisers, their stricking voltage being around 90 volts. But that was in the days when 120 volt primary cell radio batteries were available.
Way back in the 60sI used a similar method to directly light a small neon lamp to see if each plug was firing.
Neon bulb and a photo transistor might be a place to experiment. Back in the "good old days" running Packard 440 ignition wires on your hot rod would let the neighborhood know you were on the move.
a little blob of luminous paint onto the back side of the neons.
Brilliant idea. I suspect that the partial ionization of the gas reduced the striking voltage slightly and made them fire. While we are on a neon nostalgia fest, I used to make audio oscillators out of a neon, resistor and capacitor to form a relaxation oscillator. But as you say you could get an 80V high tension battery for your valve portable radio in those days.